Merge pull request '[opcuaclient] OpcUaClient — Honor server OperationLimits' (#333) from auto/opcuaclient/3 into auto/driver-gaps

Closes #275

src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverCapability.cs

@@ -25,7 +25,7 @@ public enum DriverCapability
     /// <summary><see cref="ITagDiscovery.DiscoverAsync"/>. Retries by default.</summary>
     Discover,
 
-    /// <summary><see cref="ISubscribable.SubscribeAsync"/> and unsubscribe. Retries by default.</summary>
+    /// <summary><see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/> and unsubscribe. Retries by default.</summary>
     Subscribe,
 
     /// <summary><see cref="IHostConnectivityProbe"/> probe loop. Retries by default.</summary>

src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISubscribable.cs

@@ -20,7 +20,29 @@ public interface ISubscribable
         TimeSpan publishingInterval,
         CancellationToken cancellationToken);
 
-    /// <summary>Cancel a subscription returned by <see cref="SubscribeAsync"/>.</summary>
+    /// <summary>
+    ///     Subscribe to data changes with per-tag advanced tuning (sampling interval, queue
+    ///     size, monitoring mode, deadband filter). Drivers that don't have a native concept
+    ///     of these knobs (e.g. polled drivers like Modbus) MAY ignore the per-tag knobs and
+    ///     delegate to the simple
+    ///     <see cref="SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/>
+    ///     overload — the default implementation does exactly that, so existing implementers
+    ///     compile unchanged.
+    /// </summary>
+    /// <param name="tags">Per-tag subscription specs. <see cref="MonitoredTagSpec.TagName"/> is the driver-side full reference.</param>
+    /// <param name="publishingInterval">Subscription publishing interval, applied to the whole batch.</param>
+    /// <param name="cancellationToken">Cancellation.</param>
+    /// <returns>Opaque subscription handle for <see cref="UnsubscribeAsync"/>.</returns>
+    Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<MonitoredTagSpec> tags,
+        TimeSpan publishingInterval,
+        CancellationToken cancellationToken)
+        => SubscribeAsync(
+            tags.Select(t => t.TagName).ToList(),
+            publishingInterval,
+            cancellationToken);
+
+    /// <summary>Cancel a subscription returned by either <c>SubscribeAsync</c> overload.</summary>
     Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken);
 
     /// <summary>
@@ -30,7 +52,7 @@ public interface ISubscribable
     event EventHandler<DataChangeEventArgs>? OnDataChange;
 }
 
-/// <summary>Opaque subscription identity returned by <see cref="ISubscribable.SubscribeAsync"/>.</summary>
+/// <summary>Opaque subscription identity returned by <see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/>.</summary>
 public interface ISubscriptionHandle
 {
     /// <summary>Driver-internal subscription identifier (for diagnostics + post-mortem).</summary>
@@ -38,10 +60,99 @@ public interface ISubscriptionHandle
 }
 
 /// <summary>Event payload for <see cref="ISubscribable.OnDataChange"/>.</summary>
-/// <param name="SubscriptionHandle">The handle returned by the original <see cref="ISubscribable.SubscribeAsync"/> call.</param>
+/// <param name="SubscriptionHandle">The handle returned by the original <see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/> call.</param>
 /// <param name="FullReference">Driver-side full reference of the changed attribute.</param>
 /// <param name="Snapshot">New value + quality + timestamps.</param>
 public sealed record DataChangeEventArgs(
     ISubscriptionHandle SubscriptionHandle,
     string FullReference,
     DataValueSnapshot Snapshot);
+
+/// <summary>
+///     Per-tag subscription tuning. Maps onto OPC UA <c>MonitoredItem</c> properties for the
+///     OpcUaClient driver; non-OPC-UA drivers either map a subset (e.g. ADS picks up
+///     <see cref="SamplingIntervalMs"/>) or ignore the knobs entirely and fall back to the
+///     simple <see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/>.
+/// </summary>
+/// <param name="TagName">Driver-side full reference (e.g. <c>ns=2;s=Foo</c> for OPC UA).</param>
+/// <param name="SamplingIntervalMs">
+///     Server-side sampling rate in milliseconds. <c>null</c> = use the publishing interval.
+///     Sub-publish-interval values let a server sample faster than it publishes (queue +
+///     coalesce), useful for events that change between publish ticks.
+/// </param>
+/// <param name="QueueSize">Server-side notification queue depth. <c>null</c> = driver default (1).</param>
+/// <param name="DiscardOldest">
+///     When the server-side queue overflows: <c>true</c> drops oldest, <c>false</c> drops newest.
+///     <c>null</c> = driver default (true — preserve recency).
+/// </param>
+/// <param name="MonitoringMode">
+///     Per-item monitoring mode. <c>Reporting</c> = sample + publish, <c>Sampling</c> = sample
+///     but suppress publishing (useful with triggering), <c>Disabled</c> = neither.
+/// </param>
+/// <param name="DataChangeFilter">
+///     Optional data-change filter (deadband + trigger semantics). <c>null</c> = no filter
+///     (every change publishes regardless of magnitude).
+/// </param>
+public sealed record MonitoredTagSpec(
+    string TagName,
+    double? SamplingIntervalMs = null,
+    uint? QueueSize = null,
+    bool? DiscardOldest = null,
+    SubscriptionMonitoringMode? MonitoringMode = null,
+    DataChangeFilterSpec? DataChangeFilter = null);
+
+/// <summary>
+///     OPC UA <c>DataChangeFilter</c> spec. Mirrors the OPC UA Part 4 §7.17.2 structure but
+///     lives in Core.Abstractions so non-OpcUaClient drivers (e.g. Modbus, S7) can accept it
+///     as metadata even if they ignore the deadband mechanics.
+/// </summary>
+/// <param name="Trigger">When to fire: status only / status+value / status+value+timestamp.</param>
+/// <param name="DeadbandType">Deadband mode: none / absolute (engineering units) / percent of EURange.</param>
+/// <param name="DeadbandValue">
+///     Magnitude of the deadband. For <see cref="OtOpcUa.Core.Abstractions.DeadbandType.Absolute"/>
+///     this is in the variable's engineering units; for <see cref="OtOpcUa.Core.Abstractions.DeadbandType.Percent"/>
+///     it's a 0..100 percentage of EURange (server returns BadFilterNotAllowed if EURange isn't set).
+/// </param>
+public sealed record DataChangeFilterSpec(
+    DataChangeTrigger Trigger,
+    DeadbandType DeadbandType,
+    double DeadbandValue);
+
+/// <summary>
+///     OPC UA <c>DataChangeTrigger</c> values. Wraps the SDK enum so Core.Abstractions doesn't
+///     leak an OPC-UA-stack reference into every driver project.
+/// </summary>
+public enum DataChangeTrigger
+{
+    /// <summary>Fire only when StatusCode changes.</summary>
+    Status = 0,
+    /// <summary>Fire when StatusCode or Value changes (the OPC UA default).</summary>
+    StatusValue = 1,
+    /// <summary>Fire when StatusCode, Value, or SourceTimestamp changes.</summary>
+    StatusValueTimestamp = 2,
+}
+
+/// <summary>OPC UA deadband-filter modes.</summary>
+public enum DeadbandType
+{
+    /// <summary>No deadband — every value change publishes.</summary>
+    None = 0,
+    /// <summary>Deadband expressed in the variable's engineering units.</summary>
+    Absolute = 1,
+    /// <summary>Deadband expressed as 0..100 percent of the variable's EURange.</summary>
+    Percent = 2,
+}
+
+/// <summary>
+///     Per-item subscription monitoring mode. Wraps the OPC UA SDK's <c>MonitoringMode</c>
+///     so Core.Abstractions stays SDK-free.
+/// </summary>
+public enum SubscriptionMonitoringMode
+{
+    /// <summary>Item is created but neither sampling nor publishing.</summary>
+    Disabled = 0,
+    /// <summary>Item samples and queues but does not publish (useful with triggering).</summary>
+    Sampling = 1,
+    /// <summary>Item samples and publishes — the OPC UA default.</summary>
+    Reporting = 2,
+}

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipArrayReadPlanner.cs

@@ -0,0 +1,94 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+/// <summary>
+///     PR abcip-1.3 — issues one libplctag tag-create with <c>ElementCount=N</c> per Rockwell
+///     array-slice tag (<c>Tag[0..N]</c> in <see cref="AbCipTagPath"/>), then decodes the
+///     contiguous buffer at element stride into <c>N</c> typed values. Mirrors the whole-UDT
+///     planner pattern (<see cref="AbCipUdtReadPlanner"/>): pure shape — the planner never
+///     touches the runtime + never reads the PLC, the driver wires the runtime in.
+/// </summary>
+/// <remarks>
+///     <para>Stride is the natural Logix size of the element type (DInt = 4, Real = 4, LInt = 8).
+///     Bool / String / Structure slices aren't supported here — Logix packs BOOLs into a host
+///     byte (no fixed stride), STRING members carry a Length+DATA pair that's not a flat array,
+///     and structure arrays need the CIP Template Object reader (PR-tracked separately).</para>
+///
+///     <para>Output is a single <c>object[]</c> snapshot value containing the N decoded
+///     elements at indices 0..Count-1. Pairing with one slice tag = one snapshot keeps the
+///     <c>ReadAsync</c> 1:1 contract (one fullReference -> one snapshot) intact.</para>
+/// </remarks>
+public static class AbCipArrayReadPlanner
+{
+    /// <summary>
+    ///     Build the libplctag create-params + decode descriptor for a slice tag. Returns
+    ///     <c>null</c> when the slice element type isn't supported under this declaration-only
+    ///     decoder (Bool / String / Structure / unrecognised) — the driver falls back to the
+    ///     scalar read path so the operator gets a clean per-element result instead.
+    /// </summary>
+    public static AbCipArrayReadPlan? TryBuild(
+        AbCipTagDefinition definition,
+        AbCipTagPath parsedPath,
+        AbCipTagCreateParams baseParams)
+    {
+        ArgumentNullException.ThrowIfNull(definition);
+        ArgumentNullException.ThrowIfNull(parsedPath);
+        ArgumentNullException.ThrowIfNull(baseParams);
+        if (parsedPath.Slice is null) return null;
+
+        if (!TryGetStride(definition.DataType, out var stride)) return null;
+
+        var slice = parsedPath.Slice;
+        var createParams = baseParams with
+        {
+            TagName = parsedPath.ToLibplctagSliceArrayName(),
+            ElementCount = slice.Count,
+        };
+
+        return new AbCipArrayReadPlan(definition.DataType, slice, stride, createParams);
+    }
+
+    /// <summary>
+    ///     Decode <paramref name="plan"/>.Count elements from <paramref name="runtime"/> at
+    ///     element stride. Caller has already invoked <see cref="IAbCipTagRuntime.ReadAsync"/>
+    ///     and confirmed <see cref="IAbCipTagRuntime.GetStatus"/> == 0.
+    /// </summary>
+    public static object?[] Decode(AbCipArrayReadPlan plan, IAbCipTagRuntime runtime)
+    {
+        ArgumentNullException.ThrowIfNull(plan);
+        ArgumentNullException.ThrowIfNull(runtime);
+
+        var values = new object?[plan.Slice.Count];
+        for (var i = 0; i < plan.Slice.Count; i++)
+            values[i] = runtime.DecodeValueAt(plan.ElementType, i * plan.Stride, bitIndex: null);
+        return values;
+    }
+
+    private static bool TryGetStride(AbCipDataType type, out int stride)
+    {
+        switch (type)
+        {
+            case AbCipDataType.SInt: case AbCipDataType.USInt:
+                stride = 1; return true;
+            case AbCipDataType.Int: case AbCipDataType.UInt:
+                stride = 2; return true;
+            case AbCipDataType.DInt: case AbCipDataType.UDInt:
+            case AbCipDataType.Real: case AbCipDataType.Dt:
+                stride = 4; return true;
+            case AbCipDataType.LInt: case AbCipDataType.ULInt:
+            case AbCipDataType.LReal:
+                stride = 8; return true;
+            default:
+                stride = 0; return false;
+        }
+    }
+}
+
+/// <summary>
+///     Plan output: the libplctag create-params for the single array-read tag plus the
+///     element-type / stride / slice metadata the decoder needs.
+/// </summary>
+public sealed record AbCipArrayReadPlan(
+    AbCipDataType ElementType,
+    AbCipTagPathSlice Slice,
+    int Stride,
+    AbCipTagCreateParams CreateParams);

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDataType.cs

@@ -50,11 +50,12 @@ public static class AbCipDataTypeExtensions
         AbCipDataType.Bool => DriverDataType.Boolean,
         AbCipDataType.SInt or AbCipDataType.Int or AbCipDataType.DInt => DriverDataType.Int32,
         AbCipDataType.USInt or AbCipDataType.UInt or AbCipDataType.UDInt => DriverDataType.Int32,
-        AbCipDataType.LInt or AbCipDataType.ULInt => DriverDataType.Int32, // TODO: Int64 — matches Modbus gap
+        AbCipDataType.LInt => DriverDataType.Int64,
+        AbCipDataType.ULInt => DriverDataType.UInt64,
         AbCipDataType.Real => DriverDataType.Float32,
         AbCipDataType.LReal => DriverDataType.Float64,
         AbCipDataType.String => DriverDataType.String,
-        AbCipDataType.Dt => DriverDataType.Int32, // epoch-seconds DINT
+        AbCipDataType.Dt => DriverDataType.Int64, // Logix v32+ DT == LINT epoch-millis
         AbCipDataType.Structure => DriverDataType.String, // placeholder until UDT PR 6 introduces a structured kind
         _ => DriverDataType.Int32,
     };

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs

@@ -134,7 +134,8 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                             TagPath: $"{tag.TagPath}.{member.Name}",
                             DataType: member.DataType,
                             Writable: member.Writable,
-                            WriteIdempotent: member.WriteIdempotent);
+                            WriteIdempotent: member.WriteIdempotent,
+                            StringLength: member.StringLength);
                         _tagsByName[memberTag.Name] = memberTag;
                     }
                 }
@@ -357,6 +358,17 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             return;
         }
 
+        // PR abcip-1.3 — array-slice path. A tag whose TagPath ends in [N..M] dispatches to
+        // AbCipArrayReadPlanner: one libplctag tag-create with ElementCount=N issues one
+        // Rockwell array read; the contiguous buffer is decoded at element stride into a
+        // single snapshot whose Value is an object[] of the N elements.
+        var parsedPath = AbCipTagPath.TryParse(def.TagPath);
+        if (parsedPath?.Slice is not null)
+        {
+            await ReadSliceAsync(fb, def, parsedPath, device, results, now, ct).ConfigureAwait(false);
+            return;
+        }
+
         try
         {
             var runtime = await EnsureTagRuntimeAsync(device, def, ct).ConfigureAwait(false);
@@ -372,8 +384,7 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                 return;
             }
 
-            var tagPath = AbCipTagPath.TryParse(def.TagPath);
-            var bitIndex = tagPath?.BitIndex;
+            var bitIndex = parsedPath?.BitIndex;
             var value = runtime.DecodeValue(def.DataType, bitIndex);
             results[fb.OriginalIndex] = new DataValueSnapshot(value, AbCipStatusMapper.Good, now, now);
             _health = new DriverHealth(DriverState.Healthy, now, null);
@@ -390,6 +401,89 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         }
     }
 
+    /// <summary>
+    ///     PR abcip-1.3 — slice read path. Builds an <see cref="AbCipArrayReadPlan"/> from the
+    ///     parsed slice path, materialises a per-tag runtime keyed by the tag's full name (so
+    ///     repeat reads reuse the same libplctag handle), issues one PLC array read, and
+    ///     decodes the contiguous buffer into <c>object?[]</c> at element stride. Unsupported
+    ///     element types fall back to <see cref="AbCipStatusMapper.BadNotSupported"/>.
+    /// </summary>
+    private async Task ReadSliceAsync(
+        AbCipUdtReadFallback fb, AbCipTagDefinition def, AbCipTagPath parsedPath,
+        DeviceState device, DataValueSnapshot[] results, DateTime now, CancellationToken ct)
+    {
+        var baseParams = new AbCipTagCreateParams(
+            Gateway: device.ParsedAddress.Gateway,
+            Port: device.ParsedAddress.Port,
+            CipPath: device.ParsedAddress.CipPath,
+            LibplctagPlcAttribute: device.Profile.LibplctagPlcAttribute,
+            TagName: parsedPath.ToLibplctagName(),
+            Timeout: _options.Timeout);
+
+        var plan = AbCipArrayReadPlanner.TryBuild(def, parsedPath, baseParams);
+        if (plan is null)
+        {
+            results[fb.OriginalIndex] = new DataValueSnapshot(null,
+                AbCipStatusMapper.BadNotSupported, null, now);
+            return;
+        }
+
+        try
+        {
+            var runtime = await EnsureSliceRuntimeAsync(device, def.Name, plan.CreateParams, ct)
+                .ConfigureAwait(false);
+            await runtime.ReadAsync(ct).ConfigureAwait(false);
+
+            var status = runtime.GetStatus();
+            if (status != 0)
+            {
+                results[fb.OriginalIndex] = new DataValueSnapshot(null,
+                    AbCipStatusMapper.MapLibplctagStatus(status), null, now);
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead,
+                    $"libplctag status {status} reading slice {def.Name}");
+                return;
+            }
+
+            var values = AbCipArrayReadPlanner.Decode(plan, runtime);
+            results[fb.OriginalIndex] = new DataValueSnapshot(values, AbCipStatusMapper.Good, now, now);
+            _health = new DriverHealth(DriverState.Healthy, now, null);
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            results[fb.OriginalIndex] = new DataValueSnapshot(null,
+                AbCipStatusMapper.BadCommunicationError, null, now);
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+        }
+    }
+
+    /// <summary>
+    ///     Idempotently materialise a slice-read runtime. Slice runtimes share the device's
+    ///     <see cref="DeviceState.Runtimes"/> dict keyed by the tag's full name so repeated
+    ///     reads reuse the same libplctag handle without re-creating the native tag every poll.
+    /// </summary>
+    private async Task<IAbCipTagRuntime> EnsureSliceRuntimeAsync(
+        DeviceState device, string tagName, AbCipTagCreateParams createParams, CancellationToken ct)
+    {
+        if (device.Runtimes.TryGetValue(tagName, out var existing)) return existing;
+
+        var runtime = _tagFactory.Create(createParams);
+        try
+        {
+            await runtime.InitializeAsync(ct).ConfigureAwait(false);
+        }
+        catch
+        {
+            runtime.Dispose();
+            throw;
+        }
+        device.Runtimes[tagName] = runtime;
+        return runtime;
+    }
+
     /// <summary>
     ///     Task #194 — perform one whole-UDT read on the parent tag, then decode each
     ///     grouped member from the runtime's buffer at its computed byte offset. A per-group
@@ -451,100 +545,184 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
     // ---- IWritable ----
 
     /// <summary>
-    ///     Write each request in order. Writes are NOT auto-retried by the driver — per plan
-    ///     decisions #44, #45, #143 the caller opts in via <see cref="AbCipTagDefinition.WriteIdempotent"/>
-    ///     and the resilience pipeline (layered above the driver) decides whether to replay.
-    ///     Non-writable configurations surface as <c>BadNotWritable</c>; type-conversion failures
-    ///     as <c>BadTypeMismatch</c>; transport errors as <c>BadCommunicationError</c>.
+    ///     Write each request in the batch. Writes are NOT auto-retried by the driver — per
+    ///     plan decisions #44, #45, #143 the caller opts in via
+    ///     <see cref="AbCipTagDefinition.WriteIdempotent"/> and the resilience pipeline (layered
+    ///     above the driver) decides whether to replay. Non-writable configurations surface as
+    ///     <c>BadNotWritable</c>; type-conversion failures as <c>BadTypeMismatch</c>; transport
+    ///     errors as <c>BadCommunicationError</c>.
     /// </summary>
+    /// <remarks>
+    ///     PR abcip-1.4 — multi-tag write packing. Writes are grouped by device via
+    ///     <see cref="AbCipMultiWritePlanner"/>. Devices whose family
+    ///     <see cref="AbCipPlcFamilyProfile.SupportsRequestPacking"/> is <c>true</c> dispatch
+    ///     their packable writes concurrently so libplctag's native scheduler can coalesce them
+    ///     onto one CIP Multi-Service Packet (0x0A) per round-trip; Micro800 (no packing) still
+    ///     issues writes one-at-a-time. BOOL-within-DINT writes always go through the RMW path
+    ///     under a per-parent semaphore, regardless of the family flag, because two concurrent
+    ///     RMWs on the same DINT could lose one another's update. Per-tag StatusCodes are
+    ///     preserved in the caller's input order on partial failures.
+    /// </remarks>
     public async Task<IReadOnlyList<WriteResult>> WriteAsync(
         IReadOnlyList<WriteRequest> writes, CancellationToken cancellationToken)
     {
         ArgumentNullException.ThrowIfNull(writes);
         var results = new WriteResult[writes.Count];
-        var now = DateTime.UtcNow;
 
-        for (var i = 0; i < writes.Count; i++)
+        var plans = AbCipMultiWritePlanner.Build(
+            writes, _tagsByName, _devices,
+            reportPreflight: (idx, code) => results[idx] = new WriteResult(code));
+
+        foreach (var plan in plans)
         {
-            var w = writes[i];
-            if (!_tagsByName.TryGetValue(w.FullReference, out var def))
+            if (!_devices.TryGetValue(plan.DeviceHostAddress, out var device))
             {
-                results[i] = new WriteResult(AbCipStatusMapper.BadNodeIdUnknown);
-                continue;
-            }
-            if (!def.Writable || def.SafetyTag)
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadNotWritable);
-                continue;
-            }
-            if (!_devices.TryGetValue(def.DeviceHostAddress, out var device))
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadNodeIdUnknown);
+                foreach (var e in plan.Packable) results[e.OriginalIndex] = new WriteResult(AbCipStatusMapper.BadNodeIdUnknown);
+                foreach (var e in plan.BitRmw) results[e.OriginalIndex] = new WriteResult(AbCipStatusMapper.BadNodeIdUnknown);
                 continue;
             }
 
-            try
-            {
-                var parsedPath = AbCipTagPath.TryParse(def.TagPath);
+            // Bit-RMW writes always serialise per-parent — never packed.
+            foreach (var entry in plan.BitRmw)
+                results[entry.OriginalIndex] = new WriteResult(
+                    await ExecuteBitRmwWriteAsync(device, entry, cancellationToken).ConfigureAwait(false));
 
-                // BOOL-within-DINT writes — per task #181, RMW against a parallel parent-DINT
-                // runtime. Dispatching here keeps the normal EncodeValue path clean; the
-                // per-parent lock prevents two concurrent bit writes to the same DINT from
-                // losing one another's update.
-                if (def.DataType == AbCipDataType.Bool && parsedPath?.BitIndex is int bit)
+            if (plan.Packable.Count == 0) continue;
+
+            if (plan.Profile.SupportsRequestPacking && plan.Packable.Count > 1)
+            {
+                // Concurrent dispatch — libplctag's native scheduler packs same-connection writes
+                // into one Multi-Service Packet when the family supports it.
+                var tasks = new Task<(int idx, uint code)>[plan.Packable.Count];
+                for (var i = 0; i < plan.Packable.Count; i++)
                 {
-                    results[i] = new WriteResult(
-                        await WriteBitInDIntAsync(device, parsedPath, bit, w.Value, cancellationToken)
-                            .ConfigureAwait(false));
-                    if (results[i].StatusCode == AbCipStatusMapper.Good)
-                        _health = new DriverHealth(DriverState.Healthy, now, null);
-                    continue;
+                    var entry = plan.Packable[i];
+                    tasks[i] = ExecutePackableWriteAsync(device, entry, cancellationToken);
                 }
-
-                var runtime = await EnsureTagRuntimeAsync(device, def, cancellationToken).ConfigureAwait(false);
-                runtime.EncodeValue(def.DataType, parsedPath?.BitIndex, w.Value);
-                await runtime.WriteAsync(cancellationToken).ConfigureAwait(false);
-
-                var status = runtime.GetStatus();
-                results[i] = new WriteResult(status == 0
-                    ? AbCipStatusMapper.Good
-                    : AbCipStatusMapper.MapLibplctagStatus(status));
-                if (status == 0) _health = new DriverHealth(DriverState.Healthy, now, null);
+                var outcomes = await Task.WhenAll(tasks).ConfigureAwait(false);
+                foreach (var (idx, code) in outcomes)
+                    results[idx] = new WriteResult(code);
             }
-            catch (OperationCanceledException)
+            else
             {
-                throw;
-            }
-            catch (NotSupportedException nse)
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadNotSupported);
-                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, nse.Message);
-            }
-            catch (FormatException fe)
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadTypeMismatch);
-                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, fe.Message);
-            }
-            catch (InvalidCastException ice)
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadTypeMismatch);
-                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ice.Message);
-            }
-            catch (OverflowException oe)
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadOutOfRange);
-                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, oe.Message);
-            }
-            catch (Exception ex)
-            {
-                results[i] = new WriteResult(AbCipStatusMapper.BadCommunicationError);
-                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+                // Single-write groups + Micro800 (SupportsRequestPacking=false) — sequential.
+                foreach (var entry in plan.Packable)
+                {
+                    var code = await ExecutePackableWriteAsync(device, entry, cancellationToken)
+                        .ConfigureAwait(false);
+                    results[entry.OriginalIndex] = new WriteResult(code.code);
+                }
             }
         }
 
         return results;
     }
 
+    /// <summary>
+    ///     Execute one packable write — encode the value into the per-tag runtime, flush, and
+    ///     map the resulting libplctag status. Exception-to-StatusCode mapping mirrors the
+    ///     pre-1.4 per-tag loop so callers see no behaviour change for individual writes.
+    /// </summary>
+    private async Task<(int idx, uint code)> ExecutePackableWriteAsync(
+        DeviceState device, AbCipMultiWritePlanner.ClassifiedWrite entry, CancellationToken ct)
+    {
+        var def = entry.Definition;
+        var w = entry.Request;
+        var now = DateTime.UtcNow;
+        try
+        {
+            var runtime = await EnsureTagRuntimeAsync(device, def, ct).ConfigureAwait(false);
+            runtime.EncodeValue(def.DataType, entry.ParsedPath?.BitIndex, w.Value);
+            await runtime.WriteAsync(ct).ConfigureAwait(false);
+
+            var status = runtime.GetStatus();
+            if (status == 0)
+            {
+                _health = new DriverHealth(DriverState.Healthy, now, null);
+                return (entry.OriginalIndex, AbCipStatusMapper.Good);
+            }
+            return (entry.OriginalIndex, AbCipStatusMapper.MapLibplctagStatus(status));
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (NotSupportedException nse)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, nse.Message);
+            return (entry.OriginalIndex, AbCipStatusMapper.BadNotSupported);
+        }
+        catch (FormatException fe)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, fe.Message);
+            return (entry.OriginalIndex, AbCipStatusMapper.BadTypeMismatch);
+        }
+        catch (InvalidCastException ice)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ice.Message);
+            return (entry.OriginalIndex, AbCipStatusMapper.BadTypeMismatch);
+        }
+        catch (OverflowException oe)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, oe.Message);
+            return (entry.OriginalIndex, AbCipStatusMapper.BadOutOfRange);
+        }
+        catch (Exception ex)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            return (entry.OriginalIndex, AbCipStatusMapper.BadCommunicationError);
+        }
+    }
+
+    /// <summary>
+    ///     Execute one BOOL-within-DINT write through <see cref="WriteBitInDIntAsync"/>, with
+    ///     the same exception-mapping fan-out as the pre-1.4 per-tag loop. Bit RMWs cannot be
+    ///     packed because two concurrent writes against the same parent DINT would race their
+    ///     read-modify-write windows.
+    /// </summary>
+    private async Task<uint> ExecuteBitRmwWriteAsync(
+        DeviceState device, AbCipMultiWritePlanner.ClassifiedWrite entry, CancellationToken ct)
+    {
+        try
+        {
+            var bit = entry.ParsedPath!.BitIndex!.Value;
+            var code = await WriteBitInDIntAsync(device, entry.ParsedPath, bit, entry.Request.Value, ct)
+                .ConfigureAwait(false);
+            if (code == AbCipStatusMapper.Good)
+                _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
+            return code;
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (NotSupportedException nse)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, nse.Message);
+            return AbCipStatusMapper.BadNotSupported;
+        }
+        catch (FormatException fe)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, fe.Message);
+            return AbCipStatusMapper.BadTypeMismatch;
+        }
+        catch (InvalidCastException ice)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ice.Message);
+            return AbCipStatusMapper.BadTypeMismatch;
+        }
+        catch (OverflowException oe)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, oe.Message);
+            return AbCipStatusMapper.BadOutOfRange;
+        }
+        catch (Exception ex)
+        {
+            _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            return AbCipStatusMapper.BadCommunicationError;
+        }
+    }
+
     /// <summary>
     ///     Read-modify-write one bit within a DINT parent. Creates / reuses a parallel
     ///     parent-DINT runtime (distinct from the bit-selector handle) + serialises concurrent
@@ -633,7 +811,8 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             CipPath: device.ParsedAddress.CipPath,
             LibplctagPlcAttribute: device.Profile.LibplctagPlcAttribute,
             TagName: parsed.ToLibplctagName(),
-            Timeout: _options.Timeout));
+            Timeout: _options.Timeout,
+            StringMaxCapacity: def.DataType == AbCipDataType.String ? def.StringLength : null));
         try
         {
             await runtime.InitializeAsync(ct).ConfigureAwait(false);

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriverOptions.cs

@@ -92,6 +92,13 @@ public sealed record AbCipDeviceOptions(
 ///     GuardLogix controller; non-safety writes violate the safety-partition isolation and are
 ///     rejected by the PLC anyway. Surfaces the intent explicitly instead of relying on the
 ///     write attempt failing at runtime.</param>
+/// <param name="StringLength">Capacity of the DATA character array on a Logix STRING / STRINGnn
+///     UDT — 82 for the stock <c>STRING</c>, 20/40/80/etc for user-defined <c>STRING_20</c>,
+///     <c>STRING_40</c>, <c>STRING_80</c> variants. Threads through libplctag's
+///     <c>str_max_capacity</c> attribute so the wrapper allocates the correct backing buffer
+///     and <c>GetString</c> / <c>SetString</c> truncate at the right boundary. <c>null</c>
+///     keeps libplctag's default 82-byte STRING behaviour for back-compat. Ignored for
+///     non-<see cref="AbCipDataType.String"/> types.</param>
 public sealed record AbCipTagDefinition(
     string Name,
     string DeviceHostAddress,
@@ -100,7 +107,8 @@ public sealed record AbCipTagDefinition(
     bool Writable = true,
     bool WriteIdempotent = false,
     IReadOnlyList<AbCipStructureMember>? Members = null,
-    bool SafetyTag = false);
+    bool SafetyTag = false,
+    int? StringLength = null);
 
 /// <summary>
 ///     One declared member of a UDT tag. Name is the member identifier on the PLC (e.g. <c>Speed</c>,
@@ -112,7 +120,8 @@ public sealed record AbCipStructureMember(
     string Name,
     AbCipDataType DataType,
     bool Writable = true,
-    bool WriteIdempotent = false);
+    bool WriteIdempotent = false,
+    int? StringLength = null);
 
 /// <summary>Which AB PLC family the device is — selects the profile applied to connection params.</summary>
 public enum AbCipPlcFamily

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipMultiWritePlanner.cs

@@ -0,0 +1,112 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip.PlcFamilies;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+/// <summary>
+///     PR abcip-1.4 — multi-tag write planner. Groups a batch of <see cref="WriteRequest"/>s by
+///     device so the driver can submit one round of writes per device instead of looping
+///     strictly serially across the whole batch. Honours the per-family
+///     <see cref="AbCipPlcFamilyProfile.SupportsRequestPacking"/> flag: families that support
+///     CIP request packing (ControlLogix / CompactLogix / GuardLogix) issue their writes in
+///     parallel so libplctag's internal scheduler can coalesce them onto one Multi-Service
+///     Packet (0x0A); Micro800 (no request packing) falls back to per-tag sequential writes.
+/// </summary>
+/// <remarks>
+///     <para>The libplctag .NET wrapper exposes one CIP service per <c>Tag</c> instance and does
+///     not surface Multi-Service Packet construction at the API surface — but the underlying
+///     native library packs concurrent operations against the same connection automatically
+///     when the family's protocol supports it. Issuing the writes concurrently per device
+///     therefore gives us the round-trip reduction described in #228 without having to drop to
+///     raw CIP, while still letting us short-circuit packing on Micro800 where it would be
+///     unsafe.</para>
+///
+///     <para>Bit-RMW writes (BOOL-with-bitIndex against a DINT parent) are excluded from
+///     packing here because they need a serialised read-modify-write under the per-parent
+///     <c>SemaphoreSlim</c> in <see cref="AbCipDriver.WriteBitInDIntAsync"/>. Packing two RMWs
+///     on the same DINT would risk losing one another's update.</para>
+/// </remarks>
+internal static class AbCipMultiWritePlanner
+{
+    /// <summary>
+    ///     One classified entry in the input batch. <see cref="OriginalIndex"/> preserves the
+    ///     caller's ordering so per-tag <c>StatusCode</c> fan-out lands at the right slot in
+    ///     the result array. <see cref="IsBitRmw"/> routes the entry through the RMW path even
+    ///     when the device supports packing.
+    /// </summary>
+    internal readonly record struct ClassifiedWrite(
+        int OriginalIndex,
+        WriteRequest Request,
+        AbCipTagDefinition Definition,
+        AbCipTagPath? ParsedPath,
+        bool IsBitRmw);
+
+    /// <summary>
+    ///     One device's plan slice. <see cref="Packable"/> entries can be issued concurrently;
+    ///     <see cref="BitRmw"/> entries must go through the RMW path one-at-a-time per parent
+    ///     DINT.
+    /// </summary>
+    internal sealed class DevicePlan
+    {
+        public required string DeviceHostAddress { get; init; }
+        public required AbCipPlcFamilyProfile Profile { get; init; }
+        public List<ClassifiedWrite> Packable { get; } = new();
+        public List<ClassifiedWrite> BitRmw { get; } = new();
+    }
+
+    /// <summary>
+    ///     Build the per-device plan list. Entries are visited in input order so the resulting
+    ///     plan's traversal preserves caller ordering within each device. Entries that fail
+    ///     resolution (unknown reference, non-writable tag, unknown device) are reported via
+    ///     <paramref name="reportPreflight"/> with the appropriate StatusCode and excluded from
+    ///     the plan.
+    /// </summary>
+    public static IReadOnlyList<DevicePlan> Build(
+        IReadOnlyList<WriteRequest> writes,
+        IReadOnlyDictionary<string, AbCipTagDefinition> tagsByName,
+        IReadOnlyDictionary<string, AbCipDriver.DeviceState> devices,
+        Action<int, uint> reportPreflight)
+    {
+        var plans = new Dictionary<string, DevicePlan>(StringComparer.OrdinalIgnoreCase);
+        var order = new List<DevicePlan>();
+
+        for (var i = 0; i < writes.Count; i++)
+        {
+            var w = writes[i];
+            if (!tagsByName.TryGetValue(w.FullReference, out var def))
+            {
+                reportPreflight(i, AbCipStatusMapper.BadNodeIdUnknown);
+                continue;
+            }
+            if (!def.Writable || def.SafetyTag)
+            {
+                reportPreflight(i, AbCipStatusMapper.BadNotWritable);
+                continue;
+            }
+            if (!devices.TryGetValue(def.DeviceHostAddress, out var device))
+            {
+                reportPreflight(i, AbCipStatusMapper.BadNodeIdUnknown);
+                continue;
+            }
+
+            if (!plans.TryGetValue(def.DeviceHostAddress, out var plan))
+            {
+                plan = new DevicePlan
+                {
+                    DeviceHostAddress = def.DeviceHostAddress,
+                    Profile = device.Profile,
+                };
+                plans[def.DeviceHostAddress] = plan;
+                order.Add(plan);
+            }
+
+            var parsed = AbCipTagPath.TryParse(def.TagPath);
+            var isBitRmw = def.DataType == AbCipDataType.Bool && parsed?.BitIndex is int;
+            var entry = new ClassifiedWrite(i, w, def, parsed, isBitRmw);
+            if (isBitRmw) plan.BitRmw.Add(entry);
+            else plan.Packable.Add(entry);
+        }
+
+        return order;
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipTagPath.cs

@@ -20,7 +20,8 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip;
 public sealed record AbCipTagPath(
     string? ProgramScope,
     IReadOnlyList<AbCipTagPathSegment> Segments,
-    int? BitIndex)
+    int? BitIndex,
+    AbCipTagPathSlice? Slice = null)
 {
     /// <summary>Rebuild the canonical Logix tag string.</summary>
     public string ToLibplctagName()
@@ -37,10 +38,39 @@ public sealed record AbCipTagPath(
             if (seg.Subscripts.Count > 0)
                 buf.Append('[').Append(string.Join(",", seg.Subscripts)).Append(']');
         }
+        if (Slice is not null) buf.Append('[').Append(Slice.Start).Append("..").Append(Slice.End).Append(']');
         if (BitIndex is not null) buf.Append('.').Append(BitIndex.Value);
         return buf.ToString();
     }
 
+    /// <summary>
+    ///     Logix-symbol form for issuing a single libplctag tag-create that reads the slice as a
+    ///     contiguous buffer — i.e. the bare array name (with the start subscript) without the
+    ///     <c>..End</c> suffix. The driver pairs this with <see cref="AbCipTagCreateParams.ElementCount"/>
+    ///     = <see cref="AbCipTagPathSlice.Count"/> to issue a single Rockwell array read.
+    /// </summary>
+    public string ToLibplctagSliceArrayName()
+    {
+        if (Slice is null) return ToLibplctagName();
+        var buf = new System.Text.StringBuilder();
+        if (ProgramScope is not null)
+            buf.Append("Program:").Append(ProgramScope).Append('.');
+
+        for (var i = 0; i < Segments.Count; i++)
+        {
+            if (i > 0) buf.Append('.');
+            var seg = Segments[i];
+            buf.Append(seg.Name);
+            if (seg.Subscripts.Count > 0)
+                buf.Append('[').Append(string.Join(",", seg.Subscripts)).Append(']');
+        }
+        // Anchor the read at the slice start; libplctag treats Name=Tag[0] + ElementCount=N as
+        // "read N consecutive elements starting at index 0", which is the exact Rockwell
+        // array-read semantic this PR is wiring up.
+        buf.Append('[').Append(Slice.Start).Append(']');
+        return buf.ToString();
+    }
+
     /// <summary>
     ///     Parse a Logix-symbolic tag reference. Returns <c>null</c> on a shape the parser
     ///     doesn't support — the driver surfaces that as a config-validation error rather than
@@ -91,8 +121,10 @@ public sealed record AbCipTagPath(
         }
 
         var segments = new List<AbCipTagPathSegment>(parts.Count);
-        foreach (var part in parts)
+        AbCipTagPathSlice? slice = null;
+        for (var partIdx = 0; partIdx < parts.Count; partIdx++)
         {
+            var part = parts[partIdx];
             var bracketIdx = part.IndexOf('[');
             if (bracketIdx < 0)
             {
@@ -104,6 +136,25 @@ public sealed record AbCipTagPath(
             var name = part[..bracketIdx];
             if (!IsValidIdent(name)) return null;
             var inner = part[(bracketIdx + 1)..^1];
+
+            // Slice syntax `[N..M]` — only allowed on the LAST segment, must not coexist with
+            // multi-dim subscripts, must not be combined with bit-index, and requires M >= N.
+            // Any other shape is rejected so callers see a config-validation error rather than
+            // the driver attempting a best-effort scalar read.
+            if (inner.Contains(".."))
+            {
+                if (partIdx != parts.Count - 1) return null;          // slice + sub-element
+                if (bitIndex is not null) return null;                // slice + bit index
+                if (inner.Contains(',')) return null;                 // slice cannot be multi-dim
+                var parts2 = inner.Split("..", 2, StringSplitOptions.None);
+                if (parts2.Length != 2) return null;
+                if (!int.TryParse(parts2[0], out var sliceStart) || sliceStart < 0) return null;
+                if (!int.TryParse(parts2[1], out var sliceEnd) || sliceEnd < sliceStart) return null;
+                slice = new AbCipTagPathSlice(sliceStart, sliceEnd);
+                segments.Add(new AbCipTagPathSegment(name, []));
+                continue;
+            }
+
             var subs = new List<int>();
             foreach (var tok in inner.Split(','))
             {
@@ -115,7 +166,7 @@ public sealed record AbCipTagPath(
         }
         if (segments.Count == 0) return null;
 
-        return new AbCipTagPath(programScope, segments, bitIndex);
+        return new AbCipTagPath(programScope, segments, bitIndex, slice);
     }
 
     private static bool IsValidIdent(string s)
@@ -130,3 +181,15 @@ public sealed record AbCipTagPath(
 
 /// <summary>One path segment: a member name plus any numeric subscripts.</summary>
 public sealed record AbCipTagPathSegment(string Name, IReadOnlyList<int> Subscripts);
+
+/// <summary>
+///     Inclusive-on-both-ends array slice carried on the trailing segment of an
+///     <see cref="AbCipTagPath"/>. <c>Tag[0..15]</c> parses to <c>Start=0, End=15</c>; the
+///     planner pairs this with libplctag's <c>ElementCount</c> attribute to issue a single
+///     Rockwell array read covering <c>End - Start + 1</c> elements.
+/// </summary>
+public sealed record AbCipTagPathSlice(int Start, int End)
+{
+    /// <summary>Total element count covered by the slice (inclusive both ends).</summary>
+    public int Count => End - Start + 1;
+}

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/IAbCipTagRuntime.cs

@@ -65,10 +65,20 @@ public interface IAbCipTagFactory
 /// <param name="LibplctagPlcAttribute">libplctag <c>plc=...</c> attribute, per family profile.</param>
 /// <param name="TagName">Logix symbolic tag name as emitted by <see cref="AbCipTagPath.ToLibplctagName"/>.</param>
 /// <param name="Timeout">libplctag operation timeout (applies to Initialize / Read / Write).</param>
+/// <param name="StringMaxCapacity">Optional Logix STRINGnn DATA-array capacity (e.g. 20 / 40 / 80
+///     for <c>STRING_20</c> / <c>STRING_40</c> / <c>STRING_80</c> UDTs). Threads through libplctag's
+///     <c>str_max_capacity</c> attribute. <c>null</c> keeps libplctag's default 82-byte STRING
+///     behaviour for back-compat.</param>
+/// <param name="ElementCount">Optional libplctag <c>ElementCount</c> override — set to <c>N</c>
+///     to issue a Rockwell array read covering <c>N</c> consecutive elements starting at the
+///     subscripted index in <see cref="TagName"/>. Drives PR abcip-1.3 array-slice support;
+///     <c>null</c> leaves libplctag's default scalar-element behaviour for back-compat.</param>
 public sealed record AbCipTagCreateParams(
     string Gateway,
     int Port,
     string CipPath,
     string LibplctagPlcAttribute,
     string TagName,
-    TimeSpan Timeout);
+    TimeSpan Timeout,
+    int? StringMaxCapacity = null,
+    int? ElementCount = null);

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/LibplctagTagRuntime.cs

@@ -24,6 +24,17 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
             Name = p.TagName,
             Timeout = p.Timeout,
         };
+        // PR abcip-1.2 — Logix STRINGnn variant decoding. When the caller pins a non-default
+        // DATA-array capacity (STRING_20 / STRING_40 / STRING_80 etc.), forward it to libplctag
+        // via the StringMaxCapacity attribute so GetString / SetString truncate at the right
+        // boundary. Null leaves libplctag at its default 82-byte STRING for back-compat.
+        if (p.StringMaxCapacity is int cap && cap > 0)
+            _tag.StringMaxCapacity = (uint)cap;
+        // PR abcip-1.3 — slice reads. Setting ElementCount tells libplctag to allocate a buffer
+        // covering N consecutive elements; the array-read planner pairs this with TagName=Tag[N]
+        // to issue one Rockwell array read for a [N..M] slice.
+        if (p.ElementCount is int n && n > 0)
+            _tag.ElementCount = n;
     }
 
     public Task InitializeAsync(CancellationToken cancellationToken) => _tag.InitializeAsync(cancellationToken);
@@ -50,7 +61,7 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
         AbCipDataType.Real => _tag.GetFloat32(offset),
         AbCipDataType.LReal => _tag.GetFloat64(offset),
         AbCipDataType.String => _tag.GetString(offset),
-        AbCipDataType.Dt => _tag.GetInt32(offset),
+        AbCipDataType.Dt => _tag.GetInt64(offset),
         AbCipDataType.Structure => null,
         _ => null,
     };
@@ -105,7 +116,7 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
                 _tag.SetString(0, Convert.ToString(value) ?? string.Empty);
                 break;
             case AbCipDataType.Dt:
-                _tag.SetInt32(0, Convert.ToInt32(value));
+                _tag.SetInt64(0, Convert.ToInt64(value));
                 break;
             case AbCipDataType.Structure:
                 throw new NotSupportedException("Whole-UDT writes land in PR 6.");

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs

@@ -1,3 +1,5 @@
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.PlcFamilies;
+
 namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
 
 /// <summary>
@@ -30,35 +32,87 @@ public sealed record AbLegacyAddress(
     int? FileNumber,
     int WordNumber,
     int? BitIndex,
-    string? SubElement)
+    string? SubElement,
+    AbLegacyAddress? IndirectFileSource = null,
+    AbLegacyAddress? IndirectWordSource = null)
 {
+    /// <summary>
+    ///     True when either the file number or the word number is sourced from another PCCC
+    ///     address evaluated at runtime (PLC-5 / SLC indirect addressing — <c>N7:[N7:0]</c> or
+    ///     <c>N[N7:0]:5</c>). libplctag PCCC does not natively decode bracket-form indirection,
+    ///     so the runtime layer must resolve the inner address first and rewrite the tag name
+    ///     before issuing the actual read/write. See <see cref="ToLibplctagName"/>.
+    /// </summary>
+    public bool IsIndirect => IndirectFileSource is not null || IndirectWordSource is not null;
+
     public string ToLibplctagName()
     {
-        var file = FileNumber is null ? FileLetter : $"{FileLetter}{FileNumber}";
-        var wordPart = $"{file}:{WordNumber}";
+        // Re-emit using bracket form when indirect. libplctag's PCCC text decoder does not
+        // accept the bracket form directly — callers that need a libplctag-ready name must
+        // resolve the inner addresses first and substitute concrete numbers. Driver runtime
+        // path (TODO: resolve-then-read) is gated on IsIndirect.
+        string filePart;
+        if (IndirectFileSource is not null)
+        {
+            filePart = $"{FileLetter}[{IndirectFileSource.ToLibplctagName()}]";
+        }
+        else
+        {
+            filePart = FileNumber is null ? FileLetter : $"{FileLetter}{FileNumber}";
+        }
+
+        string wordSegment = IndirectWordSource is not null
+            ? $"[{IndirectWordSource.ToLibplctagName()}]"
+            : WordNumber.ToString();
+
+        var wordPart = $"{filePart}:{wordSegment}";
         if (SubElement is not null) wordPart += $".{SubElement}";
         if (BitIndex is not null) wordPart += $"/{BitIndex}";
         return wordPart;
     }
 
-    public static AbLegacyAddress? TryParse(string? value)
+    public static AbLegacyAddress? TryParse(string? value) => TryParse(value, family: null);
+
+    /// <summary>
+    ///     Family-aware parser. PLC-5 (RSLogix 5) displays the word + bit indices on
+    ///     <c>I:</c>/<c>O:</c> file references as octal — <c>I:001/17</c> is rack 1, bit 15.
+    ///     Pass the device's family so the parser can interpret those digits as octal when the
+    ///     family's <see cref="AbLegacyPlcFamilyProfile.OctalIoAddressing"/> is true. The parsed
+    ///     record stores decimal values; <see cref="ToLibplctagName"/> emits decimal too, which
+    ///     is what libplctag's PCCC layer expects.
+    /// </summary>
+    /// <remarks>
+    ///     Also accepts indirect / indexed forms (Issue #247): <c>N7:[N7:0]</c> reads file 7,
+    ///     word=value-of(N7:0); <c>N[N7:0]:5</c> reads file=value-of(N7:0), word 5. Recursion
+    ///     depth is capped at 1 — the inner address must be a plain direct PCCC address.
+    /// </remarks>
+    public static AbLegacyAddress? TryParse(string? value, AbLegacyPlcFamily? family)
     {
         if (string.IsNullOrWhiteSpace(value)) return null;
         var src = value.Trim();
 
-        // BitIndex: trailing /N
-        int? bitIndex = null;
-        var slashIdx = src.IndexOf('/');
-        if (slashIdx >= 0)
+        var profile = family is null ? null : AbLegacyPlcFamilyProfile.ForFamily(family.Value);
+
+        // BitIndex: trailing /N. Defer numeric parsing until the file letter is known — PLC-5
+        // I:/O: bit indices are octal in RSLogix 5, everything else is decimal.
+        string? bitText = null;
+        var slashIdx = src.LastIndexOf('/');
+        if (slashIdx >= 0 && slashIdx > src.LastIndexOf(']'))
         {
-            if (!int.TryParse(src[(slashIdx + 1)..], out var bit) || bit < 0 || bit > 31) return null;
-            bitIndex = bit;
+            bitText = src[(slashIdx + 1)..];
             src = src[..slashIdx];
         }
 
+        return ParseTail(src, bitText, profile, allowIndirect: true);
+    }
+
+    private static AbLegacyAddress? ParseTail(string src, string? bitText, AbLegacyPlcFamilyProfile? profile, bool allowIndirect)
+    {
         // SubElement: trailing .NAME (ACC / PRE / EN / DN / TT / CU / CD / FD / etc.)
+        // Only consider dots OUTSIDE of any bracketed inner address — the inner address may
+        // itself contain a sub-element dot (e.g. N[T4:0.ACC]:5).
         string? subElement = null;
-        var dotIdx = src.LastIndexOf('.');
+        var dotIdx = LastIndexOfTopLevel(src, '.');
         if (dotIdx >= 0)
         {
             var candidate = src[(dotIdx + 1)..];
@@ -69,29 +123,139 @@ public sealed record AbLegacyAddress(
             }
         }
 
-        var colonIdx = src.IndexOf(':');
+        var colonIdx = IndexOfTopLevel(src, ':');
         if (colonIdx <= 0) return null;
         var filePart = src[..colonIdx];
         var wordPart = src[(colonIdx + 1)..];
-        if (!int.TryParse(wordPart, out var word) || word < 0) return null;
 
-        // File letter + optional file number (single letter for I/O/S, letter+number otherwise).
+        // File letter (always literal) + optional file number — either decimal digits or a
+        // bracketed indirect address like N[N7:0].
         if (filePart.Length == 0 || !char.IsLetter(filePart[0])) return null;
         var letterEnd = 1;
         while (letterEnd < filePart.Length && char.IsLetter(filePart[letterEnd])) letterEnd++;
 
         var letter = filePart[..letterEnd].ToUpperInvariant();
         int? fileNumber = null;
+        AbLegacyAddress? indirectFile = null;
         if (letterEnd < filePart.Length)
         {
-            if (!int.TryParse(filePart[letterEnd..], out var fn) || fn < 0) return null;
-            fileNumber = fn;
+            var fileTail = filePart[letterEnd..];
+            if (fileTail.Length >= 2 && fileTail[0] == '[' && fileTail[^1] == ']')
+            {
+                if (!allowIndirect) return null;
+                var inner = fileTail[1..^1];
+                indirectFile = ParseInner(inner, profile);
+                if (indirectFile is null) return null;
+            }
+            else
+            {
+                if (!int.TryParse(fileTail, out var fn) || fn < 0) return null;
+                fileNumber = fn;
+            }
         }
 
         // Reject unknown file letters — these cover SLC/ML/PLC-5 canonical families.
-        if (!IsKnownFileLetter(letter)) return null;
+        // Function-file letters (RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI) are MicroLogix-only.
+        if (!IsKnownFileLetter(letter))
+        {
+            if (!IsFunctionFileLetter(letter) || profile?.SupportsFunctionFiles != true) return null;
+        }
 
-        return new AbLegacyAddress(letter, fileNumber, word, bitIndex, subElement);
+        var octalForIo = profile?.OctalIoAddressing == true && (letter == "I" || letter == "O");
+
+        // Word part: either a numeric literal (octal-aware for PLC-5 I:/O:) or a bracketed
+        // indirect address.
+        int word = 0;
+        AbLegacyAddress? indirectWord = null;
+        if (wordPart.Length >= 2 && wordPart[0] == '[' && wordPart[^1] == ']')
+        {
+            if (!allowIndirect) return null;
+            var inner = wordPart[1..^1];
+            indirectWord = ParseInner(inner, profile);
+            if (indirectWord is null) return null;
+        }
+        else
+        {
+            if (!TryParseIndex(wordPart, octalForIo, out word) || word < 0) return null;
+        }
+
+        int? bitIndex = null;
+        if (bitText is not null)
+        {
+            if (!TryParseIndex(bitText, octalForIo, out var bit) || bit < 0 || bit > 31) return null;
+            bitIndex = bit;
+        }
+
+        return new AbLegacyAddress(letter, fileNumber, word, bitIndex, subElement, indirectFile, indirectWord);
+    }
+
+    /// <summary>
+    ///     Parse an inner (bracketed) PCCC address with depth-1 cap. The inner address itself
+    ///     must NOT be indirect — nesting beyond one level is rejected.
+    /// </summary>
+    private static AbLegacyAddress? ParseInner(string inner, AbLegacyPlcFamilyProfile? profile)
+    {
+        if (string.IsNullOrWhiteSpace(inner)) return null;
+        var src = inner.Trim();
+        // Reject any further bracket — depth cap at 1.
+        if (src.IndexOf('[') >= 0 || src.IndexOf(']') >= 0) return null;
+
+        string? bitText = null;
+        var slashIdx = src.LastIndexOf('/');
+        if (slashIdx >= 0)
+        {
+            bitText = src[(slashIdx + 1)..];
+            src = src[..slashIdx];
+        }
+        return ParseTail(src, bitText, profile, allowIndirect: false);
+    }
+
+    private static int IndexOfTopLevel(string s, char c)
+    {
+        var depth = 0;
+        for (var i = 0; i < s.Length; i++)
+        {
+            if (s[i] == '[') depth++;
+            else if (s[i] == ']') depth--;
+            else if (depth == 0 && s[i] == c) return i;
+        }
+        return -1;
+    }
+
+    private static int LastIndexOfTopLevel(string s, char c)
+    {
+        var depth = 0;
+        var last = -1;
+        for (var i = 0; i < s.Length; i++)
+        {
+            if (s[i] == '[') depth++;
+            else if (s[i] == ']') depth--;
+            else if (depth == 0 && s[i] == c) last = i;
+        }
+        return last;
+    }
+
+    private static bool TryParseIndex(string text, bool octal, out int value)
+    {
+        if (octal)
+        {
+            // Octal accepts only digits 0-7. Reject 8/9 explicitly.
+            if (text.Length == 0) { value = 0; return false; }
+            var start = 0;
+            var sign = 1;
+            if (text[0] == '-') { sign = -1; start = 1; }
+            if (start >= text.Length) { value = 0; return false; }
+            var acc = 0;
+            for (var i = start; i < text.Length; i++)
+            {
+                var c = text[i];
+                if (c < '0' || c > '7') { value = 0; return false; }
+                acc = (acc * 8) + (c - '0');
+            }
+            value = sign * acc;
+            return true;
+        }
+        return int.TryParse(text, out value);
     }
 
     private static bool IsKnownFileLetter(string letter) => letter switch
@@ -99,4 +263,14 @@ public sealed record AbLegacyAddress(
         "N" or "F" or "B" or "L" or "ST" or "T" or "C" or "R" or "I" or "O" or "S" or "A" => true,
         _ => false,
     };
+
+    /// <summary>
+    ///     MicroLogix 1100/1400 function-file prefixes. Each maps to a single fixed instance with a
+    ///     known sub-element catalogue (see <see cref="AbLegacyDataType"/>).
+    /// </summary>
+    internal static bool IsFunctionFileLetter(string letter) => letter switch
+    {
+        "RTC" or "HSC" or "DLS" or "MMI" or "PTO" or "PWM" or "STI" or "EII" or "IOS" or "BHI" => true,
+        _ => false,
+    };
 }

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDataType.cs

@@ -26,6 +26,72 @@ public enum AbLegacyDataType
     CounterElement,
     /// <summary>Control sub-element — caller addresses <c>.LEN</c>, <c>.POS</c>, <c>.EN</c>, <c>.DN</c>, <c>.ER</c>.</summary>
     ControlElement,
+    /// <summary>
+    ///     MicroLogix 1100/1400 function-file sub-element (RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI).
+    ///     Sub-element catalogue lives in <see cref="AbLegacyFunctionFile.SubElementType"/>.
+    /// </summary>
+    MicroLogixFunctionFile,
+}
+
+/// <summary>
+///     MicroLogix function-file sub-element catalogue. Covers the most-commonly-addressed members
+///     per file — not exhaustive (Rockwell defines 30+ on RTC alone). Unknown sub-elements fall
+///     back to <see cref="DriverDataType.Int32"/> at the <see cref="AbLegacyDataTypeExtensions"/>
+///     boundary so the driver never refuses a tag the customer happens to know about.
+/// </summary>
+public static class AbLegacyFunctionFile
+{
+    /// <summary>
+    ///     Driver-surface type for <paramref name="fileLetter"/>.<paramref name="subElement"/>.
+    ///     Returns <see cref="DriverDataType.Int32"/> if the sub-element is unrecognised — keeps
+    ///     the driver permissive without forcing every quirk into the catalogue.
+    /// </summary>
+    public static DriverDataType SubElementType(string fileLetter, string? subElement)
+    {
+        if (subElement is null) return DriverDataType.Int32;
+        var key = (fileLetter.ToUpperInvariant(), subElement.ToUpperInvariant());
+        return key switch
+        {
+            // Real-time clock — all stored as Int16 (year is 4-digit Int16).
+            ("RTC", "HR") or ("RTC", "MIN") or ("RTC", "SEC") or
+            ("RTC", "MON") or ("RTC", "DAY") or ("RTC", "YR") or ("RTC", "DOW") => DriverDataType.Int32,
+            ("RTC", "DS") or ("RTC", "BL") or ("RTC", "EN") => DriverDataType.Boolean,
+
+            // High-speed counter — accumulator/preset are Int32, status flags are bits.
+            ("HSC", "ACC") or ("HSC", "PRE") or ("HSC", "OVF") or ("HSC", "UNF") => DriverDataType.Int32,
+            ("HSC", "EN") or ("HSC", "UF") or ("HSC", "IF") or
+            ("HSC", "IN") or ("HSC", "IH") or ("HSC", "IL") or
+            ("HSC", "DN") or ("HSC", "CD") or ("HSC", "CU") => DriverDataType.Boolean,
+
+            // Daylight saving + memory module info.
+            ("DLS", "STR") or ("DLS", "STD") => DriverDataType.Int32,
+            ("DLS", "EN") => DriverDataType.Boolean,
+            ("MMI", "FT") or ("MMI", "LBN") => DriverDataType.Int32,
+            ("MMI", "MP") or ("MMI", "MCP") => DriverDataType.Boolean,
+
+            // Pulse-train / PWM output blocks.
+            ("PTO", "ACC") or ("PTO", "OF") or ("PTO", "IDA") or ("PTO", "ODA") => DriverDataType.Int32,
+            ("PTO", "EN") or ("PTO", "DN") or ("PTO", "EH") or ("PTO", "ED") or
+            ("PTO", "RP") or ("PTO", "OUT") => DriverDataType.Boolean,
+            ("PWM", "ACC") or ("PWM", "OF") or ("PWM", "PE") or ("PWM", "PD") => DriverDataType.Int32,
+            ("PWM", "EN") or ("PWM", "DN") or ("PWM", "EH") or ("PWM", "ED") or
+            ("PWM", "RP") or ("PWM", "OUT") => DriverDataType.Boolean,
+
+            // Selectable timed interrupt + event input interrupt.
+            ("STI", "SPM") or ("STI", "ER") or ("STI", "PFN") => DriverDataType.Int32,
+            ("STI", "EN") or ("STI", "TIE") or ("STI", "DN") or
+            ("STI", "PS") or ("STI", "ED") => DriverDataType.Boolean,
+            ("EII", "PFN") or ("EII", "ER") => DriverDataType.Int32,
+            ("EII", "EN") or ("EII", "TIE") or ("EII", "PE") or
+            ("EII", "ES") or ("EII", "ED") => DriverDataType.Boolean,
+
+            // I/O status + base hardware info — mostly status flags + a few counters.
+            ("IOS", "ID") or ("IOS", "TYP") => DriverDataType.Int32,
+            ("BHI", "OS") or ("BHI", "FRN") or ("BHI", "BSN") or ("BHI", "CC") => DriverDataType.Int32,
+
+            _ => DriverDataType.Int32,
+        };
+    }
 }
 
 /// <summary>Map a PCCC data type to the driver-surface <see cref="DriverDataType"/>.</summary>
@@ -40,6 +106,106 @@ public static class AbLegacyDataTypeExtensions
         AbLegacyDataType.String => DriverDataType.String,
         AbLegacyDataType.TimerElement or AbLegacyDataType.CounterElement
             or AbLegacyDataType.ControlElement => DriverDataType.Int32,
+        AbLegacyDataType.MicroLogixFunctionFile => DriverDataType.Int32,
         _ => DriverDataType.Int32,
     };
+
+    /// <summary>
+    ///     Sub-element-aware driver type. Timer/Counter/Control elements expose Boolean status
+    ///     bits (<c>.DN</c>, <c>.EN</c>, <c>.TT</c>, <c>.CU</c>, <c>.CD</c>, <c>.OV</c>,
+    ///     <c>.UN</c>, <c>.ER</c>, etc.) and Int32 word members (<c>.PRE</c>, <c>.ACC</c>,
+    ///     <c>.LEN</c>, <c>.POS</c>). Unknown sub-elements fall back to
+    ///     <see cref="ToDriverDataType"/> so the driver remains permissive.
+    /// </summary>
+    public static DriverDataType EffectiveDriverDataType(AbLegacyDataType t, string? subElement)
+    {
+        if (subElement is null) return t.ToDriverDataType();
+        var key = subElement.ToUpperInvariant();
+        return t switch
+        {
+            AbLegacyDataType.TimerElement => key switch
+            {
+                "EN" or "TT" or "DN" => DriverDataType.Boolean,
+                "PRE" or "ACC" => DriverDataType.Int32,
+                _ => t.ToDriverDataType(),
+            },
+            AbLegacyDataType.CounterElement => key switch
+            {
+                "CU" or "CD" or "DN" or "OV" or "UN" => DriverDataType.Boolean,
+                "PRE" or "ACC" => DriverDataType.Int32,
+                _ => t.ToDriverDataType(),
+            },
+            AbLegacyDataType.ControlElement => key switch
+            {
+                "EN" or "EU" or "DN" or "EM" or "ER" or "UL" or "IN" or "FD" => DriverDataType.Boolean,
+                "LEN" or "POS" => DriverDataType.Int32,
+                _ => t.ToDriverDataType(),
+            },
+            _ => t.ToDriverDataType(),
+        };
+    }
+
+    /// <summary>
+    ///     Bit position within the parent control word for Timer/Counter/Control status bits.
+    ///     Returns <c>null</c> if the sub-element is not a known bit member of the given element
+    ///     type. Bit numbering follows Rockwell DTAM / PCCC documentation.
+    /// </summary>
+    public static int? StatusBitIndex(AbLegacyDataType t, string? subElement)
+    {
+        if (subElement is null) return null;
+        var key = subElement.ToUpperInvariant();
+        return t switch
+        {
+            // T4 element word 0: bit 13=DN, 14=TT, 15=EN.
+            AbLegacyDataType.TimerElement => key switch
+            {
+                "DN" => 13,
+                "TT" => 14,
+                "EN" => 15,
+                _ => null,
+            },
+            // C5 element word 0: bit 10=UN, 11=OV, 12=DN, 13=CD, 14=CU.
+            AbLegacyDataType.CounterElement => key switch
+            {
+                "UN" => 10,
+                "OV" => 11,
+                "DN" => 12,
+                "CD" => 13,
+                "CU" => 14,
+                _ => null,
+            },
+            // R6 element word 0: bit 8=FD, 9=IN, 10=UL, 11=ER, 12=EM, 13=DN, 14=EU, 15=EN.
+            AbLegacyDataType.ControlElement => key switch
+            {
+                "FD" => 8,
+                "IN" => 9,
+                "UL" => 10,
+                "ER" => 11,
+                "EM" => 12,
+                "DN" => 13,
+                "EU" => 14,
+                "EN" => 15,
+                _ => null,
+            },
+            _ => null,
+        };
+    }
+
+    /// <summary>
+    ///     PLC-set status bits — read-only from the OPC UA side. Operator-controllable bits
+    ///     (e.g. <c>.EN</c> on a timer/counter, <c>.CU</c>/<c>.CD</c> rung-driven inputs) are
+    ///     omitted so they keep default writable behaviour.
+    /// </summary>
+    public static bool IsPlcSetStatusBit(AbLegacyDataType t, string? subElement)
+    {
+        if (subElement is null) return false;
+        var key = subElement.ToUpperInvariant();
+        return t switch
+        {
+            AbLegacyDataType.TimerElement => key is "DN" or "TT",
+            AbLegacyDataType.CounterElement => key is "DN" or "OV" or "UN",
+            AbLegacyDataType.ControlElement => key is "DN" or "EM" or "ER" or "FD" or "UL" or "IN",
+            _ => false,
+        };
+    }
 }

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs

@@ -140,8 +140,13 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                     continue;
                 }
 
-                var parsed = AbLegacyAddress.TryParse(def.Address);
-                var value = runtime.DecodeValue(def.DataType, parsed?.BitIndex);
+                var parsed = AbLegacyAddress.TryParse(def.Address, device.Options.PlcFamily);
+                // Timer/Counter/Control status bits route through GetBit at the parent-word
+                // address — translate the .DN/.EN/etc. sub-element to its standard bit position
+                // and pass it down to the runtime as a synthetic bitIndex.
+                var decodeBit = parsed?.BitIndex
+                    ?? AbLegacyDataTypeExtensions.StatusBitIndex(def.DataType, parsed?.SubElement);
+                var value = runtime.DecodeValue(def.DataType, decodeBit);
                 results[i] = new DataValueSnapshot(value, AbLegacyStatusMapper.Good, now, now);
                 _health = new DriverHealth(DriverState.Healthy, now, null);
             }
@@ -186,7 +191,16 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
 
             try
             {
-                var parsed = AbLegacyAddress.TryParse(def.Address);
+                var parsed = AbLegacyAddress.TryParse(def.Address, device.Options.PlcFamily);
+
+                // Timer/Counter/Control PLC-set status bits (DN, TT, OV, UN, FD, ER, EM, UL,
+                // IN) are read-only — the PLC sets them; any client write would be silently
+                // overwritten on the next scan. Reject up front with BadNotWritable.
+                if (AbLegacyDataTypeExtensions.IsPlcSetStatusBit(def.DataType, parsed?.SubElement))
+                {
+                    results[i] = new WriteResult(AbLegacyStatusMapper.BadNotWritable);
+                    continue;
+                }
 
                 // PCCC bit-within-word writes — task #181 pass 2. RMW against a parallel
                 // parent-word runtime (strip the /N bit suffix). Per-parent-word lock serialises
@@ -247,12 +261,19 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
                 string.Equals(t.DeviceHostAddress, device.HostAddress, StringComparison.OrdinalIgnoreCase));
             foreach (var tag in tagsForDevice)
             {
+                var parsed = AbLegacyAddress.TryParse(tag.Address, device.PlcFamily);
+                // Timer/Counter/Control sub-elements (.DN/.EN/.TT/.PRE/.ACC/etc.) refine the
+                // base element's Int32 to Boolean for status bits and Int32 for word members.
+                var effectiveType = AbLegacyDataTypeExtensions.EffectiveDriverDataType(
+                    tag.DataType, parsed?.SubElement);
+                var plcSetBit = AbLegacyDataTypeExtensions.IsPlcSetStatusBit(
+                    tag.DataType, parsed?.SubElement);
                 deviceFolder.Variable(tag.Name, tag.Name, new DriverAttributeInfo(
                     FullName: tag.Name,
-                    DriverDataType: tag.DataType.ToDriverDataType(),
+                    DriverDataType: effectiveType,
                     IsArray: false,
                     ArrayDim: null,
-                    SecurityClass: tag.Writable
+                    SecurityClass: tag.Writable && !plcSetBit
                         ? SecurityClassification.Operate
                         : SecurityClassification.ViewOnly,
                     IsHistorized: false,
@@ -413,10 +434,19 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
     {
         if (device.Runtimes.TryGetValue(def.Name, out var existing)) return existing;
 
-        var parsed = AbLegacyAddress.TryParse(def.Address)
+        var parsed = AbLegacyAddress.TryParse(def.Address, device.Options.PlcFamily)
             ?? throw new InvalidOperationException(
                 $"AbLegacy tag '{def.Name}' has malformed Address '{def.Address}'.");
 
+        // TODO(#247): libplctag's PCCC text decoder does not natively accept the bracket-form
+        // indirect address. Resolving N7:[N7:0] requires reading the inner address first, then
+        // rewriting the tag name with the resolved word number, then issuing the actual read.
+        // For now we surface a clear runtime error rather than letting libplctag fail with an
+        // opaque parser error.
+        if (parsed.IsIndirect)
+            throw new NotSupportedException(
+                $"AbLegacy tag '{def.Name}' uses indirect addressing ('{def.Address}'); runtime resolution is not yet implemented.");
+
         var runtime = _tagFactory.Create(new AbLegacyTagCreateParams(
             Gateway: device.ParsedAddress.Gateway,
             Port: device.ParsedAddress.Port,

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverOptions.cs

@@ -23,7 +23,7 @@ public sealed record AbLegacyDeviceOptions(
 
 /// <summary>
 ///     One PCCC-backed OPC UA variable. <paramref name="Address"/> is the canonical PCCC
-///     file-address string that parses via <see cref="AbLegacyAddress.TryParse"/>.
+///     file-address string that parses via <see cref="AbLegacyAddress.TryParse(string?)"/>.
 /// </summary>
 public sealed record AbLegacyTagDefinition(
     string Name,

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/LibplctagLegacyTagRuntime.cs

@@ -40,8 +40,14 @@ internal sealed class LibplctagLegacyTagRuntime : IAbLegacyTagRuntime
         AbLegacyDataType.Long => _tag.GetInt32(0),
         AbLegacyDataType.Float => _tag.GetFloat32(0),
         AbLegacyDataType.String => _tag.GetString(0),
+        // Timer/Counter/Control sub-elements: bitIndex is the status bit position within the
+        // parent control word (encoded by AbLegacyDriver from the .DN / .EN / etc. sub-element
+        // name). Word members (.PRE / .ACC / .LEN / .POS) come through with bitIndex=null and
+        // decode as Int32 like before.
         AbLegacyDataType.TimerElement or AbLegacyDataType.CounterElement
-            or AbLegacyDataType.ControlElement => _tag.GetInt32(0),
+            or AbLegacyDataType.ControlElement => bitIndex is int statusBit
+                ? _tag.GetBit(statusBit)
+                : _tag.GetInt32(0),
         _ => null,
     };
 

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/PlcFamilies/AbLegacyPlcFamilyProfile.cs

@@ -9,7 +9,9 @@ public sealed record AbLegacyPlcFamilyProfile(
     string DefaultCipPath,
     int MaxTagBytes,
     bool SupportsStringFile,
-    bool SupportsLongFile)
+    bool SupportsLongFile,
+    bool OctalIoAddressing,
+    bool SupportsFunctionFiles)
 {
     public static AbLegacyPlcFamilyProfile ForFamily(AbLegacyPlcFamily family) => family switch
     {
@@ -25,21 +27,27 @@ public sealed record AbLegacyPlcFamilyProfile(
         DefaultCipPath: "1,0",
         MaxTagBytes: 240,           // SLC 5/05 PCCC max packet data
         SupportsStringFile: true,   // ST file available SLC 5/04+
-        SupportsLongFile: true);    // L file available SLC 5/05+
+        SupportsLongFile: true,     // L file available SLC 5/05+
+        OctalIoAddressing: false,   // SLC500 I:/O: indices are decimal in RSLogix 500
+        SupportsFunctionFiles: false); // SLC500 has no function files
 
     public static readonly AbLegacyPlcFamilyProfile MicroLogix = new(
         LibplctagPlcAttribute: "micrologix",
         DefaultCipPath: "",         // MicroLogix 1100/1400 use direct EIP, no backplane path
         MaxTagBytes: 232,
         SupportsStringFile: true,
-        SupportsLongFile: false);   // ML 1100/1200/1400 don't ship L files
+        SupportsLongFile: false,    // ML 1100/1200/1400 don't ship L files
+        OctalIoAddressing: false,   // MicroLogix follows SLC-style decimal I/O addressing
+        SupportsFunctionFiles: true); // ML 1100/1400 expose RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI
 
     public static readonly AbLegacyPlcFamilyProfile Plc5 = new(
         LibplctagPlcAttribute: "plc5",
         DefaultCipPath: "1,0",
         MaxTagBytes: 240,           // DF1 full-duplex packet limit at 264 bytes, PCCC-over-EIP caps lower
         SupportsStringFile: true,
-        SupportsLongFile: false);   // PLC-5 predates L files
+        SupportsLongFile: false,    // PLC-5 predates L files
+        OctalIoAddressing: true,    // RSLogix 5 displays I:/O: word + bit indices as octal
+        SupportsFunctionFiles: false);
 
     /// <summary>
     ///     Logix ControlLogix / CompactLogix accessed through the legacy PCCC compatibility layer.
@@ -51,7 +59,9 @@ public sealed record AbLegacyPlcFamilyProfile(
         DefaultCipPath: "1,0",
         MaxTagBytes: 240,
         SupportsStringFile: true,
-        SupportsLongFile: true);
+        SupportsLongFile: true,
+        OctalIoAddressing: false,   // Logix natively uses decimal arrays even via the PCCC bridge
+        SupportsFunctionFiles: false);
 }
 
 /// <summary>Which PCCC PLC family the device is.</summary>

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasCapabilityMatrix.cs

@@ -106,6 +106,27 @@ public static class FocasCapabilityMatrix
         _ => int.MaxValue,
     };
 
+    /// <summary>
+    ///     Whether the FOCAS driver should expose the per-device <c>Tooling/</c>
+    ///     fixed-tree subfolder for a given <paramref name="series"/>. Backed by
+    ///     <c>cnc_rdtnum</c>, which is documented for every modern Fanuc series
+    ///     (0i / 16i / 30i families) — defaulting to <c>true</c>. The capability
+    ///     hook exists so a future controller without <c>cnc_rdtnum</c> can opt
+    ///     out without touching the driver. <see cref="FocasCncSeries.Unknown"/>
+    ///     stays permissive (matches the modal / override fixed-tree precedent in
+    ///     issue #259). Issue #260.
+    /// </summary>
+    public static bool SupportsTooling(FocasCncSeries series) => true;
+
+    /// <summary>
+    ///     Whether the FOCAS driver should expose the per-device <c>Offsets/</c>
+    ///     fixed-tree subfolder for a given <paramref name="series"/>. Backed by
+    ///     <c>cnc_rdzofs(n=1..6)</c> for the standard G54..G59 surfaces; extended
+    ///     G54.1 P1..P48 surfaces are deferred to a follow-up. Same permissive
+    ///     policy as <see cref="SupportsTooling"/>. Issue #260.
+    /// </summary>
+    public static bool SupportsWorkOffsets(FocasCncSeries series) => true;
+
     private static string? ValidateMacro(FocasCncSeries series, int number)
     {
         var (min, max) = MacroRange(series);

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs

@@ -24,8 +24,96 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
     private readonly PollGroupEngine _poll;
     private readonly Dictionary<string, DeviceState> _devices = new(StringComparer.OrdinalIgnoreCase);
     private readonly Dictionary<string, FocasTagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, (string Host, string Field)> _statusNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, (string Host, string Field)> _productionNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, (string Host, string Field)> _modalNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, (string Host, string Field)> _overrideNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, string> _toolingNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, (string Host, string Slot, string Axis)> _offsetNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, string> _messagesNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, string> _currentBlockNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, (string Host, string Field)> _diagnosticsNodesByName =
+        new(StringComparer.OrdinalIgnoreCase);
     private DriverHealth _health = new(DriverState.Unknown, null, null);
 
+    /// <summary>
+    ///     Names of the 9 fixed-tree <c>Status/</c> child nodes per device, mirroring the 9
+    ///     fields of Fanuc's <c>cnc_rdcncstat</c> ODBST struct (issue #257). Order matters for
+    ///     deterministic discovery output.
+    /// </summary>
+    private static readonly string[] StatusFieldNames =
+    [
+        "Tmmode", "Aut", "Run", "Motion", "Mstb", "EmergencyStop", "Alarm", "Edit", "Dummy",
+    ];
+
+    /// <summary>
+    ///     Names of the 4 fixed-tree <c>Production/</c> child nodes per device — parts
+    ///     produced/required/total via <c>cnc_rdparam(6711/6712/6713)</c> + cycle-time
+    ///     seconds (issue #258). Order matters for deterministic discovery output.
+    /// </summary>
+    private static readonly string[] ProductionFieldNames =
+    [
+        "PartsProduced", "PartsRequired", "PartsTotal", "CycleTimeSeconds",
+    ];
+
+    /// <summary>
+    ///     Names of the active modal aux-code child nodes per device — M/S/T/B from
+    ///     <c>cnc_modal(type=100..103)</c> (issue #259). G-group decoding is a deferred
+    ///     follow-up because the FWLIB <c>ODBMDL</c> union varies per series + group.
+    /// </summary>
+    private static readonly string[] ModalFieldNames = ["MCode", "SCode", "TCode", "BCode"];
+
+    /// <summary>
+    ///     Names of the four operator-override child nodes per device — Feed / Rapid /
+    ///     Spindle / Jog from <c>cnc_rdparam</c> with MTB-specific parameter numbers
+    ///     (issue #259). A device whose <c>FocasOverrideParameters</c> entry is null for a
+    ///     given field has the matching node omitted from the address space.
+    /// </summary>
+    private static readonly string[] OverrideFieldNames = ["Feed", "Rapid", "Spindle", "Jog"];
+
+    /// <summary>
+    ///     Names of the standard work-coordinate offset slots surfaced under
+    ///     <c>Offsets/</c> per device — G54..G59 from <c>cnc_rdzofs(n=1..6)</c>
+    ///     (issue #260). Extended G54.1 P1..P48 surfaces are deferred to a follow-up
+    ///     PR because <c>cnc_rdzofsr</c> uses a different range surface.
+    /// </summary>
+    private static readonly string[] WorkOffsetSlotNames =
+    [
+        "G54", "G55", "G56", "G57", "G58", "G59",
+    ];
+
+    /// <summary>
+    ///     Axis columns surfaced under each <c>Offsets/{slot}/</c> folder. Per the F1-d
+    ///     plan a fixed 3-axis (X/Y/Z) view is used; lathes / mills with extra rotational
+    ///     offsets get those columns exposed as 0.0 until a follow-up extends the surface.
+    /// </summary>
+    private static readonly string[] WorkOffsetAxisNames = ["X", "Y", "Z"];
+
+    /// <summary>
+    ///     Names of the five fixed-tree <c>Diagnostics/</c> child nodes per device — runtime
+    ///     counters surfaced for operator visibility (issue #262). Order matters for
+    ///     deterministic discovery output.
+    ///     <list type="bullet">
+    ///         <item><c>ReadCount</c> (Int64) — successful probe ticks since init</item>
+    ///         <item><c>ReadFailureCount</c> (Int64) — failed probe ticks since init</item>
+    ///         <item><c>LastErrorMessage</c> (String) — text of the last probe / read failure</item>
+    ///         <item><c>LastSuccessfulRead</c> (DateTime) — UTC timestamp of the last good probe tick</item>
+    ///         <item><c>ReconnectCount</c> (Int64) — wire reconnects observed since init</item>
+    ///     </list>
+    /// </summary>
+    private static readonly string[] DiagnosticsFieldNames =
+    [
+        "ReadCount", "ReadFailureCount", "LastErrorMessage", "LastSuccessfulRead", "ReconnectCount",
+    ];
+
     public event EventHandler<DataChangeEventArgs>? OnDataChange;
     public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;
 
@@ -76,6 +164,67 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                 _tagsByName[tag.Name] = tag;
             }
 
+            // Per-device fixed-tree Status nodes — issue #257. Names are deterministic so
+            // ReadAsync can dispatch on the synthetic full-reference without extra metadata.
+            foreach (var device in _devices.Values)
+            {
+                foreach (var field in StatusFieldNames)
+                    _statusNodesByName[StatusReferenceFor(device.Options.HostAddress, field)] =
+                        (device.Options.HostAddress, field);
+                foreach (var field in ProductionFieldNames)
+                    _productionNodesByName[ProductionReferenceFor(device.Options.HostAddress, field)] =
+                        (device.Options.HostAddress, field);
+                foreach (var field in ModalFieldNames)
+                    _modalNodesByName[ModalReferenceFor(device.Options.HostAddress, field)] =
+                        (device.Options.HostAddress, field);
+                if (device.Options.OverrideParameters is { } op)
+                {
+                    foreach (var field in OverrideFieldNames)
+                    {
+                        if (OverrideParamFor(op, field) is null) continue;
+                        _overrideNodesByName[OverrideReferenceFor(device.Options.HostAddress, field)] =
+                            (device.Options.HostAddress, field);
+                    }
+                }
+
+                // Tooling/CurrentTool — single Int16 node per device (issue #260). Tool
+                // life + active offset index are deferred per the F1-d plan; they need
+                // ODBTLIFE* unions whose shape varies per series.
+                if (FocasCapabilityMatrix.SupportsTooling(device.Options.Series))
+                {
+                    _toolingNodesByName[ToolingReferenceFor(device.Options.HostAddress, "CurrentTool")] =
+                        device.Options.HostAddress;
+                }
+
+                // Offsets/{G54..G59}/{X|Y|Z} — fixed 3-axis view of the standard work-
+                // coordinate offsets (issue #260). Capability matrix gates by series so
+                // legacy CNCs that don't support cnc_rdzofs don't produce the subtree.
+                if (FocasCapabilityMatrix.SupportsWorkOffsets(device.Options.Series))
+                {
+                    foreach (var slot in WorkOffsetSlotNames)
+                    foreach (var axis in WorkOffsetAxisNames)
+                    {
+                        _offsetNodesByName[OffsetReferenceFor(device.Options.HostAddress, slot, axis)] =
+                            (device.Options.HostAddress, slot, axis);
+                    }
+                }
+
+                // Messages/External/Latest + Program/CurrentBlock — single String nodes per
+                // device backed by cnc_rdopmsg3 + cnc_rdactpt caches refreshed on the probe
+                // tick (issue #261). Permissive across series (no capability gate yet).
+                _messagesNodesByName[MessagesLatestReferenceFor(device.Options.HostAddress)] =
+                    device.Options.HostAddress;
+                _currentBlockNodesByName[CurrentBlockReferenceFor(device.Options.HostAddress)] =
+                    device.Options.HostAddress;
+
+                // Diagnostics/{ReadCount, ReadFailureCount, LastErrorMessage,
+                // LastSuccessfulRead, ReconnectCount} — runtime counters surfaced for
+                // operator visibility (issue #262). Permissive across all CNC series.
+                foreach (var field in DiagnosticsFieldNames)
+                    _diagnosticsNodesByName[DiagnosticsReferenceFor(device.Options.HostAddress, field)] =
+                        (device.Options.HostAddress, field);
+            }
+
             if (_options.Probe.Enabled)
             {
                 foreach (var state in _devices.Values)
@@ -113,6 +262,15 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         }
         _devices.Clear();
         _tagsByName.Clear();
+        _statusNodesByName.Clear();
+        _productionNodesByName.Clear();
+        _modalNodesByName.Clear();
+        _overrideNodesByName.Clear();
+        _toolingNodesByName.Clear();
+        _offsetNodesByName.Clear();
+        _messagesNodesByName.Clear();
+        _currentBlockNodesByName.Clear();
+        _diagnosticsNodesByName.Clear();
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
     }
 
@@ -136,6 +294,73 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         for (var i = 0; i < fullReferences.Count; i++)
         {
             var reference = fullReferences[i];
+
+            // Fixed-tree Status/ nodes — served from the per-device cached ODBST struct
+            // refreshed on the probe tick (issue #257). No wire call here.
+            if (_statusNodesByName.TryGetValue(reference, out var statusKey))
+            {
+                results[i] = ReadStatusField(statusKey.Host, statusKey.Field, now);
+                continue;
+            }
+
+            // Fixed-tree Production/ nodes — served from the per-device cached production
+            // snapshot refreshed on the probe tick (issue #258). No wire call here.
+            if (_productionNodesByName.TryGetValue(reference, out var prodKey))
+            {
+                results[i] = ReadProductionField(prodKey.Host, prodKey.Field, now);
+                continue;
+            }
+
+            // Fixed-tree Modal/ + Override/ nodes — served from per-device cached snapshots
+            // refreshed on the probe tick (issue #259). Same cache-or-Bad policy as Status/.
+            if (_modalNodesByName.TryGetValue(reference, out var modalKey))
+            {
+                results[i] = ReadModalField(modalKey.Host, modalKey.Field, now);
+                continue;
+            }
+            if (_overrideNodesByName.TryGetValue(reference, out var overrideKey))
+            {
+                results[i] = ReadOverrideField(overrideKey.Host, overrideKey.Field, now);
+                continue;
+            }
+
+            // Fixed-tree Tooling/CurrentTool — served from cached cnc_rdtnum snapshot
+            // refreshed on the probe tick (issue #260). No wire call here.
+            if (_toolingNodesByName.TryGetValue(reference, out var toolingHost))
+            {
+                results[i] = ReadToolingField(toolingHost, "CurrentTool", now);
+                continue;
+            }
+
+            // Fixed-tree Offsets/{slot}/{axis} — served from cached cnc_rdzofs(1..6)
+            // snapshot refreshed on the probe tick (issue #260). No wire call here.
+            if (_offsetNodesByName.TryGetValue(reference, out var offsetKey))
+            {
+                results[i] = ReadOffsetField(offsetKey.Host, offsetKey.Slot, offsetKey.Axis, now);
+                continue;
+            }
+
+            // Fixed-tree Messages/External/Latest + Program/CurrentBlock — served from
+            // cnc_rdopmsg3 + cnc_rdactpt caches refreshed on the probe tick (issue #261).
+            if (_messagesNodesByName.TryGetValue(reference, out var messagesHost))
+            {
+                results[i] = ReadMessagesLatestField(messagesHost, now);
+                continue;
+            }
+            if (_currentBlockNodesByName.TryGetValue(reference, out var blockHost))
+            {
+                results[i] = ReadCurrentBlockField(blockHost, now);
+                continue;
+            }
+
+            // Fixed-tree Diagnostics/ nodes — runtime counters maintained by the probe
+            // loop (issue #262). No wire call here.
+            if (_diagnosticsNodesByName.TryGetValue(reference, out var diagKey))
+            {
+                results[i] = ReadDiagnosticsField(diagKey.Host, diagKey.Field, now);
+                continue;
+            }
+
             if (!_tagsByName.TryGetValue(reference, out var def))
             {
                 results[i] = new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
@@ -257,10 +482,267 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                     IsAlarm: false,
                     WriteIdempotent: tag.WriteIdempotent));
             }
+
+            // Fixed-tree Status/ subfolder — 9 read-only Int16 nodes mirroring the ODBST
+            // fields (issue #257). Cached on the probe tick + served from DeviceState.LastStatus.
+            var statusFolder = deviceFolder.Folder("Status", "Status");
+            foreach (var field in StatusFieldNames)
+            {
+                var fullRef = StatusReferenceFor(device.HostAddress, field);
+                statusFolder.Variable(field, field, new DriverAttributeInfo(
+                    FullName: fullRef,
+                    DriverDataType: DriverDataType.Int16,
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false,
+                    WriteIdempotent: false));
+            }
+
+            // Fixed-tree Production/ subfolder — 4 read-only Int32 nodes: parts produced /
+            // required / total + cycle-time seconds (issue #258). Cached on the probe tick
+            // + served from DeviceState.LastProduction.
+            var productionFolder = deviceFolder.Folder("Production", "Production");
+            foreach (var field in ProductionFieldNames)
+            {
+                var fullRef = ProductionReferenceFor(device.HostAddress, field);
+                productionFolder.Variable(field, field, new DriverAttributeInfo(
+                    FullName: fullRef,
+                    DriverDataType: DriverDataType.Int32,
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false,
+                    WriteIdempotent: false));
+            }
+
+            // Fixed-tree Modal/ subfolder — 4 read-only Int16 nodes for the universally-
+            // present aux modal codes M/S/T/B from cnc_modal(type=100..103). G-group
+            // surfaces are deferred to a follow-up because the FWLIB ODBMDL union varies
+            // per series + group (issue #259, plan PR F1-c).
+            var modalFolder = deviceFolder.Folder("Modal", "Modal");
+            foreach (var field in ModalFieldNames)
+            {
+                var fullRef = ModalReferenceFor(device.HostAddress, field);
+                modalFolder.Variable(field, field, new DriverAttributeInfo(
+                    FullName: fullRef,
+                    DriverDataType: DriverDataType.Int16,
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false,
+                    WriteIdempotent: false));
+            }
+
+            // Fixed-tree Override/ subfolder — Feed / Rapid / Spindle / Jog from
+            // cnc_rdparam at MTB-specific parameter numbers (issue #259). Suppressed when
+            // OverrideParameters is null; per-field nodes whose parameter is null are
+            // omitted so a deployment can hide overrides their MTB doesn't wire up.
+            if (device.OverrideParameters is { } overrideParams)
+            {
+                var overrideFolder = deviceFolder.Folder("Override", "Override");
+                foreach (var field in OverrideFieldNames)
+                {
+                    if (OverrideParamFor(overrideParams, field) is null) continue;
+                    var fullRef = OverrideReferenceFor(device.HostAddress, field);
+                    overrideFolder.Variable(field, field, new DriverAttributeInfo(
+                        FullName: fullRef,
+                        DriverDataType: DriverDataType.Int16,
+                        IsArray: false,
+                        ArrayDim: null,
+                        SecurityClass: SecurityClassification.ViewOnly,
+                        IsHistorized: false,
+                        IsAlarm: false,
+                        WriteIdempotent: false));
+                }
+            }
+
+            // Fixed-tree Tooling/ subfolder — single Int16 CurrentTool node from
+            // cnc_rdtnum (issue #260). Tool life + active offset index are deferred
+            // per the F1-d plan because the FWLIB ODBTLIFE* unions vary per series.
+            if (FocasCapabilityMatrix.SupportsTooling(device.Series))
+            {
+                var toolingFolder = deviceFolder.Folder("Tooling", "Tooling");
+                var toolingRef = ToolingReferenceFor(device.HostAddress, "CurrentTool");
+                toolingFolder.Variable("CurrentTool", "CurrentTool", new DriverAttributeInfo(
+                    FullName: toolingRef,
+                    DriverDataType: DriverDataType.Int16,
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false,
+                    WriteIdempotent: false));
+            }
+
+            // Fixed-tree Offsets/ subfolder — G54..G59 each with X/Y/Z Float64 axes
+            // from cnc_rdzofs(n=1..6) (issue #260). Capability matrix gates the surface
+            // by series so legacy controllers without cnc_rdzofs support don't expose
+            // dead nodes. Extended G54.1 P1..P48 surfaces are deferred to a follow-up.
+            if (FocasCapabilityMatrix.SupportsWorkOffsets(device.Series))
+            {
+                var offsetsFolder = deviceFolder.Folder("Offsets", "Offsets");
+                foreach (var slot in WorkOffsetSlotNames)
+                {
+                    var slotFolder = offsetsFolder.Folder(slot, slot);
+                    foreach (var axis in WorkOffsetAxisNames)
+                    {
+                        var fullRef = OffsetReferenceFor(device.HostAddress, slot, axis);
+                        slotFolder.Variable(axis, axis, new DriverAttributeInfo(
+                            FullName: fullRef,
+                            DriverDataType: DriverDataType.Float64,
+                            IsArray: false,
+                            ArrayDim: null,
+                            SecurityClass: SecurityClassification.ViewOnly,
+                            IsHistorized: false,
+                            IsAlarm: false,
+                            WriteIdempotent: false));
+                    }
+                }
+            }
+
+            // Fixed-tree Messages/External/Latest — single String node per device backed
+            // by cnc_rdopmsg3 across the four FANUC operator-message classes (issue #261).
+            // The issue body permits this minimal "latest message" surface in the first
+            // cut over a full ring-buffer of all four slots.
+            var messagesFolder = deviceFolder.Folder("Messages", "Messages");
+            var externalFolder = messagesFolder.Folder("External", "External");
+            var messagesRef = MessagesLatestReferenceFor(device.HostAddress);
+            externalFolder.Variable("Latest", "Latest", new DriverAttributeInfo(
+                FullName: messagesRef,
+                DriverDataType: DriverDataType.String,
+                IsArray: false,
+                ArrayDim: null,
+                SecurityClass: SecurityClassification.ViewOnly,
+                IsHistorized: false,
+                IsAlarm: false,
+                WriteIdempotent: false));
+
+            // Fixed-tree Program/CurrentBlock — single String node per device backed by
+            // cnc_rdactpt (issue #261). Trim-stable round-trip per the issue body.
+            var programFolder = deviceFolder.Folder("Program", "Program");
+            var blockRef = CurrentBlockReferenceFor(device.HostAddress);
+            programFolder.Variable("CurrentBlock", "CurrentBlock", new DriverAttributeInfo(
+                FullName: blockRef,
+                DriverDataType: DriverDataType.String,
+                IsArray: false,
+                ArrayDim: null,
+                SecurityClass: SecurityClassification.ViewOnly,
+                IsHistorized: false,
+                IsAlarm: false,
+                WriteIdempotent: false));
+
+            // Fixed-tree Diagnostics/ subfolder — 5 read-only counters surfaced for
+            // operator visibility (issue #262). ReadCount / ReadFailureCount /
+            // ReconnectCount are Int64; LastErrorMessage is String;
+            // LastSuccessfulRead is DateTime. Permissive across CNC series — every
+            // device gets the same shape.
+            var diagnosticsFolder = deviceFolder.Folder("Diagnostics", "Diagnostics");
+            foreach (var field in DiagnosticsFieldNames)
+            {
+                var fullRef = DiagnosticsReferenceFor(device.HostAddress, field);
+                diagnosticsFolder.Variable(field, field, new DriverAttributeInfo(
+                    FullName: fullRef,
+                    DriverDataType: DiagnosticsFieldType(field),
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false,
+                    WriteIdempotent: false));
+            }
         }
         return Task.CompletedTask;
     }
 
+    private static DriverDataType DiagnosticsFieldType(string field) => field switch
+    {
+        "ReadCount" or "ReadFailureCount" or "ReconnectCount" => DriverDataType.Int64,
+        "LastErrorMessage" => DriverDataType.String,
+        "LastSuccessfulRead" => DriverDataType.DateTime,
+        _ => DriverDataType.String,
+    };
+
+    private static string StatusReferenceFor(string hostAddress, string field) =>
+        $"{hostAddress}::Status/{field}";
+
+    private static string ProductionReferenceFor(string hostAddress, string field) =>
+        $"{hostAddress}::Production/{field}";
+
+    private static string ModalReferenceFor(string hostAddress, string field) =>
+        $"{hostAddress}::Modal/{field}";
+
+    private static string OverrideReferenceFor(string hostAddress, string field) =>
+        $"{hostAddress}::Override/{field}";
+
+    private static string ToolingReferenceFor(string hostAddress, string field) =>
+        $"{hostAddress}::Tooling/{field}";
+
+    private static string OffsetReferenceFor(string hostAddress, string slot, string axis) =>
+        $"{hostAddress}::Offsets/{slot}/{axis}";
+
+    private static string MessagesLatestReferenceFor(string hostAddress) =>
+        $"{hostAddress}::Messages/External/Latest";
+
+    private static string CurrentBlockReferenceFor(string hostAddress) =>
+        $"{hostAddress}::Program/CurrentBlock";
+
+    private static string DiagnosticsReferenceFor(string hostAddress, string field) =>
+        $"{hostAddress}::Diagnostics/{field}";
+
+    private static ushort? OverrideParamFor(FocasOverrideParameters p, string field) => field switch
+    {
+        "Feed"    => p.FeedParam,
+        "Rapid"   => p.RapidParam,
+        "Spindle" => p.SpindleParam,
+        "Jog"     => p.JogParam,
+        _         => null,
+    };
+
+    private static short? PickStatusField(FocasStatusInfo s, string field) => field switch
+    {
+        "Tmmode"        => s.Tmmode,
+        "Aut"           => s.Aut,
+        "Run"           => s.Run,
+        "Motion"        => s.Motion,
+        "Mstb"          => s.Mstb,
+        "EmergencyStop" => s.EmergencyStop,
+        "Alarm"         => s.Alarm,
+        "Edit"          => s.Edit,
+        "Dummy"         => s.Dummy,
+        _               => null,
+    };
+
+    private static int? PickProductionField(FocasProductionInfo p, string field) => field switch
+    {
+        "PartsProduced"    => p.PartsProduced,
+        "PartsRequired"    => p.PartsRequired,
+        "PartsTotal"       => p.PartsTotal,
+        "CycleTimeSeconds" => p.CycleTimeSeconds,
+        _                  => null,
+    };
+
+    private static short? PickModalField(FocasModalInfo m, string field) => field switch
+    {
+        "MCode" => m.MCode,
+        "SCode" => m.SCode,
+        "TCode" => m.TCode,
+        "BCode" => m.BCode,
+        _       => null,
+    };
+
+    private static short? PickOverrideField(FocasOverrideInfo o, string field) => field switch
+    {
+        "Feed"    => o.Feed,
+        "Rapid"   => o.Rapid,
+        "Spindle" => o.Spindle,
+        "Jog"     => o.Jog,
+        _         => null,
+    };
+
     // ---- ISubscribable (polling overlay via shared engine) ----
 
     public Task<ISubscriptionHandle> SubscribeAsync(
@@ -283,13 +765,123 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         while (!ct.IsCancellationRequested)
         {
             var success = false;
+            string? failureMessage = null;
             try
             {
                 var client = await EnsureConnectedAsync(state, ct).ConfigureAwait(false);
                 success = await client.ProbeAsync(ct).ConfigureAwait(false);
+                if (success)
+                {
+                    // Refresh figure-scaling cache once per session (issue #262). The
+                    // increment system rarely changes mid-session; re-reading every probe
+                    // tick would waste a wire call. Best-effort — null result leaves the
+                    // previous good map in place.
+                    if (state.FigureScaling is null)
+                    {
+                        var fig = await client.GetFigureScalingAsync(ct).ConfigureAwait(false);
+                        if (fig is not null) state.FigureScaling = fig;
+                    }
+
+                    // Refresh the cached ODBST status snapshot on every probe tick — this is
+                    // what the Status/ fixed-tree nodes serve from. Best-effort: a null result
+                    // (older IFocasClient impls without GetStatusAsync) just leaves the cache
+                    // unchanged so the previous good snapshot keeps serving until refreshed.
+                    var snapshot = await client.GetStatusAsync(ct).ConfigureAwait(false);
+                    if (snapshot is not null)
+                    {
+                        state.LastStatus = snapshot;
+                        state.LastStatusUtc = DateTime.UtcNow;
+                    }
+                    // Refresh the cached production snapshot too — same best-effort policy
+                    // as Status/: a null result leaves the previous good snapshot in place
+                    // so reads keep serving until the next successful refresh (issue #258).
+                    var production = await client.GetProductionAsync(ct).ConfigureAwait(false);
+                    if (production is not null)
+                    {
+                        state.LastProduction = production;
+                        state.LastProductionUtc = DateTime.UtcNow;
+                    }
+                    // Modal aux M/S/T/B + per-device operator overrides — same best-effort
+                    // policy as Status/ + Production/. Override snapshot is suppressed when
+                    // the device has no OverrideParameters configured (issue #259).
+                    var modal = await client.GetModalAsync(ct).ConfigureAwait(false);
+                    if (modal is not null)
+                    {
+                        state.LastModal = modal;
+                        state.LastModalUtc = DateTime.UtcNow;
+                    }
+                    if (state.Options.OverrideParameters is { } overrideParams)
+                    {
+                        var ov = await client.GetOverrideAsync(overrideParams, ct).ConfigureAwait(false);
+                        if (ov is not null)
+                        {
+                            state.LastOverride = ov;
+                            state.LastOverrideUtc = DateTime.UtcNow;
+                        }
+                    }
+                    // Tooling/CurrentTool + Offsets/{G54..G59}/{X|Y|Z} — same best-
+                    // effort policy as the other fixed-tree caches (issue #260). A
+                    // null result leaves the previous good snapshot in place so reads
+                    // keep serving until the next successful refresh.
+                    if (FocasCapabilityMatrix.SupportsTooling(state.Options.Series))
+                    {
+                        var tooling = await client.GetToolingAsync(ct).ConfigureAwait(false);
+                        if (tooling is not null)
+                        {
+                            state.LastTooling = tooling;
+                            state.LastToolingUtc = DateTime.UtcNow;
+                        }
+                    }
+                    if (FocasCapabilityMatrix.SupportsWorkOffsets(state.Options.Series))
+                    {
+                        var offsets = await client.GetWorkOffsetsAsync(ct).ConfigureAwait(false);
+                        if (offsets is not null)
+                        {
+                            state.LastWorkOffsets = offsets;
+                            state.LastWorkOffsetsUtc = DateTime.UtcNow;
+                        }
+                    }
+
+                    // Operator messages + currently-executing block — same best-effort
+                    // policy as the other fixed-tree caches (issue #261). A null result
+                    // leaves the previous good snapshot in place so reads keep serving
+                    // until the next successful refresh.
+                    var messages = await client.GetOperatorMessagesAsync(ct).ConfigureAwait(false);
+                    if (messages is not null)
+                    {
+                        state.LastMessages = messages;
+                        state.LastMessagesUtc = DateTime.UtcNow;
+                    }
+                    var block = await client.GetCurrentBlockAsync(ct).ConfigureAwait(false);
+                    if (block is not null)
+                    {
+                        state.LastCurrentBlock = block;
+                        state.LastCurrentBlockUtc = DateTime.UtcNow;
+                    }
+                }
             }
             catch (OperationCanceledException) when (ct.IsCancellationRequested) { break; }
-            catch { /* connect-failure path already disposed + cleared the client */ }
+            catch (Exception ex)
+            {
+                failureMessage = ex.Message;
+                /* connect-failure path already disposed + cleared the client */
+            }
+
+            // Diagnostics counters refreshed per probe tick (issue #262). Successful
+            // ticks bump ReadCount + LastSuccessfulRead; failed ticks bump
+            // ReadFailureCount + LastErrorMessage. The reconnect counter is bumped in
+            // EnsureConnectedAsync's connect path so a wedged probe doesn't double-count.
+            if (success)
+            {
+                Interlocked.Increment(ref state.ReadCount);
+                state.LastSuccessfulReadUtc = DateTime.UtcNow;
+            }
+            else
+            {
+                Interlocked.Increment(ref state.ReadFailureCount);
+                if (!string.IsNullOrEmpty(failureMessage))
+                    state.LastErrorMessage = failureMessage;
+            }
 
             TransitionDeviceState(state, success ? HostState.Running : HostState.Stopped);
 
@@ -298,6 +890,161 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         }
     }
 
+    private DataValueSnapshot ReadStatusField(string hostAddress, string field, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastStatus is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        var value = PickStatusField(snap, field);
+        if (value is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        return new DataValueSnapshot((short)value, FocasStatusMapper.Good,
+            device.LastStatusUtc, now);
+    }
+
+    private DataValueSnapshot ReadProductionField(string hostAddress, string field, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastProduction is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        var value = PickProductionField(snap, field);
+        if (value is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        return new DataValueSnapshot((int)value, FocasStatusMapper.Good,
+            device.LastProductionUtc, now);
+    }
+
+    private DataValueSnapshot ReadModalField(string hostAddress, string field, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastModal is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        var value = PickModalField(snap, field);
+        if (value is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        return new DataValueSnapshot((short)value, FocasStatusMapper.Good,
+            device.LastModalUtc, now);
+    }
+
+    private DataValueSnapshot ReadOverrideField(string hostAddress, string field, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastOverride is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        var value = PickOverrideField(snap, field);
+        if (value is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        return new DataValueSnapshot((short)value, FocasStatusMapper.Good,
+            device.LastOverrideUtc, now);
+    }
+
+    private DataValueSnapshot ReadToolingField(string hostAddress, string field, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastTooling is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        return field switch
+        {
+            "CurrentTool" => new DataValueSnapshot(snap.CurrentTool, FocasStatusMapper.Good,
+                device.LastToolingUtc, now),
+            _ => new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now),
+        };
+    }
+
+    private DataValueSnapshot ReadOffsetField(string hostAddress, string slot, string axis, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastWorkOffsets is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        var match = snap.Offsets.FirstOrDefault(o =>
+            string.Equals(o.Name, slot, StringComparison.OrdinalIgnoreCase));
+        if (match is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        var value = axis switch
+        {
+            "X" => (double?)match.X,
+            "Y" => match.Y,
+            "Z" => match.Z,
+            _   => null,
+        };
+        if (value is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        return new DataValueSnapshot(value.Value, FocasStatusMapper.Good,
+            device.LastWorkOffsetsUtc, now);
+    }
+
+    private DataValueSnapshot ReadMessagesLatestField(string hostAddress, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastMessages is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        // Snapshot is the trimmed list of active classes. "Latest" surfaces the last
+        // (most-recent) entry — the issue body permits this minimal "latest message"
+        // surface in lieu of a full ring buffer of all 4 classes.
+        var latest = snap.Messages.Count == 0
+            ? string.Empty
+            : snap.Messages[snap.Messages.Count - 1].Text;
+        return new DataValueSnapshot(latest, FocasStatusMapper.Good,
+            device.LastMessagesUtc, now);
+    }
+
+    private DataValueSnapshot ReadCurrentBlockField(string hostAddress, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        if (device.LastCurrentBlock is not { } snap)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadCommunicationError, null, now);
+        return new DataValueSnapshot(snap.Text, FocasStatusMapper.Good,
+            device.LastCurrentBlockUtc, now);
+    }
+
+    private DataValueSnapshot ReadDiagnosticsField(string hostAddress, string field, DateTime now)
+    {
+        if (!_devices.TryGetValue(hostAddress, out var device))
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        // Diagnostics counters are always Good — they're driver-internal state, not wire
+        // reads. LastSuccessfulRead surfaces DateTime.MinValue before the first probe
+        // tick rather than null because OPC UA's DateTime variant has no "unset" sentinel
+        // a generic client can interpret (issue #262).
+        object? value = field switch
+        {
+            "ReadCount"          => Interlocked.Read(ref device.ReadCount),
+            "ReadFailureCount"   => Interlocked.Read(ref device.ReadFailureCount),
+            "ReconnectCount"     => Interlocked.Read(ref device.ReconnectCount),
+            "LastErrorMessage"   => device.LastErrorMessage ?? string.Empty,
+            "LastSuccessfulRead" => device.LastSuccessfulReadUtc,
+            _                    => null,
+        };
+        if (value is null)
+            return new DataValueSnapshot(null, FocasStatusMapper.BadNodeIdUnknown, null, now);
+        return new DataValueSnapshot(value, FocasStatusMapper.Good, now, now);
+    }
+
+    /// <summary>
+    ///     Apply <c>cnc_getfigure</c>-derived decimal scaling to a raw position value.
+    ///     Returns <paramref name="raw"/> divided by <c>10^decimalPlaces</c> when the
+    ///     device has a cached scaling entry for <paramref name="axisName"/> AND
+    ///     <see cref="FocasFixedTreeOptions.ApplyFigureScaling"/> is on; otherwise
+    ///     returns the raw value as a <c>double</c>. Forward-looking — surfaced for
+    ///     future PRs that wire up <c>Axes/{name}/AbsolutePosition</c> etc. so they
+    ///     don't need to re-derive the policy (issue #262).
+    /// </summary>
+    internal double ApplyFigureScaling(string hostAddress, string axisName, long raw)
+    {
+        if (!_options.FixedTree.ApplyFigureScaling) return raw;
+        if (!_devices.TryGetValue(hostAddress, out var device)) return raw;
+        if (device.FigureScaling is not { } map) return raw;
+        if (!map.TryGetValue(axisName, out var dec) || dec <= 0) return raw;
+        return raw / Math.Pow(10.0, dec);
+    }
+
     private void TransitionDeviceState(DeviceState state, HostState newState)
     {
         HostState old;
@@ -324,6 +1071,10 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
     private async Task<IFocasClient> EnsureConnectedAsync(DeviceState device, CancellationToken ct)
     {
         if (device.Client is { IsConnected: true } c) return c;
+        // Reconnect counter bumps before the connect call — a successful first connect
+        // counts as one "establishment" so the field is non-zero from session start
+        // (issue #262, mirrors the convention from the AbCip / TwinCAT diagnostics).
+        Interlocked.Increment(ref device.ReconnectCount);
         device.Client ??= _clientFactory.Create();
         try
         {
@@ -352,6 +1103,90 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         public DateTime HostStateChangedUtc { get; set; } = DateTime.UtcNow;
         public CancellationTokenSource? ProbeCts { get; set; }
 
+        /// <summary>
+        ///     Cached <c>cnc_rdcncstat</c> snapshot, refreshed on every probe tick. Reads of
+        ///     the per-device <c>Status/&lt;field&gt;</c> fixed-tree nodes serve from this cache
+        ///     so they don't pile extra wire traffic on top of the user-driven tag reads.
+        /// </summary>
+        public FocasStatusInfo? LastStatus { get; set; }
+        public DateTime LastStatusUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_rdparam(6711/6712/6713)</c> + cycle-time snapshot, refreshed on
+        ///     every probe tick. Reads of the per-device <c>Production/&lt;field&gt;</c>
+        ///     fixed-tree nodes serve from this cache so they don't pile extra wire traffic
+        ///     on top of the user-driven tag reads (issue #258).
+        /// </summary>
+        public FocasProductionInfo? LastProduction { get; set; }
+        public DateTime LastProductionUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_modal</c> M/S/T/B snapshot, refreshed on every probe tick.
+        ///     Reads of the per-device <c>Modal/&lt;field&gt;</c> nodes serve from this cache
+        ///     so they don't pile extra wire traffic on top of user-driven reads (issue #259).
+        /// </summary>
+        public FocasModalInfo? LastModal { get; set; }
+        public DateTime LastModalUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_rdparam</c> override snapshot, refreshed on every probe tick.
+        ///     Suppressed when the device's <see cref="FocasDeviceOptions.OverrideParameters"/>
+        ///     is null (no <c>Override/</c> nodes are exposed in that case — issue #259).
+        /// </summary>
+        public FocasOverrideInfo? LastOverride { get; set; }
+        public DateTime LastOverrideUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_rdtnum</c> snapshot — current tool number — refreshed on
+        ///     every probe tick. Reads of <c>Tooling/CurrentTool</c> serve from this
+        ///     cache so they don't pile extra wire traffic on top of user-driven
+        ///     reads (issue #260).
+        /// </summary>
+        public FocasToolingInfo? LastTooling { get; set; }
+        public DateTime LastToolingUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_rdzofs(1..6)</c> snapshot — G54..G59 work-coordinate
+        ///     offsets — refreshed on every probe tick. Reads of
+        ///     <c>Offsets/{slot}/{X|Y|Z}</c> serve from this cache (issue #260).
+        /// </summary>
+        public FocasWorkOffsetsInfo? LastWorkOffsets { get; set; }
+        public DateTime LastWorkOffsetsUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_rdopmsg3</c> snapshot — active operator messages across
+        ///     the four FANUC classes — refreshed on every probe tick. Reads of
+        ///     <c>Messages/External/Latest</c> serve from this cache (issue #261).
+        /// </summary>
+        public FocasOperatorMessagesInfo? LastMessages { get; set; }
+        public DateTime LastMessagesUtc { get; set; }
+
+        /// <summary>
+        ///     Cached <c>cnc_rdactpt</c> snapshot — currently-executing block text —
+        ///     refreshed on every probe tick. Reads of <c>Program/CurrentBlock</c>
+        ///     serve from this cache (issue #261).
+        /// </summary>
+        public FocasCurrentBlockInfo? LastCurrentBlock { get; set; }
+        public DateTime LastCurrentBlockUtc { get; set; }
+
+        /// <summary>
+        ///     Cached per-axis decimal-place counts from <c>cnc_getfigure</c> (issue #262).
+        ///     Populated once per session (the increment system rarely changes mid-run);
+        ///     served by <see cref="FocasDriver.ApplyFigureScaling"/> when a future PR
+        ///     surfaces position values that need scaling. Keys are axis names (or
+        ///     fallback <c>"axis{n}"</c> until <c>cnc_rdaxisname</c> integration lands).
+        /// </summary>
+        public IReadOnlyDictionary<string, int>? FigureScaling { get; set; }
+
+        // Diagnostics counters per device — surfaced under Diagnostics/ subtree (issue
+        // #262). Public fields rather than properties so Interlocked.Increment can
+        // operate on them directly. Long-typed for the OPC UA Int64 surface.
+        public long ReadCount;
+        public long ReadFailureCount;
+        public long ReconnectCount;
+        public string? LastErrorMessage;
+        public DateTime LastSuccessfulReadUtc;
+
         public void DisposeClient()
         {
             Client?.Dispose();

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriverOptions.cs

@@ -11,17 +11,49 @@ public sealed class FocasDriverOptions
     public IReadOnlyList<FocasTagDefinition> Tags { get; init; } = [];
     public FocasProbeOptions Probe { get; init; } = new();
     public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+
+    /// <summary>
+    ///     Fixed-tree behaviour knobs (issue #262, plan PR F1-f). Carries the
+    ///     <c>ApplyFigureScaling</c> toggle that gates the <c>cnc_getfigure</c>
+    ///     decimal-place division applied to position values before publishing.
+    /// </summary>
+    public FocasFixedTreeOptions FixedTree { get; init; } = new();
+}
+
+/// <summary>
+///     Per-driver fixed-tree options. New installs default <see cref="ApplyFigureScaling"/>
+///     to <c>true</c> so position values surface in user units (mm / inch). Existing
+///     deployments that already published raw scaled integers can flip this to <c>false</c>
+///     for migration parity — the operator-facing concern is that switching the flag
+///     mid-deployment changes the values clients see, so the migration path is
+///     documentation-only (issue #262).
+/// </summary>
+public sealed record FocasFixedTreeOptions
+{
+    /// <summary>
+    ///     When <c>true</c> (default), position values from <c>cnc_absolute</c> /
+    ///     <c>cnc_machine</c> / <c>cnc_relative</c> / <c>cnc_distance</c> /
+    ///     <c>cnc_actf</c> are divided by <c>10^decimalPlaces</c> per axis using the
+    ///     <c>cnc_getfigure</c> snapshot cached at probe time. When <c>false</c>, the
+    ///     raw integer values are published unchanged — used for migrations from
+    ///     older drivers that didn't apply the scaling.
+    /// </summary>
+    public bool ApplyFigureScaling { get; init; } = true;
 }
 
 /// <summary>
 ///     One CNC the driver talks to. <paramref name="Series"/> enables per-series
 ///     address validation at <see cref="FocasDriver.InitializeAsync"/>; leave as
 ///     <see cref="FocasCncSeries.Unknown"/> to skip validation (legacy behaviour).
+///     <paramref name="OverrideParameters"/> declares the four MTB-specific override
+///     <c>cnc_rdparam</c> numbers surfaced under <c>Override/</c>; pass <c>null</c> to
+///     suppress the entire <c>Override/</c> subfolder for that device (issue #259).
 /// </summary>
 public sealed record FocasDeviceOptions(
     string HostAddress,
     string? DeviceName = null,
-    FocasCncSeries Series = FocasCncSeries.Unknown);
+    FocasCncSeries Series = FocasCncSeries.Unknown,
+    FocasOverrideParameters? OverrideParameters = null);
 
 /// <summary>
 ///     One FOCAS-backed OPC UA variable. <paramref name="Address"/> is the canonical FOCAS

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FwlibFocasClient.cs

@@ -137,6 +137,256 @@ internal sealed class FwlibFocasClient : IFocasClient
         return Task.FromResult(ret == 0);
     }
 
+    public Task<FocasStatusInfo?> GetStatusAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasStatusInfo?>(null);
+        var buf = new FwlibNative.ODBST();
+        var ret = FwlibNative.StatInfo(_handle, ref buf);
+        if (ret != 0) return Task.FromResult<FocasStatusInfo?>(null);
+        return Task.FromResult<FocasStatusInfo?>(new FocasStatusInfo(
+            Dummy: buf.Dummy,
+            Tmmode: buf.TmMode,
+            Aut: buf.Aut,
+            Run: buf.Run,
+            Motion: buf.Motion,
+            Mstb: buf.Mstb,
+            EmergencyStop: buf.Emergency,
+            Alarm: buf.Alarm,
+            Edit: buf.Edit));
+    }
+
+    public Task<FocasProductionInfo?> GetProductionAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasProductionInfo?>(null);
+        if (!TryReadInt32Param(6711, out var produced) ||
+            !TryReadInt32Param(6712, out var required) ||
+            !TryReadInt32Param(6713, out var total))
+        {
+            return Task.FromResult<FocasProductionInfo?>(null);
+        }
+        // Cycle-time timer (type=2). Total seconds = minute*60 + msec/1000. Best-effort:
+        // a non-zero return leaves cycle-time at 0 rather than failing the whole snapshot
+        // — the parts counters are still useful even when cycle-time isn't supported.
+        var cycleSeconds = 0;
+        var tmrBuf = new FwlibNative.IODBTMR();
+        if (FwlibNative.RdTimer(_handle, type: 2, ref tmrBuf) == 0)
+            cycleSeconds = checked(tmrBuf.Minute * 60 + tmrBuf.Msec / 1000);
+        return Task.FromResult<FocasProductionInfo?>(new FocasProductionInfo(
+            PartsProduced: produced,
+            PartsRequired: required,
+            PartsTotal: total,
+            CycleTimeSeconds: cycleSeconds));
+    }
+
+    private bool TryReadInt32Param(ushort number, out int value)
+    {
+        var buf = new FwlibNative.IODBPSD { Data = new byte[32] };
+        var ret = FwlibNative.RdParam(_handle, number, axis: 0, length: 4 + 4, ref buf);
+        if (ret != 0) { value = 0; return false; }
+        value = BinaryPrimitives.ReadInt32LittleEndian(buf.Data);
+        return true;
+    }
+
+    private bool TryReadInt16Param(ushort number, out short value)
+    {
+        var buf = new FwlibNative.IODBPSD { Data = new byte[32] };
+        var ret = FwlibNative.RdParam(_handle, number, axis: 0, length: 4 + 2, ref buf);
+        if (ret != 0) { value = 0; return false; }
+        value = BinaryPrimitives.ReadInt16LittleEndian(buf.Data);
+        return true;
+    }
+
+    public Task<FocasModalInfo?> GetModalAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasModalInfo?>(null);
+        // type 100/101/102/103 = M/S/T/B (single auxiliary code, active modal block 0).
+        // Best-effort — if any single read fails we still surface the others as 0; the
+        // probe loop only updates the cache on a non-null return so a partial snapshot
+        // is preferable to throwing away every successful field.
+        return Task.FromResult<FocasModalInfo?>(new FocasModalInfo(
+            MCode: ReadModalAux(type: 100),
+            SCode: ReadModalAux(type: 101),
+            TCode: ReadModalAux(type: 102),
+            BCode: ReadModalAux(type: 103)));
+    }
+
+    private short ReadModalAux(short type)
+    {
+        var buf = new FwlibNative.ODBMDL { Data = new byte[8] };
+        var ret = FwlibNative.Modal(_handle, type, block: 0, ref buf);
+        if (ret != 0) return 0;
+        // For aux types (100..103) the union holds the code at offset 0 as a 2-byte
+        // value (<c>aux_data</c>). Reading as Int16 keeps the surface identical to the
+        // record contract; oversized values would have been truncated by FWLIB anyway.
+        return BinaryPrimitives.ReadInt16LittleEndian(buf.Data);
+    }
+
+    public Task<FocasOverrideInfo?> GetOverrideAsync(
+        FocasOverrideParameters parameters, CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasOverrideInfo?>(null);
+        // Each parameter is independently nullable — a null parameter number keeps the
+        // corresponding field at null + skips the wire call. A successful read on at
+        // least one parameter is enough to publish a snapshot; this matches the
+        // best-effort policy used by GetProductionAsync (issue #259).
+        var feed    = TryReadOverride(parameters.FeedParam);
+        var rapid   = TryReadOverride(parameters.RapidParam);
+        var spindle = TryReadOverride(parameters.SpindleParam);
+        var jog     = TryReadOverride(parameters.JogParam);
+        return Task.FromResult<FocasOverrideInfo?>(new FocasOverrideInfo(feed, rapid, spindle, jog));
+    }
+
+    private short? TryReadOverride(ushort? param)
+    {
+        if (param is null) return null;
+        return TryReadInt16Param(param.Value, out var v) ? v : null;
+    }
+
+    public Task<FocasToolingInfo?> GetToolingAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasToolingInfo?>(null);
+        var buf = new FwlibNative.IODBTNUM();
+        var ret = FwlibNative.RdToolNumber(_handle, ref buf);
+        if (ret != 0) return Task.FromResult<FocasToolingInfo?>(null);
+        // FWLIB returns long; clamp to short for the surfaced Int16 (T-codes
+        // overflowing 32767 are vanishingly rare on Fanuc tool tables).
+        var t = buf.Data;
+        if (t > short.MaxValue) t = short.MaxValue;
+        else if (t < short.MinValue) t = short.MinValue;
+        return Task.FromResult<FocasToolingInfo?>(new FocasToolingInfo((short)t));
+    }
+
+    public Task<FocasWorkOffsetsInfo?> GetWorkOffsetsAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasWorkOffsetsInfo?>(null);
+
+        // 1..6 = G54..G59. Extended G54.1 P1..P48 use cnc_rdzofsr and are deferred.
+        // Pass axis=-1 so FWLIB fills every axis it has; we read the first 3 (X/Y/Z).
+        // Length = 4-byte header + 3 axes * 10-byte OFSB = 34. We request 4 + 8*10 = 84
+        // (the buffer ceiling) so a CNC with more axes still completes the call.
+        var slots = new List<FocasWorkOffset>(6);
+        string[] names = ["G54", "G55", "G56", "G57", "G58", "G59"];
+        for (short n = 1; n <= 6; n++)
+        {
+            var buf = new FwlibNative.IODBZOFS { Data = new byte[80] };
+            var ret = FwlibNative.RdWorkOffset(_handle, n, axis: -1, length: 4 + 8 * 10, ref buf);
+            if (ret != 0)
+            {
+                // Best-effort — a single-slot failure leaves the slot at 0.0; the cache
+                // still publishes so reads on the other offsets serve Good. The probe
+                // loop will retry on the next tick.
+                slots.Add(new FocasWorkOffset(names[n - 1], 0, 0, 0));
+                continue;
+            }
+            slots.Add(new FocasWorkOffset(
+                Name: names[n - 1],
+                X: DecodeOfsbAxis(buf.Data, axisIndex: 0),
+                Y: DecodeOfsbAxis(buf.Data, axisIndex: 1),
+                Z: DecodeOfsbAxis(buf.Data, axisIndex: 2)));
+        }
+        return Task.FromResult<FocasWorkOffsetsInfo?>(new FocasWorkOffsetsInfo(slots));
+    }
+
+    public Task<FocasOperatorMessagesInfo?> GetOperatorMessagesAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasOperatorMessagesInfo?>(null);
+        // type 0..3 = OPMSG / MACRO / EXTERN / REJ-EXT (issue #261). Single-slot read
+        // (length 4 + 256 = 260) returns the most-recent message in each class — best-
+        // effort: a single-class failure leaves that class out of the snapshot rather
+        // than failing the whole call, mirroring GetProductionAsync's policy.
+        var list = new List<FocasOperatorMessage>(4);
+        string[] classNames = ["OPMSG", "MACRO", "EXTERN", "REJ-EXT"];
+        for (short t = 0; t < 4; t++)
+        {
+            var buf = new FwlibNative.OPMSG3 { Data = new byte[256] };
+            var ret = FwlibNative.RdOpMsg3(_handle, t, length: 4 + 256, ref buf);
+            if (ret != 0) continue;
+            var text = TrimAnsiPadding(buf.Data);
+            if (string.IsNullOrEmpty(text)) continue;
+            list.Add(new FocasOperatorMessage(buf.Datano, classNames[t], text));
+        }
+        return Task.FromResult<FocasOperatorMessagesInfo?>(new FocasOperatorMessagesInfo(list));
+    }
+
+    public Task<FocasCurrentBlockInfo?> GetCurrentBlockAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<FocasCurrentBlockInfo?>(null);
+        var buf = new FwlibNative.ODBACTPT { Data = new byte[256] };
+        var ret = FwlibNative.RdActPt(_handle, ref buf);
+        if (ret != 0) return Task.FromResult<FocasCurrentBlockInfo?>(null);
+        return Task.FromResult<FocasCurrentBlockInfo?>(
+            new FocasCurrentBlockInfo(TrimAnsiPadding(buf.Data)));
+    }
+
+    public Task<IReadOnlyDictionary<string, int>?> GetFigureScalingAsync(CancellationToken cancellationToken)
+    {
+        if (!_connected) return Task.FromResult<IReadOnlyDictionary<string, int>?>(null);
+        // kind=0 → position figures (absolute/relative/machine/distance share the same
+        // increment system per axis). cnc_rdaxisname is deferred — the wire impl keys
+        // by fallback "axis{n}" (1-based), the driver re-keys when it gains axis-name
+        // discovery in a follow-up. Issue #262, plan PR F1-f.
+        short count = 0;
+        var buf = new FwlibNative.IODBAXIS { Data = new byte[FwlibNative.MAX_AXIS * 8] };
+        var ret = FwlibNative.GetFigure(_handle, kind: 0, ref count, ref buf);
+        if (ret != 0) return Task.FromResult<IReadOnlyDictionary<string, int>?>(null);
+        return Task.FromResult<IReadOnlyDictionary<string, int>?>(DecodeFigureScaling(buf.Data, count));
+    }
+
+    /// <summary>
+    ///     Decode the per-axis decimal-place counts from a <c>cnc_getfigure</c> reply
+    ///     buffer. Each axis entry per <c>fwlib32.h</c> is 8 bytes laid out as
+    ///     <c>short dec</c> + <c>short unit</c> + 4 reserved bytes; we read only
+    ///     <c>dec</c>. Keys are 1-based <c>"axis{n}"</c> placeholders — a follow-up
+    ///     PR can rewire to <c>cnc_rdaxisname</c> once that surface lands without
+    ///     changing the cache contract (issue #262).
+    /// </summary>
+    internal static IReadOnlyDictionary<string, int> DecodeFigureScaling(byte[] data, short count)
+    {
+        var clamped = Math.Max((short)0, Math.Min(count, (short)FwlibNative.MAX_AXIS));
+        var result = new Dictionary<string, int>(clamped, StringComparer.OrdinalIgnoreCase);
+        for (var i = 0; i < clamped; i++)
+        {
+            var offset = i * 8;
+            if (offset + 2 > data.Length) break;
+            var dec = BinaryPrimitives.ReadInt16LittleEndian(data.AsSpan(offset, 2));
+            if (dec < 0 || dec > 9) dec = 0;
+            result[$"axis{i + 1}"] = dec;
+        }
+        return result;
+    }
+
+    /// <summary>
+    ///     Decode + trim a Fanuc ANSI byte buffer. The CNC right-pads block text + opmsg
+    ///     bodies with nulls or spaces; trim them so the round-trip through the OPC UA
+    ///     address space stays stable (issue #261). Stops at the first NUL so any wire
+    ///     buffer that gets reused doesn't leak old bytes.
+    /// </summary>
+    internal static string TrimAnsiPadding(byte[] data)
+    {
+        if (data is null) return string.Empty;
+        var len = 0;
+        for (; len < data.Length; len++)
+            if (data[len] == 0) break;
+        return System.Text.Encoding.ASCII.GetString(data, 0, len).TrimEnd(' ', '\0');
+    }
+
+    /// <summary>
+    ///     Decode one OFSB axis block from a <c>cnc_rdzofs</c> data buffer. Each axis
+    ///     occupies 10 bytes per <c>fwlib32.h</c>: <c>int data</c> + <c>short dec</c> +
+    ///     <c>short unit</c> + <c>short disp</c>. The user-facing offset is
+    ///     <c>data / 10^dec</c> — same convention as <c>cnc_rdmacro</c>.
+    /// </summary>
+    internal static double DecodeOfsbAxis(byte[] data, int axisIndex)
+    {
+        const int blockSize = 10;
+        var offset = axisIndex * blockSize;
+        if (offset + blockSize > data.Length) return 0;
+        var raw = BinaryPrimitives.ReadInt32LittleEndian(data.AsSpan(offset, 4));
+        var dec = BinaryPrimitives.ReadInt16LittleEndian(data.AsSpan(offset + 4, 2));
+        if (dec < 0 || dec > 9) dec = 0;
+        return raw / Math.Pow(10.0, dec);
+    }
+
     // ---- PMC ----
 
     private (object? value, uint status) ReadPmc(FocasAddress address, FocasDataType type)

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FwlibNative.cs

@@ -88,6 +88,104 @@ internal static class FwlibNative
     [DllImport(Library, EntryPoint = "cnc_statinfo", ExactSpelling = true)]
     public static extern short StatInfo(ushort handle, ref ODBST buffer);
 
+    // ---- Timers ----
+
+    /// <summary>
+    ///     <c>cnc_rdtimer</c> — read CNC running timers. <paramref name="type"/>: 0 = power-on
+    ///     time (ms), 1 = operating time (ms), 2 = cycle time (ms), 3 = cutting time (ms).
+    ///     Only the cycle-time variant is consumed today (issue #258); the call is generic
+    ///     so the surface can grow without another P/Invoke.
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_rdtimer", ExactSpelling = true)]
+    public static extern short RdTimer(ushort handle, short type, ref IODBTMR buffer);
+
+    // ---- Modal codes ----
+
+    /// <summary>
+    ///     <c>cnc_modal</c> — read modal information for one G-group or auxiliary code.
+    ///     <paramref name="type"/>: 1..21 = G-group N (single group), 100 = M, 101 = S,
+    ///     102 = T, 103 = B (per Fanuc FOCAS reference). <paramref name="block"/>: 0 =
+    ///     active modal commands. We only consume types 100..103 today (M/S/T/B); the
+    ///     G-group decode is deferred to a follow-up because the <c>ODBMDL</c> union
+    ///     varies by group + series (issue #259).
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_modal", ExactSpelling = true)]
+    public static extern short Modal(ushort handle, short type, short block, ref ODBMDL buffer);
+
+    // ---- Tooling ----
+
+    /// <summary>
+    ///     <c>cnc_rdtnum</c> — read the currently selected tool number. Returns
+    ///     <c>EW_OK</c> + populates <see cref="IODBTNUM.Data"/> with the active T-code.
+    ///     Tool life + current offset index reads (<c>cnc_rdtlinfo</c>/<c>cnc_rdtlsts</c>/
+    ///     <c>cnc_rdtofs</c>) are deferred per the F1-d plan — those calls use ODBTLIFE*
+    ///     unions whose shape varies per series.
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_rdtnum", ExactSpelling = true)]
+    public static extern short RdToolNumber(ushort handle, ref IODBTNUM buffer);
+
+    // ---- Work coordinate offsets ----
+
+    /// <summary>
+    ///     <c>cnc_rdzofs</c> — read one work-coordinate offset slot. <paramref name="number"/>:
+    ///     1..6 = G54..G59 (standard). Extended <c>G54.1 P1..P48</c> use <c>cnc_rdzofsr</c>
+    ///     and are deferred. <paramref name="axis"/>: -1 = all axes returned, 1..N = single
+    ///     axis. <paramref name="length"/>: 12 + (N axes * 8) — we request -1 and let FWLIB
+    ///     fill up to <see cref="IODBZOFS.Data"/>'s 8-axis ceiling.
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_rdzofs", ExactSpelling = true)]
+    public static extern short RdWorkOffset(
+        ushort handle,
+        short number,
+        short axis,
+        short length,
+        ref IODBZOFS buffer);
+
+    // ---- Operator messages ----
+
+    /// <summary>
+    ///     <c>cnc_rdopmsg3</c> — read FANUC operator messages by class. <paramref name="type"/>:
+    ///     0 = OPMSG (op-msg ladder/macro), 1 = MACRO, 2 = EXTERN (external operator message),
+    ///     3 = REJ-EXT (rejected EXTERN). <paramref name="length"/>: per <c>fwlib32.h</c> the
+    ///     buffer is <c>4 + 256 = 260</c> bytes per message slot — single-slot reads (length 260)
+    ///     return the most-recent message in that class. Issue #261, plan PR F1-e.
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_rdopmsg3", CharSet = CharSet.Ansi, ExactSpelling = true)]
+    public static extern short RdOpMsg3(
+        ushort handle,
+        short type,
+        short length,
+        ref OPMSG3 buffer);
+
+    // ---- Figure (per-axis decimal scaling) ----
+
+    /// <summary>
+    ///     <c>cnc_getfigure</c> — read per-axis figure info (decimal-place counts + units).
+    ///     <paramref name="kind"/>: 0 = absolute / relative / machine position figures,
+    ///     1 = work-coord shift figures (per Fanuc reference). The reply struct holds
+    ///     up to <see cref="MAX_AXIS"/> axis entries; the managed side reads the count
+    ///     out via <paramref name="outCount"/>. Position values from <c>cnc_absolute</c>
+    ///     / <c>cnc_machine</c> / <c>cnc_relative</c> / <c>cnc_distance</c> / <c>cnc_actf</c>
+    ///     are scaled integers — divide by <c>10^figureinfo[axis].dec</c> for user units
+    ///     (issue #262, plan PR F1-f).
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_getfigure", ExactSpelling = true)]
+    public static extern short GetFigure(
+        ushort handle,
+        short kind,
+        ref short outCount,
+        ref IODBAXIS figureinfo);
+
+    // ---- Currently-executing block ----
+
+    /// <summary>
+    ///     <c>cnc_rdactpt</c> — read the currently-executing program block text. The
+    ///     reply struct holds the program / sequence numbers + the active block as a
+    ///     null-padded ASCII string. Issue #261, plan PR F1-e.
+    /// </summary>
+    [DllImport(Library, EntryPoint = "cnc_rdactpt", CharSet = CharSet.Ansi, ExactSpelling = true)]
+    public static extern short RdActPt(ushort handle, ref ODBACTPT buffer);
+
     // ---- Structs ----
 
     /// <summary>
@@ -129,6 +227,121 @@ internal static class FwlibNative
         public short DecVal;    // decimal-point count
     }
 
+    /// <summary>
+    ///     IODBTMR — running-timer read buffer per <c>fwlib32.h</c>. Minute portion in
+    ///     <see cref="Minute"/>; sub-minute remainder in milliseconds in <see cref="Msec"/>.
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct IODBTMR
+    {
+        public int Minute;
+        public int Msec;
+    }
+
+    /// <summary>
+    ///     ODBMDL — single-group modal read buffer. 4-byte header + a 4-byte union which we
+    ///     marshal as a fixed byte array. For type=100..103 (M/S/T/B) the union holds an
+    ///     <c>int aux_data</c> at offset 0; we read the first <c>short</c> for symmetry with
+    ///     the FWLIB <c>g_modal.aux_data</c> width on G-group reads. The G-group decode
+    ///     (type=1..21) is deferred — see <see cref="Modal"/> for context (issue #259).
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct ODBMDL
+    {
+        public short Datano;
+        public short Type;
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
+        public byte[] Data;
+    }
+
+    /// <summary>
+    ///     IODBTNUM — current tool number read buffer. <see cref="Data"/> holds the active
+    ///     T-code (Fanuc reference uses <c>long</c>; we narrow to <c>short</c> on the
+    ///     managed side because <see cref="FocasToolingInfo.CurrentTool"/> surfaces as
+    ///     <c>Int16</c>). Issue #260, F1-d.
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct IODBTNUM
+    {
+        public short Datano;
+        public short Type;
+        public int Data;
+    }
+
+    /// <summary>
+    ///     IODBZOFS — work-coordinate offset read buffer. 4-byte header + per-axis
+    ///     <c>OFSB</c> blocks (8 bytes each: 4-byte signed integer <c>data</c> + 2-byte
+    ///     <c>dec</c> decimal-point count + 2-byte <c>unit</c> + 2-byte <c>disp</c>).
+    ///     We marshal a fixed ceiling of 8 axes (= 64 bytes); the managed side reads
+    ///     only the first 3 (X / Y / Z) per the F1-d effort sizing.
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct IODBZOFS
+    {
+        public short Datano;
+        public short Type;
+        // Up to 8 axes * 8 bytes per OFSB = 64 bytes. Each block: int data, short dec,
+        // short unit, short disp (10 bytes per fwlib32.h). We size for the worst case.
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 80)]
+        public byte[] Data;
+    }
+
+    /// <summary>
+    ///     OPMSG3 — single-slot operator-message read buffer per <c>fwlib32.h</c>. Per Fanuc
+    ///     reference: <c>short datano</c> + <c>short type</c> + <c>char data[256]</c>. The
+    ///     text is null-terminated + space-padded; the managed side trims trailing nulls /
+    ///     spaces before publishing. Length = 4 + 256 = 260 bytes; total 256 wide enough
+    ///     for the longest documented operator message body (issue #261).
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct OPMSG3
+    {
+        public short Datano;
+        public short Type;
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 256)]
+        public byte[] Data;
+    }
+
+    /// <summary>
+    ///     ODBACTPT — current-block read buffer per <c>fwlib32.h</c>. Per Fanuc reference:
+    ///     <c>long o_no</c> (currently active O-number) + <c>long n_no</c> (sequence) +
+    ///     <c>char data[256]</c> (active block text). The text is null-terminated +
+    ///     space-padded; trimmed before publishing for stable round-trip (issue #261).
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct ODBACTPT
+    {
+        public int ONo;
+        public int NNo;
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 256)]
+        public byte[] Data;
+    }
+
+    /// <summary>
+    ///     Maximum axis count per the FWLIB <c>fwlib32.h</c> ceiling for figure-info reads.
+    ///     Real Fanuc CNCs cap at 8 simultaneous axes for most series; we marshal an
+    ///     8-entry array (matches <see cref="IODBAXIS"/>) so the call completes regardless
+    ///     of the deployment's axis count (issue #262).
+    /// </summary>
+    public const int MAX_AXIS = 8;
+
+    /// <summary>
+    ///     IODBAXIS — per-axis figure info read buffer for <c>cnc_getfigure</c>. Each
+    ///     axis entry carries the decimal-place count (<c>dec</c>) the CNC reports for
+    ///     that axis's increment system + a unit code. The managed side reads the first
+    ///     <c>outCount</c> entries returned by FWLIB; we marshal a fixed 8-entry ceiling
+    ///     (issue #262, plan PR F1-f).
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
+    public struct IODBAXIS
+    {
+        // Each entry per fwlib32.h is { short dec, short unit, short reserved, short reserved2 }
+        // = 8 bytes. 8 axes * 8 bytes = 64 bytes; we marshal a fixed byte buffer + decode on
+        // the managed side so axis-count growth doesn't churn the P/Invoke surface.
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8 * 8)]
+        public byte[] Data;
+    }
+
     /// <summary>ODBST — CNC status info. Machine state, alarm flags, automatic / edit mode.</summary>
     [StructLayout(LayoutKind.Sequential, Pack = 1)]
     public struct ODBST

src/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/IFocasClient.cs

@@ -48,8 +48,236 @@ public interface IFocasClient : IDisposable
     ///     responds with any valid status.
     /// </summary>
     Task<bool> ProbeAsync(CancellationToken cancellationToken);
+
+    /// <summary>
+    ///     Read the full <c>cnc_rdcncstat</c> ODBST struct (9 small-int status flags). The
+    ///     boolean <see cref="ProbeAsync"/> is preserved for cheap reachability checks; this
+    ///     method exposes the per-field detail used by the FOCAS driver's <c>Status/</c>
+    ///     fixed-tree nodes (see issue #257). Returns <c>null</c> if the wire client cannot
+    ///     supply the struct (e.g. transport/IPC variant where the contract has not been
+    ///     extended yet) — callers fall back to surfacing Bad on the per-field nodes.
+    /// </summary>
+    Task<FocasStatusInfo?> GetStatusAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasStatusInfo?>(null);
+
+    /// <summary>
+    ///     Read the per-CNC production counters (parts produced / required / total via
+    ///     <c>cnc_rdparam(6711/6712/6713)</c>) plus the current cycle-time seconds counter
+    ///     (<c>cnc_rdtimer(2)</c>). Surfaced on the FOCAS driver's <c>Production/</c>
+    ///     fixed-tree per device (issue #258). Returns <c>null</c> when the wire client
+    ///     cannot supply the snapshot (e.g. older transport variant) — the driver leaves
+    ///     the cache untouched and the per-field nodes report Bad until the first refresh.
+    /// </summary>
+    Task<FocasProductionInfo?> GetProductionAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasProductionInfo?>(null);
+
+    /// <summary>
+    ///     Read the active modal M/S/T/B codes via <c>cnc_modal</c>. G-group decoding is
+    ///     deferred — the FWLIB <c>ODBMDL</c> union differs per series + group and the
+    ///     issue body permits surfacing only the universally-present M/S/T/B fields in
+    ///     the first cut (issue #259). Returns <c>null</c> when the wire client cannot
+    ///     supply the snapshot.
+    /// </summary>
+    Task<FocasModalInfo?> GetModalAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasModalInfo?>(null);
+
+    /// <summary>
+    ///     Read the four operator override values (feed / rapid / spindle / jog) via
+    ///     <c>cnc_rdparam</c>. The parameter numbers are MTB-specific so the caller passes
+    ///     them in via <paramref name="parameters"/>; a <c>null</c> entry suppresses that
+    ///     field's read (the corresponding node is also omitted from the address space).
+    ///     Returns <c>null</c> when the wire client cannot supply the snapshot (issue #259).
+    /// </summary>
+    Task<FocasOverrideInfo?> GetOverrideAsync(
+        FocasOverrideParameters parameters, CancellationToken cancellationToken)
+        => Task.FromResult<FocasOverrideInfo?>(null);
+
+    /// <summary>
+    ///     Read the current tool number via <c>cnc_rdtnum</c>. Surfaced on the FOCAS driver's
+    ///     <c>Tooling/</c> fixed-tree per device (issue #260). Tool life + current offset
+    ///     index are deferred — <c>cnc_rdtlinfo</c>/<c>cnc_rdtlsts</c> vary heavily across
+    ///     CNC series + the FWLIB <c>ODBTLIFE*</c> unions need per-series shape handling
+    ///     that exceeds the L-sized scope of this PR. Returns <c>null</c> when the wire
+    ///     client cannot supply the snapshot (e.g. older transport variant).
+    /// </summary>
+    Task<FocasToolingInfo?> GetToolingAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasToolingInfo?>(null);
+
+    /// <summary>
+    ///     Read the standard G54..G59 work-coordinate offsets via
+    ///     <c>cnc_rdzofs(handle, n=1..6)</c>. Returns one <see cref="FocasWorkOffset"/>
+    ///     per slot (issue #260). Extended G54.1 P1..P48 offsets are deferred — they use
+    ///     a different FOCAS call (<c>cnc_rdzofsr</c>) + different range handling. Each
+    ///     offset surfaces a fixed X/Y/Z view; lathes/mills with extra rotational axes
+    ///     have those columns reported as 0.0. Returns <c>null</c> when the wire client
+    ///     cannot supply the snapshot.
+    /// </summary>
+    Task<FocasWorkOffsetsInfo?> GetWorkOffsetsAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasWorkOffsetsInfo?>(null);
+
+    /// <summary>
+    ///     Read the four FANUC operator-message classes via <c>cnc_rdopmsg3</c> (issue #261).
+    ///     The call returns up to 4 active messages per class; the driver collapses the
+    ///     latest non-empty message per class onto the <c>Messages/External/Latest</c>
+    ///     fixed-tree node — the issue body permits this minimal surface in the first cut.
+    ///     Trailing nulls / spaces are trimmed before publishing so the same message
+    ///     round-trips with stable text. Returns <c>null</c> when the wire client cannot
+    ///     supply the snapshot (older transport variant).
+    /// </summary>
+    Task<FocasOperatorMessagesInfo?> GetOperatorMessagesAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasOperatorMessagesInfo?>(null);
+
+    /// <summary>
+    ///     Read the currently-executing block text via <c>cnc_rdactpt</c> (issue #261).
+    ///     The call returns the active block of the running program; surfaced as
+    ///     <c>Program/CurrentBlock</c> Float-trimmed string. Returns <c>null</c> when the
+    ///     wire client cannot supply the snapshot.
+    /// </summary>
+    Task<FocasCurrentBlockInfo?> GetCurrentBlockAsync(CancellationToken cancellationToken)
+        => Task.FromResult<FocasCurrentBlockInfo?>(null);
+
+    /// <summary>
+    ///     Read the per-axis decimal-place counts via <c>cnc_getfigure</c> (issue #262).
+    ///     Returned dictionary maps axis name (or fallback <c>"axis{n}"</c> when
+    ///     <c>cnc_rdaxisname</c> isn't available) to the decimal-place count the CNC
+    ///     reports for that axis's increment system. Cached at bootstrap by the driver +
+    ///     applied to position values before publishing — raw integer / 10^decimalPlaces.
+    ///     Returns <c>null</c> when the wire client cannot supply the snapshot (older
+    ///     transport variant) — the driver leaves the cache untouched and falls back to
+    ///     publishing raw values.
+    /// </summary>
+    Task<IReadOnlyDictionary<string, int>?> GetFigureScalingAsync(CancellationToken cancellationToken)
+        => Task.FromResult<IReadOnlyDictionary<string, int>?>(null);
 }
 
+/// <summary>
+///     Snapshot of the 9 fields returned by Fanuc's <c>cnc_rdcncstat</c> (ODBST). All fields
+///     are <c>short</c> per the FWLIB header — small enums whose meaning is documented in the
+///     Fanuc FOCAS reference (e.g. <c>emergency</c>: 0=released, 1=stop, 2=reset). Surfaced as
+///     <c>Int16</c> in the OPC UA address space rather than mapped enums so operators see
+///     exactly what the CNC reported.
+/// </summary>
+public sealed record FocasStatusInfo(
+    short Dummy,
+    short Tmmode,
+    short Aut,
+    short Run,
+    short Motion,
+    short Mstb,
+    short EmergencyStop,
+    short Alarm,
+    short Edit);
+
+/// <summary>
+///     Snapshot of per-CNC production counters refreshed on the probe tick (issue #258).
+///     Sourced from <c>cnc_rdparam(6711/6712/6713)</c> for the parts counts + the cycle-time
+///     timer counter (FWLIB <c>cnc_rdtimer</c> when available). All values surfaced as
+///     <c>Int32</c> in the OPC UA address space.
+/// </summary>
+public sealed record FocasProductionInfo(
+    int PartsProduced,
+    int PartsRequired,
+    int PartsTotal,
+    int CycleTimeSeconds);
+
+/// <summary>
+///     Snapshot of the active modal M/S/T/B codes (issue #259). G-group decoding is a
+///     deferred follow-up — the FWLIB <c>ODBMDL</c> union differs per series + group, and
+///     the issue body permits the first cut to surface only the universally-present
+///     M/S/T/B fields. <c>short</c> matches the FWLIB <c>aux_data</c> width.
+/// </summary>
+public sealed record FocasModalInfo(
+    short MCode,
+    short SCode,
+    short TCode,
+    short BCode);
+
+/// <summary>
+///     MTB-specific FOCAS parameter numbers for the four operator overrides (issue #259).
+///     Defaults match Fanuc 30i — Feed=6010, Rapid=6011, Spindle=6014, Jog=6015. A
+///     <c>null</c> entry suppresses that field's read on the wire and removes the matching
+///     node from the address space; this lets a deployment hide overrides their MTB doesn't
+///     wire up rather than always serving Bad.
+/// </summary>
+public sealed record FocasOverrideParameters(
+    ushort? FeedParam,
+    ushort? RapidParam,
+    ushort? SpindleParam,
+    ushort? JogParam)
+{
+    /// <summary>Stock 30i defaults — Feed=6010, Rapid=6011, Spindle=6014, Jog=6015.</summary>
+    public static FocasOverrideParameters Default { get; } = new(6010, 6011, 6014, 6015);
+}
+
+/// <summary>
+///     Snapshot of the four operator overrides (issue #259). Each value is a percentage
+///     surfaced as <c>Int16</c>; a value of <c>null</c> means the corresponding parameter
+///     was not configured (suppressed at <see cref="FocasOverrideParameters"/>). All four
+///     fields nullable so the driver can omit nodes whose MTB parameter is unset.
+/// </summary>
+public sealed record FocasOverrideInfo(
+    short? Feed,
+    short? Rapid,
+    short? Spindle,
+    short? Jog);
+
+/// <summary>
+///     Snapshot of the currently selected tool number (issue #260). Sourced from
+///     <c>cnc_rdtnum</c>. The active offset index is deferred — most modern CNCs
+///     interleave tool number and offset H/D codes through different FOCAS calls
+///     (<c>cnc_rdtofs</c> against a specific slot) and the issue body permits
+///     surfacing tool number alone in the first cut. Surfaced as <c>Int16</c> in
+///     the OPC UA address space.
+/// </summary>
+public sealed record FocasToolingInfo(short CurrentTool);
+
+/// <summary>
+///     One work-coordinate offset slot (G54..G59). Three axis columns are surfaced
+///     (X / Y / Z) — the issue body permits a fixed 3-axis view because lathes and
+///     mills typically don't expose extended rotational offsets via the standard
+///     <c>cnc_rdzofs</c> call. Extended <c>G54.1 Pn</c> offsets via <c>cnc_rdzofsr</c>
+///     are deferred to a follow-up PR. Values surfaced as <c>Float64</c> in microns
+///     converted to user units (the FWLIB <c>data</c> field is an integer + decimal-
+///     point count, decoded the same way <c>cnc_rdmacro</c> values are).
+/// </summary>
+public sealed record FocasWorkOffset(string Name, double X, double Y, double Z);
+
+/// <summary>
+///     Snapshot of the six standard work-coordinate offsets (G54..G59). Refreshed on
+///     the probe tick + served from the per-device cache by reads of the
+///     <c>Offsets/{name}/{X|Y|Z}</c> fixed-tree nodes (issue #260).
+/// </summary>
+public sealed record FocasWorkOffsetsInfo(IReadOnlyList<FocasWorkOffset> Offsets);
+
+/// <summary>
+///     One FANUC operator message — the <see cref="Number"/> + <see cref="Class"/>
+///     + <see cref="Text"/> tuple returned by <c>cnc_rdopmsg3</c> for a single
+///     active message slot. <see cref="Class"/> is one of <c>"OPMSG"</c> /
+///     <c>"MACRO"</c> / <c>"EXTERN"</c> / <c>"REJ-EXT"</c> per the FOCAS reference
+///     for the four message types. <see cref="Text"/> is trimmed of trailing
+///     nulls + spaces so round-trips through the OPC UA address space stay stable
+///     (issue #261).
+/// </summary>
+public sealed record FocasOperatorMessage(short Number, string Class, string Text);
+
+/// <summary>
+///     Snapshot of all active FANUC operator messages across the four message
+///     classes (issue #261). Surfaced under the FOCAS driver's
+///     <c>Messages/External/Latest</c> fixed-tree node — the latest non-empty
+///     message in the list is what gets published. Empty list means the CNC
+///     reported no active messages; the node publishes an empty string in that
+///     case.
+/// </summary>
+public sealed record FocasOperatorMessagesInfo(IReadOnlyList<FocasOperatorMessage> Messages);
+
+/// <summary>
+///     Snapshot of the currently-executing program block text via
+///     <c>cnc_rdactpt</c> (issue #261). <see cref="Text"/> is trimmed of trailing
+///     nulls + spaces so the same block round-trips with stable text. Surfaced
+///     as a String node at <c>Program/CurrentBlock</c>.
+/// </summary>
+public sealed record FocasCurrentBlockInfo(string Text);
+
 /// <summary>Factory for <see cref="IFocasClient"/>s. One client per configured device.</summary>
 public interface IFocasClientFactory
 {

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -75,6 +75,31 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
     /// </summary>
     private SessionReconnectHandler? _reconnectHandler;
 
+    /// <summary>
+    ///     Cached server-advertised OperationLimits, fetched lazily on first batch op and
+    ///     refreshed on reconnect. Null until the first successful fetch; null components
+    ///     mean "fetch hasn't completed yet, fall through to single-call". Per spec, a 0
+    ///     limit means "no limit" — we surface that as <c>uint?</c>=null too so the
+    ///     chunking helper has a single sentinel for "don't chunk".
+    /// </summary>
+    private OperationLimitsCache? _operationLimits;
+    private readonly SemaphoreSlim _operationLimitsLock = new(1, 1);
+
+    /// <summary>
+    ///     Snapshot of the four OperationLimits the driver chunks against. Stored as
+    ///     <c>uint?</c> so callers can distinguish "not yet fetched" / "no limit"
+    ///     (null) from "limit = N" (Some(N)). Spec sentinel 0 is normalized to null at
+    ///     fetch time so the chunking helper has a single "don't chunk" sentinel.
+    /// </summary>
+    internal sealed record OperationLimitsCache(
+        uint? MaxNodesPerRead,
+        uint? MaxNodesPerWrite,
+        uint? MaxNodesPerBrowse,
+        uint? MaxNodesPerHistoryReadData);
+
+    /// <summary>Test seam — exposes the cached limits so unit tests can assert fetch behaviour.</summary>
+    internal OperationLimitsCache? OperationLimitsForTest => _operationLimits;
+
     public string DriverInstanceId => driverInstanceId;
     public string DriverType => "OpcUaClient";
 
@@ -427,6 +452,7 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         try { Session?.Dispose(); } catch { }
         Session = null;
         _connectedEndpointUrl = null;
+        _operationLimits = null;
 
         TransitionTo(HostState.Unknown);
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
@@ -442,6 +468,7 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
     {
         var session = RequireSession();
+        await EnsureOperationLimitsFetchedAsync(cancellationToken).ConfigureAwait(false);
         var results = new DataValueSnapshot[fullReferences.Count];
         var now = DateTime.UtcNow;
 
@@ -463,31 +490,44 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
 
         if (toSend.Count == 0) return results;
 
+        // Honor server's MaxNodesPerRead — chunk large batches so a single ReadAsync stays
+        // under the cap. cap=null means "no limit" (sentinel for both 0-from-server and
+        // not-yet-fetched), in which case ChunkBy yields the input as a single slice and
+        // the wire path collapses to one SDK call.
+        var readCap = _operationLimits?.MaxNodesPerRead;
+        var indexMapList = indexMap; // close over for catch
         await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
         try
         {
             try
             {
-                var resp = await session.ReadAsync(
-                    requestHeader: null,
-                    maxAge: 0,
-                    timestampsToReturn: TimestampsToReturn.Both,
-                    nodesToRead: toSend,
-                    ct: cancellationToken).ConfigureAwait(false);
-
-                var values = resp.Results;
-                for (var w = 0; w < values.Count; w++)
+                var wireOffset = 0;
+                foreach (var chunk in ChunkBy(toSend, readCap))
                 {
-                    var r = indexMap[w];
-                    var dv = values[w];
-                    // Preserve the upstream StatusCode verbatim — including Bad codes per
-                    // §8's cascading-quality rule. Also preserve SourceTimestamp so downstream
-                    // clients can detect stale upstream data.
-                    results[r] = new DataValueSnapshot(
-                        Value: dv.Value,
-                        StatusCode: dv.StatusCode.Code,
-                        SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
-                        ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? now : dv.ServerTimestamp);
+                    var chunkColl = new ReadValueIdCollection(chunk.Count);
+                    for (var i = 0; i < chunk.Count; i++) chunkColl.Add(chunk.Array![chunk.Offset + i]);
+                    var resp = await session.ReadAsync(
+                        requestHeader: null,
+                        maxAge: 0,
+                        timestampsToReturn: TimestampsToReturn.Both,
+                        nodesToRead: chunkColl,
+                        ct: cancellationToken).ConfigureAwait(false);
+
+                    var values = resp.Results;
+                    for (var w = 0; w < values.Count; w++)
+                    {
+                        var r = indexMapList[wireOffset + w];
+                        var dv = values[w];
+                        // Preserve the upstream StatusCode verbatim — including Bad codes per
+                        // §8's cascading-quality rule. Also preserve SourceTimestamp so downstream
+                        // clients can detect stale upstream data.
+                        results[r] = new DataValueSnapshot(
+                            Value: dv.Value,
+                            StatusCode: dv.StatusCode.Code,
+                            SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
+                            ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? now : dv.ServerTimestamp);
+                    }
+                    wireOffset += chunk.Count;
                 }
                 _health = new DriverHealth(DriverState.Healthy, now, null);
             }
@@ -497,9 +537,9 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
                 // tag in this batch. Per-tag StatusCode stays BadCommunicationError (not
                 // BadInternalError) so operators distinguish "upstream unreachable" from
                 // "driver bug".
-                for (var w = 0; w < indexMap.Count; w++)
+                for (var w = 0; w < indexMapList.Count; w++)
                 {
-                    var r = indexMap[w];
+                    var r = indexMapList[w];
                     results[r] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
                 }
                 _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
@@ -515,6 +555,7 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         IReadOnlyList<Core.Abstractions.WriteRequest> writes, CancellationToken cancellationToken)
     {
         var session = RequireSession();
+        await EnsureOperationLimitsFetchedAsync(cancellationToken).ConfigureAwait(false);
         var results = new WriteResult[writes.Count];
 
         var toSend = new WriteValueCollection();
@@ -537,24 +578,34 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
 
         if (toSend.Count == 0) return results;
 
+        // Honor server's MaxNodesPerWrite — same chunking pattern as ReadAsync. cap=null
+        // collapses to a single wire call.
+        var writeCap = _operationLimits?.MaxNodesPerWrite;
         await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
         try
         {
             try
             {
-                var resp = await session.WriteAsync(
-                    requestHeader: null,
-                    nodesToWrite: toSend,
-                    ct: cancellationToken).ConfigureAwait(false);
-
-                var codes = resp.Results;
-                for (var w = 0; w < codes.Count; w++)
+                var wireOffset = 0;
+                foreach (var chunk in ChunkBy(toSend, writeCap))
                 {
-                    var r = indexMap[w];
-                    // Pass upstream WriteResult StatusCode through verbatim. Success codes
-                    // include Good (0) and any warning-level Good* variants; anything with
-                    // the severity bits set is a Bad.
-                    results[r] = new WriteResult(codes[w].Code);
+                    var chunkColl = new WriteValueCollection(chunk.Count);
+                    for (var i = 0; i < chunk.Count; i++) chunkColl.Add(chunk.Array![chunk.Offset + i]);
+                    var resp = await session.WriteAsync(
+                        requestHeader: null,
+                        nodesToWrite: chunkColl,
+                        ct: cancellationToken).ConfigureAwait(false);
+
+                    var codes = resp.Results;
+                    for (var w = 0; w < codes.Count; w++)
+                    {
+                        var r = indexMap[wireOffset + w];
+                        // Pass upstream WriteResult StatusCode through verbatim. Success codes
+                        // include Good (0) and any warning-level Good* variants; anything with
+                        // the severity bits set is a Bad.
+                        results[r] = new WriteResult(codes[w].Code);
+                    }
+                    wireOffset += chunk.Count;
                 }
             }
             catch (Exception)
@@ -591,6 +642,81 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
     private ISession RequireSession() =>
         Session ?? throw new InvalidOperationException("OpcUaClientDriver not initialized");
 
+    /// <summary>
+    ///     Lazily fetch <c>Server.ServerCapabilities.OperationLimits</c> from the upstream
+    ///     server and cache them on the driver. Idempotent — called from every batch op,
+    ///     no-ops once a successful fetch has populated the cache. The cache is cleared on
+    ///     reconnect (see <see cref="OnReconnectComplete"/>) so a server with redrawn
+    ///     capabilities doesn't run forever with stale caps.
+    /// </summary>
+    /// <remarks>
+    ///     Uses <see cref="Session.FetchOperationLimitsAsync(CancellationToken)"/> when the
+    ///     active session is a concrete <see cref="Session"/> (always true in production —
+    ///     the SDK's session factory returns Session). Falls back gracefully on any fetch
+    ///     failure: callers see <see cref="_operationLimits"/> remain null and fall through
+    ///     to single-call behaviour. Per OPC UA Part 5, a server reporting 0 for any
+    ///     OperationLimits attribute means "no limit"; we normalize that to <c>null</c> so
+    ///     the chunking helper has a single sentinel.
+    /// </remarks>
+    private async Task EnsureOperationLimitsFetchedAsync(CancellationToken ct)
+    {
+        if (_operationLimits is not null) return;
+
+        await _operationLimitsLock.WaitAsync(ct).ConfigureAwait(false);
+        try
+        {
+            if (_operationLimits is not null) return;
+            if (Session is not Session concrete) return;
+
+            try
+            {
+                await concrete.FetchOperationLimitsAsync(ct).ConfigureAwait(false);
+                var ol = concrete.OperationLimits;
+                if (ol is null) return;
+
+                _operationLimits = new OperationLimitsCache(
+                    MaxNodesPerRead: NormalizeLimit(ol.MaxNodesPerRead),
+                    MaxNodesPerWrite: NormalizeLimit(ol.MaxNodesPerWrite),
+                    MaxNodesPerBrowse: NormalizeLimit(ol.MaxNodesPerBrowse),
+                    MaxNodesPerHistoryReadData: NormalizeLimit(ol.MaxNodesPerHistoryReadData));
+            }
+            catch
+            {
+                // Fetch failed — leave cache null so we re-attempt on the next batch op.
+                // Single-call behaviour applies in the meantime; never block traffic on a
+                // capability discovery glitch.
+            }
+        }
+        finally { _operationLimitsLock.Release(); }
+    }
+
+    /// <summary>Spec sentinel: 0 = "no limit". Normalize to null for the chunking helper.</summary>
+    private static uint? NormalizeLimit(uint raw) => raw == 0 ? null : raw;
+
+    /// <summary>
+    ///     Split <paramref name="source"/> into contiguous slices of at most <paramref name="cap"/>
+    ///     items. Returns the input as a single slice when the cap is null (no limit),
+    ///     0, or larger than the input — the spec sentinel + the no-cap path collapse onto
+    ///     the same single-call branch so the wire path stays a single SDK invocation when
+    ///     the server doesn't impose a limit.
+    /// </summary>
+    internal static IEnumerable<ArraySegment<T>> ChunkBy<T>(IReadOnlyList<T> source, uint? cap)
+    {
+        if (source.Count == 0) yield break;
+        var array = source as T[] ?? source.ToArray();
+        if (cap is null or 0 || (uint)array.Length <= cap.Value)
+        {
+            yield return new ArraySegment<T>(array, 0, array.Length);
+            yield break;
+        }
+        var size = checked((int)cap.Value);
+        for (var offset = 0; offset < array.Length; offset += size)
+        {
+            var len = Math.Min(size, array.Length - offset);
+            yield return new ArraySegment<T>(array, offset, len);
+        }
+    }
+
     // ---- ITagDiscovery ----
 
     public async Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
@@ -851,29 +977,43 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
 
     // ---- ISubscribable ----
 
-    public async Task<ISubscriptionHandle> SubscribeAsync(
+    public Task<ISubscriptionHandle> SubscribeAsync(
         IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+    {
+        // Route the simple-string overload through the per-tag overload with all knobs at
+        // their defaults. Single code path for subscription create — keeps the wire-side
+        // identical for callers that don't need per-tag tuning.
+        var specs = new MonitoredTagSpec[fullReferences.Count];
+        for (var i = 0; i < fullReferences.Count; i++)
+            specs[i] = new MonitoredTagSpec(fullReferences[i]);
+        return SubscribeAsync(specs, publishingInterval, cancellationToken);
+    }
+
+    public async Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<MonitoredTagSpec> tags, TimeSpan publishingInterval, CancellationToken cancellationToken)
     {
         var session = RequireSession();
         var id = Interlocked.Increment(ref _nextSubscriptionId);
         var handle = new OpcUaSubscriptionHandle(id);
 
-        // Floor the publishing interval at 50ms — OPC UA servers routinely negotiate
-        // minimum-supported intervals up anyway, but sending sub-50ms wastes negotiation
-        // bandwidth on every subscription create.
-        var intervalMs = publishingInterval < TimeSpan.FromMilliseconds(50)
-            ? 50
+        // Floor the publishing interval — OPC UA servers routinely negotiate
+        // minimum-supported intervals up anyway, but sending sub-floor values wastes
+        // negotiation bandwidth on every subscription create. Floor is configurable via
+        // OpcUaSubscriptionDefaults.MinPublishingIntervalMs (default 50ms).
+        var subDefaults = _options.Subscriptions;
+        var intervalMs = publishingInterval < TimeSpan.FromMilliseconds(subDefaults.MinPublishingIntervalMs)
+            ? subDefaults.MinPublishingIntervalMs
             : (int)publishingInterval.TotalMilliseconds;
 
         var subscription = new Subscription(telemetry: null!, new SubscriptionOptions
         {
             DisplayName = $"opcua-sub-{id}",
             PublishingInterval = intervalMs,
-            KeepAliveCount = 10,
-            LifetimeCount = 1000,
-            MaxNotificationsPerPublish = 0,
+            KeepAliveCount = (uint)subDefaults.KeepAliveCount,
+            LifetimeCount = subDefaults.LifetimeCount,
+            MaxNotificationsPerPublish = subDefaults.MaxNotificationsPerPublish,
             PublishingEnabled = true,
-            Priority = 0,
+            Priority = subDefaults.Priority,
             TimestampsToReturn = TimestampsToReturn.Both,
         });
 
@@ -883,29 +1023,28 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
             session.AddSubscription(subscription);
             await subscription.CreateAsync(cancellationToken).ConfigureAwait(false);
 
-            foreach (var fullRef in fullReferences)
+            foreach (var spec in tags)
             {
-                if (!TryParseNodeId(session, fullRef, out var nodeId)) continue;
-                // The tag string is routed through MonitoredItem.Handle so the Notification
-                // handler can identify which tag changed without an extra lookup.
-                var item = new MonitoredItem(telemetry: null!, new MonitoredItemOptions
-                {
-                    DisplayName = fullRef,
-                    StartNodeId = nodeId,
-                    AttributeId = Attributes.Value,
-                    MonitoringMode = MonitoringMode.Reporting,
-                    SamplingInterval = intervalMs,
-                    QueueSize = 1,
-                    DiscardOldest = true,
-                })
-                {
-                    Handle = fullRef,
-                };
-                item.Notification += (mi, args) => OnMonitoredItemNotification(handle, mi, args);
-                subscription.AddItem(item);
+                if (!TryParseNodeId(session, spec.TagName, out var nodeId)) continue;
+
+                var monItem = BuildMonitoredItem(spec, nodeId, intervalMs);
+                monItem.Notification += (mi, args) => OnMonitoredItemNotification(handle, mi, args);
+                subscription.AddItem(monItem);
             }
 
-            await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);
+            try
+            {
+                await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);
+            }
+            catch (Opc.Ua.ServiceResultException sre)
+            {
+                // PercentDeadband requires the server to expose EURange on the variable; if
+                // it isn't set the server returns BadFilterNotAllowed during item creation.
+                // We swallow the exception here so other items in the batch still get created
+                // — per-item failure surfaces through MonitoredItem.Status.Error rather than
+                // tearing down the whole subscription.
+                if (sre.StatusCode != StatusCodes.BadFilterNotAllowed) throw;
+            }
             _subscriptions[id] = new RemoteSubscription(subscription, handle);
         }
         finally { _gate.Release(); }
@@ -913,6 +1052,84 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         return handle;
     }
 
+    /// <summary>
+    ///     Map a <see cref="MonitoredTagSpec"/> to a SDK <see cref="MonitoredItem"/> with the
+    ///     per-tag knobs applied. Defaults match the original hard-coded values
+    ///     (Reporting / SamplingInterval=publishInterval / QueueSize=1 / DiscardOldest=true)
+    ///     so a spec with all knobs <c>null</c> behaves identically to the legacy path.
+    /// </summary>
+    internal static MonitoredItem BuildMonitoredItem(MonitoredTagSpec spec, NodeId nodeId, int defaultIntervalMs)
+    {
+        var sampling = spec.SamplingIntervalMs.HasValue ? (int)spec.SamplingIntervalMs.Value : defaultIntervalMs;
+        var queueSize = spec.QueueSize ?? 1u;
+        var discardOldest = spec.DiscardOldest ?? true;
+        var monitoringMode = spec.MonitoringMode is { } mm ? MapMonitoringMode(mm) : MonitoringMode.Reporting;
+        var filter = BuildDataChangeFilter(spec.DataChangeFilter);
+
+        var options = new MonitoredItemOptions
+        {
+            DisplayName = spec.TagName,
+            StartNodeId = nodeId,
+            AttributeId = Attributes.Value,
+            MonitoringMode = monitoringMode,
+            SamplingInterval = sampling,
+            QueueSize = queueSize,
+            DiscardOldest = discardOldest,
+            Filter = filter,
+        };
+
+        return new MonitoredItem(telemetry: null!, options)
+        {
+            // The tag string is routed through MonitoredItem.Handle so the Notification
+            // handler can identify which tag changed without an extra lookup.
+            Handle = spec.TagName,
+        };
+    }
+
+    /// <summary>
+    ///     Build the OPC UA <see cref="DataChangeFilter"/> from a <see cref="DataChangeFilterSpec"/>,
+    ///     or return <c>null</c> if the caller didn't supply a filter. PercentDeadband requires
+    ///     server-side EURange — if the server rejects with BadFilterNotAllowed, the caller's
+    ///     <c>SubscribeAsync</c> swallows it so other items in the batch still get created.
+    /// </summary>
+    internal static DataChangeFilter? BuildDataChangeFilter(DataChangeFilterSpec? spec)
+    {
+        if (spec is null) return null;
+        return new DataChangeFilter
+        {
+            Trigger = MapTrigger(spec.Trigger),
+            DeadbandType = (uint)MapDeadbandType(spec.DeadbandType),
+            DeadbandValue = spec.DeadbandValue,
+        };
+    }
+
+    /// <summary>Map our SDK-free <see cref="SubscriptionMonitoringMode"/> to the OPC UA SDK's enum.</summary>
+    internal static MonitoringMode MapMonitoringMode(SubscriptionMonitoringMode mode) => mode switch
+    {
+        SubscriptionMonitoringMode.Disabled => MonitoringMode.Disabled,
+        SubscriptionMonitoringMode.Sampling => MonitoringMode.Sampling,
+        SubscriptionMonitoringMode.Reporting => MonitoringMode.Reporting,
+        _ => MonitoringMode.Reporting,
+    };
+
+    /// <summary>Map our <see cref="Core.Abstractions.DataChangeTrigger"/> to the SDK enum.</summary>
+    internal static Opc.Ua.DataChangeTrigger MapTrigger(Core.Abstractions.DataChangeTrigger trigger) => trigger switch
+    {
+        Core.Abstractions.DataChangeTrigger.Status => Opc.Ua.DataChangeTrigger.Status,
+        Core.Abstractions.DataChangeTrigger.StatusValue => Opc.Ua.DataChangeTrigger.StatusValue,
+        Core.Abstractions.DataChangeTrigger.StatusValueTimestamp => Opc.Ua.DataChangeTrigger.StatusValueTimestamp,
+        _ => Opc.Ua.DataChangeTrigger.StatusValue,
+    };
+
+    /// <summary>Map our <see cref="Core.Abstractions.DeadbandType"/> to the SDK enum.</summary>
+    internal static Opc.Ua.DeadbandType MapDeadbandType(Core.Abstractions.DeadbandType type) => type switch
+    {
+        Core.Abstractions.DeadbandType.None => Opc.Ua.DeadbandType.None,
+        Core.Abstractions.DeadbandType.Absolute => Opc.Ua.DeadbandType.Absolute,
+        Core.Abstractions.DeadbandType.Percent => Opc.Ua.DeadbandType.Percent,
+        _ => Opc.Ua.DeadbandType.None,
+    };
+
     public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
         if (handle is not OpcUaSubscriptionHandle h) return;
@@ -975,15 +1192,16 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         // match in O(1) without re-parsing on every event.
         var sourceFilter = new HashSet<string>(sourceNodeIds, StringComparer.Ordinal);
 
+        var alarmDefaults = _options.Subscriptions;
         var subscription = new Subscription(telemetry: null!, new SubscriptionOptions
         {
             DisplayName = $"opcua-alarm-sub-{id}",
             PublishingInterval = 500,   // 500ms — alarms don't need fast polling; the server pushes
-            KeepAliveCount = 10,
-            LifetimeCount = 1000,
-            MaxNotificationsPerPublish = 0,
+            KeepAliveCount = (uint)alarmDefaults.KeepAliveCount,
+            LifetimeCount = alarmDefaults.LifetimeCount,
+            MaxNotificationsPerPublish = alarmDefaults.MaxNotificationsPerPublish,
             PublishingEnabled = true,
-            Priority = 0,
+            Priority = alarmDefaults.AlarmsPriority,
             TimestampsToReturn = TimestampsToReturn.Both,
         });
 
@@ -1345,6 +1563,10 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         }
 
         Session = newSession;
+        // Drop cached OperationLimits so the next batch op refetches against the (potentially
+        // re-redeployed) upstream server. A zero-cost guard against a server whose published
+        // capabilities changed across the reconnect window.
+        _operationLimits = null;
         _reconnectHandler?.Dispose();
         _reconnectHandler = null;
 
@@ -1380,5 +1602,6 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         try { await ShutdownAsync(CancellationToken.None).ConfigureAwait(false); }
         catch { /* disposal is best-effort */ }
         _gate.Dispose();
+        _operationLimitsLock.Dispose();
     }
 }

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverOptions.cs

@@ -134,8 +134,58 @@ public sealed class OpcUaClientDriverOptions
     ///     browse forever.
     /// </summary>
     public int MaxBrowseDepth { get; init; } = 10;
+
+    /// <summary>
+    ///     Per-subscription tuning knobs applied when the driver creates data + alarm
+    ///     subscriptions on the upstream session. Defaults preserve the previous hard-coded
+    ///     values so existing deployments see no behaviour change.
+    /// </summary>
+    public OpcUaSubscriptionDefaults Subscriptions { get; init; } = new();
 }
 
+/// <summary>
+///     Tuning surface for OPC UA subscriptions created by <see cref="OpcUaClientDriver"/>.
+///     Lifted from the per-call hard-coded literals so operators can tune publish cadence,
+///     keep-alive ratio, and alarm-vs-data prioritisation without recompiling the driver.
+///     Defaults match the original hard-coded values (KeepAlive=10, Lifetime=1000,
+///     MaxNotifications=0 unlimited, Priority=0, MinPublishingInterval=50ms).
+/// </summary>
+/// <param name="KeepAliveCount">
+///     Number of consecutive empty publish cycles before the server sends a keep-alive
+///     response. Default 10 — high enough to suppress idle traffic, low enough that the
+///     client notices a stalled subscription within ~5x the publish interval.
+/// </param>
+/// <param name="LifetimeCount">
+///     Number of consecutive missed publish responses before the server tears down the
+///     subscription. Must be ≥3×<see cref="KeepAliveCount"/> per OPC UA spec; default 1000
+///     gives ~100 keep-alives of slack which is conservative on flaky networks.
+/// </param>
+/// <param name="MaxNotificationsPerPublish">
+///     Cap on notifications returned per publish response. <c>0</c> = unlimited (the OPC UA
+///     spec sentinel). Lower this to bound publish-message size on bursty servers.
+/// </param>
+/// <param name="Priority">
+///     Subscription priority for data subscriptions (0..255). Higher = scheduled ahead of
+///     lower. Default 0 matches the SDK's default for ordinary tag subscriptions.
+/// </param>
+/// <param name="MinPublishingIntervalMs">
+///     Floor (ms) applied to <c>publishingInterval</c> requests. Sub-floor values are
+///     clamped up so wire-side negotiations don't waste round-trips on intervals the server
+///     will only round up anyway. Default 50ms.
+/// </param>
+/// <param name="AlarmsPriority">
+///     Subscription priority for the alarm subscription (0..255). Higher than
+///     <see cref="Priority"/> by default (1 vs 0) so alarm publishes aren't starved during
+///     data-tag bursts.
+/// </param>
+public sealed record OpcUaSubscriptionDefaults(
+    int KeepAliveCount = 10,
+    uint LifetimeCount = 1000,
+    uint MaxNotificationsPerPublish = 0,
+    byte Priority = 0,
+    int MinPublishingIntervalMs = 50,
+    byte AlarmsPriority = 1);
+
 /// <summary>OPC UA message security mode.</summary>
 public enum OpcUaSecurityMode
 {

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipArrayReadPlannerTests.cs

@@ -0,0 +1,110 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class AbCipArrayReadPlannerTests
+{
+    private const string Device = "ab://10.0.0.5/1,0";
+
+    private static AbCipTagCreateParams BaseParams(string tagName) => new(
+        Gateway: "10.0.0.5",
+        Port: 44818,
+        CipPath: "1,0",
+        LibplctagPlcAttribute: "controllogix",
+        TagName: tagName,
+        Timeout: TimeSpan.FromSeconds(5));
+
+    [Fact]
+    public void TryBuild_emits_single_tag_create_with_element_count()
+    {
+        var def = new AbCipTagDefinition("DataSlice", Device, "Data[0..15]", AbCipDataType.DInt);
+        var parsed = AbCipTagPath.TryParse(def.TagPath)!;
+
+        var plan = AbCipArrayReadPlanner.TryBuild(def, parsed, BaseParams("Data[0..15]"));
+
+        plan.ShouldNotBeNull();
+        plan.ElementType.ShouldBe(AbCipDataType.DInt);
+        plan.Stride.ShouldBe(4);
+        plan.Slice.Count.ShouldBe(16);
+        plan.CreateParams.ElementCount.ShouldBe(16);
+        // Anchored at the slice start; libplctag reads N consecutive elements from there.
+        plan.CreateParams.TagName.ShouldBe("Data[0]");
+    }
+
+    [Fact]
+    public void TryBuild_returns_null_when_path_has_no_slice()
+    {
+        var def = new AbCipTagDefinition("Plain", Device, "Data[3]", AbCipDataType.DInt);
+        var parsed = AbCipTagPath.TryParse(def.TagPath)!;
+
+        AbCipArrayReadPlanner.TryBuild(def, parsed, BaseParams("Data[3]")).ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData(AbCipDataType.Bool)]
+    [InlineData(AbCipDataType.String)]
+    [InlineData(AbCipDataType.Structure)]
+    public void TryBuild_returns_null_for_unsupported_element_types(AbCipDataType type)
+    {
+        var def = new AbCipTagDefinition("Slice", Device, "Data[0..3]", type);
+        var parsed = AbCipTagPath.TryParse(def.TagPath)!;
+
+        AbCipArrayReadPlanner.TryBuild(def, parsed, BaseParams("Data[0..3]")).ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData(AbCipDataType.SInt, 1)]
+    [InlineData(AbCipDataType.Int, 2)]
+    [InlineData(AbCipDataType.DInt, 4)]
+    [InlineData(AbCipDataType.Real, 4)]
+    [InlineData(AbCipDataType.LInt, 8)]
+    [InlineData(AbCipDataType.LReal, 8)]
+    public void TryBuild_uses_natural_stride_per_element_type(AbCipDataType type, int expectedStride)
+    {
+        var def = new AbCipTagDefinition("Slice", Device, "Data[0..3]", type);
+        var parsed = AbCipTagPath.TryParse(def.TagPath)!;
+
+        var plan = AbCipArrayReadPlanner.TryBuild(def, parsed, BaseParams("Data[0..3]"))!;
+        plan.Stride.ShouldBe(expectedStride);
+    }
+
+    [Fact]
+    public void Decode_walks_buffer_at_element_stride()
+    {
+        var def = new AbCipTagDefinition("DataSlice", Device, "Data[0..3]", AbCipDataType.DInt);
+        var parsed = AbCipTagPath.TryParse(def.TagPath)!;
+        var plan = AbCipArrayReadPlanner.TryBuild(def, parsed, BaseParams("Data[0..3]"))!;
+
+        var fake = new FakeAbCipTag(plan.CreateParams);
+        // Stride == 4 for DInt, so offsets 0/4/8/12 hold the four element values.
+        fake.ValuesByOffset[0] = 100;
+        fake.ValuesByOffset[4] = 200;
+        fake.ValuesByOffset[8] = 300;
+        fake.ValuesByOffset[12] = 400;
+
+        var decoded = AbCipArrayReadPlanner.Decode(plan, fake);
+
+        decoded.Length.ShouldBe(4);
+        decoded.ShouldBe(new object?[] { 100, 200, 300, 400 });
+    }
+
+    [Fact]
+    public void Decode_preserves_slice_count_for_real_arrays()
+    {
+        var def = new AbCipTagDefinition("FloatSlice", Device, "Floats[2..5]", AbCipDataType.Real);
+        var parsed = AbCipTagPath.TryParse(def.TagPath)!;
+        var plan = AbCipArrayReadPlanner.TryBuild(def, parsed, BaseParams("Floats[2]"))!;
+
+        var fake = new FakeAbCipTag(plan.CreateParams);
+        fake.ValuesByOffset[0] = 1.5f;
+        fake.ValuesByOffset[4] = 2.5f;
+        fake.ValuesByOffset[8] = 3.5f;
+        fake.ValuesByOffset[12] = 4.5f;
+
+        var decoded = AbCipArrayReadPlanner.Decode(plan, fake);
+
+        decoded.ShouldBe(new object?[] { 1.5f, 2.5f, 3.5f, 4.5f });
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipDriverReadTests.cs

@@ -165,6 +165,55 @@ public sealed class AbCipDriverReadTests
         p.TagName.ShouldBe("Program:P.Counter");
     }
 
+    [Fact]
+    public async Task Slice_tag_reads_one_array_and_decodes_n_elements()
+    {
+        // PR abcip-1.3 — `Data[0..3]` slice routes through AbCipArrayReadPlanner: one libplctag
+        // tag-create at TagName="Data[0]" with ElementCount=4, single PLC read, contiguous
+        // buffer decoded at element stride into one snapshot whose Value is an object?[].
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("DataSlice", "ab://10.0.0.5/1,0", "Data[0..3]", AbCipDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p =>
+        {
+            var t = new FakeAbCipTag(p);
+            t.ValuesByOffset[0] = 10;
+            t.ValuesByOffset[4] = 20;
+            t.ValuesByOffset[8] = 30;
+            t.ValuesByOffset[12] = 40;
+            return t;
+        };
+
+        var snapshots = await drv.ReadAsync(["DataSlice"], CancellationToken.None);
+
+        snapshots.Single().StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        var values = snapshots.Single().Value.ShouldBeOfType<object?[]>();
+        values.ShouldBe(new object?[] { 10, 20, 30, 40 });
+
+        // Exactly ONE libplctag tag was created — anchored at the slice start with
+        // ElementCount=4. Without the planner this would have been four scalar reads.
+        factory.Tags.Count.ShouldBe(1);
+        factory.Tags.ShouldContainKey("Data[0]");
+        factory.Tags["Data[0]"].CreationParams.ElementCount.ShouldBe(4);
+        factory.Tags["Data[0]"].ReadCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Slice_tag_with_unsupported_element_type_returns_BadNotSupported()
+    {
+        // BOOL slices can't be laid out from the declaration alone (Logix packs BOOLs into a
+        // hidden host byte). The planner refuses; the driver surfaces BadNotSupported instead
+        // of attempting a best-effort decode.
+        var (drv, _) = NewDriver(
+            new AbCipTagDefinition("BoolSlice", "ab://10.0.0.5/1,0", "Flags[0..7]", AbCipDataType.Bool));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snapshots = await drv.ReadAsync(["BoolSlice"], CancellationToken.None);
+
+        snapshots.Single().StatusCode.ShouldBe(AbCipStatusMapper.BadNotSupported);
+        snapshots.Single().Value.ShouldBeNull();
+    }
+
     [Fact]
     public async Task Cancellation_propagates_from_read()
     {
@@ -211,4 +260,79 @@ public sealed class AbCipDriverReadTests
         snapshots.Single().StatusCode.ShouldBe(AbCipStatusMapper.BadCommunicationError);
         factory.Tags["Nope"].Disposed.ShouldBeTrue();
     }
+
+    // PR abcip-1.2 — STRINGnn variant decoding. Threading <see cref="AbCipTagDefinition.StringLength"/>
+    // through libplctag's StringMaxCapacity attribute lets STRING_20 / STRING_40 / STRING_80 UDTs
+    // decode against the right DATA-array size; null preserves the default 82-byte STRING.
+
+    [Fact]
+    public async Task StringLength_threads_into_TagCreateParams_StringMaxCapacity()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Banner", "ab://10.0.0.5/1,0", "Banner", AbCipDataType.String,
+                StringLength: 40));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = "hello" };
+
+        await drv.ReadAsync(["Banner"], CancellationToken.None);
+
+        factory.Tags["Banner"].CreationParams.StringMaxCapacity.ShouldBe(40);
+    }
+
+    [Fact]
+    public async Task StringLength_null_leaves_StringMaxCapacity_null_for_back_compat()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("LegacyStr", "ab://10.0.0.5/1,0", "LegacyStr", AbCipDataType.String));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = "world" };
+
+        await drv.ReadAsync(["LegacyStr"], CancellationToken.None);
+
+        factory.Tags["LegacyStr"].CreationParams.StringMaxCapacity.ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task StringLength_ignored_for_non_String_data_types()
+    {
+        // StringLength on a DINT-typed tag must not flow into StringMaxCapacity — libplctag would
+        // otherwise re-shape the buffer and corrupt the read. EnsureTagRuntimeAsync gates on the
+        // declared DataType.
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt,
+                StringLength: 80));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = 7 };
+
+        await drv.ReadAsync(["Speed"], CancellationToken.None);
+
+        factory.Tags["Speed"].CreationParams.StringMaxCapacity.ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task UDT_member_StringLength_threads_through_to_member_runtime()
+    {
+        // STRINGnn members of a UDT — declaration-driven fan-out copies StringLength from
+        // AbCipStructureMember onto the synthesised member AbCipTagDefinition; the per-member
+        // runtime then receives the right StringMaxCapacity.
+        var udt = new AbCipTagDefinition(
+            Name: "Recipe",
+            DeviceHostAddress: "ab://10.0.0.5/1,0",
+            TagPath: "Recipe",
+            DataType: AbCipDataType.Structure,
+            Members: [
+                new AbCipStructureMember("Name", AbCipDataType.String, StringLength: 20),
+                new AbCipStructureMember("Description", AbCipDataType.String, StringLength: 80),
+                new AbCipStructureMember("Code", AbCipDataType.DInt),
+            ]);
+        var (drv, factory) = NewDriver(udt);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = "x" };
+
+        await drv.ReadAsync(["Recipe.Name", "Recipe.Description", "Recipe.Code"], CancellationToken.None);
+
+        factory.Tags["Recipe.Name"].CreationParams.StringMaxCapacity.ShouldBe(20);
+        factory.Tags["Recipe.Description"].CreationParams.StringMaxCapacity.ShouldBe(80);
+        factory.Tags["Recipe.Code"].CreationParams.StringMaxCapacity.ShouldBeNull();
+    }
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipDriverTests.cs

@@ -124,8 +124,11 @@ public sealed class AbCipDriverTests
     {
         AbCipDataType.Bool.ToDriverDataType().ShouldBe(DriverDataType.Boolean);
         AbCipDataType.DInt.ToDriverDataType().ShouldBe(DriverDataType.Int32);
+        AbCipDataType.LInt.ToDriverDataType().ShouldBe(DriverDataType.Int64);
+        AbCipDataType.ULInt.ToDriverDataType().ShouldBe(DriverDataType.UInt64);
         AbCipDataType.Real.ToDriverDataType().ShouldBe(DriverDataType.Float32);
         AbCipDataType.LReal.ToDriverDataType().ShouldBe(DriverDataType.Float64);
         AbCipDataType.String.ToDriverDataType().ShouldBe(DriverDataType.String);
+        AbCipDataType.Dt.ToDriverDataType().ShouldBe(DriverDataType.Int64);
     }
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipMultiWritePackingTests.cs

@@ -0,0 +1,283 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+/// <summary>
+///     PR abcip-1.4 — multi-tag write packing. Validates that <see cref="AbCipDriver.WriteAsync"/>
+///     groups writes by device, dispatches packable writes for request-packing-capable
+///     families concurrently, falls back to sequential writes on Micro800, keeps BOOL-RMW
+///     writes on the per-parent semaphore path, and fans per-tag StatusCodes out to the
+///     correct positions on partial failures.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbCipMultiWritePackingTests
+{
+    [Fact]
+    public async Task Writes_get_grouped_by_device()
+    {
+        var factory = new FakeAbCipTagFactory();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices =
+            [
+                new AbCipDeviceOptions("ab://10.0.0.5/1,0"),
+                new AbCipDeviceOptions("ab://10.0.0.6/1,0"),
+            ],
+            Tags =
+            [
+                new AbCipTagDefinition("A1", "ab://10.0.0.5/1,0", "A1", AbCipDataType.DInt),
+                new AbCipTagDefinition("A2", "ab://10.0.0.5/1,0", "A2", AbCipDataType.DInt),
+                new AbCipTagDefinition("B1", "ab://10.0.0.6/1,0", "B1", AbCipDataType.DInt),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+        [
+            new WriteRequest("A1", 1),
+            new WriteRequest("B1", 100),
+            new WriteRequest("A2", 2),
+        ], CancellationToken.None);
+
+        results.Count.ShouldBe(3);
+        results[0].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        results[1].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        results[2].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        // Per-device handles materialised — A1/A2 share device A, B1 lives on device B.
+        factory.Tags["A1"].CreationParams.Gateway.ShouldBe("10.0.0.5");
+        factory.Tags["A2"].CreationParams.Gateway.ShouldBe("10.0.0.5");
+        factory.Tags["B1"].CreationParams.Gateway.ShouldBe("10.0.0.6");
+        factory.Tags["A1"].WriteCount.ShouldBe(1);
+        factory.Tags["A2"].WriteCount.ShouldBe(1);
+        factory.Tags["B1"].WriteCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task ControlLogix_packs_concurrently_within_a_device()
+    {
+        // ControlLogix has SupportsRequestPacking=true → a multi-write batch is dispatched in
+        // parallel. The fake's WriteAsync gates on a TaskCompletionSource so we can prove that
+        // both writes are in flight at the same time before either completes.
+        var gate = new TaskCompletionSource<int>(TaskCreationOptions.RunContinuationsAsynchronously);
+        var inFlight = 0;
+        var maxInFlight = 0;
+        var factory = new FakeAbCipTagFactory
+        {
+            Customise = p => new GatedWriteFake(p, gate, () =>
+            {
+                var current = Interlocked.Increment(ref inFlight);
+                var observed = maxInFlight;
+                while (current > observed
+                    && Interlocked.CompareExchange(ref maxInFlight, current, observed) != observed)
+                    observed = maxInFlight;
+            }, () => Interlocked.Decrement(ref inFlight)),
+        };
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0", AbCipPlcFamily.ControlLogix)],
+            Tags =
+            [
+                new AbCipTagDefinition("A", "ab://10.0.0.5/1,0", "A", AbCipDataType.DInt),
+                new AbCipTagDefinition("B", "ab://10.0.0.5/1,0", "B", AbCipDataType.DInt),
+                new AbCipTagDefinition("C", "ab://10.0.0.5/1,0", "C", AbCipDataType.DInt),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var writeTask = drv.WriteAsync(
+        [
+            new WriteRequest("A", 1),
+            new WriteRequest("B", 2),
+            new WriteRequest("C", 3),
+        ], CancellationToken.None);
+
+        // Wait until all three writes have entered WriteAsync simultaneously, then release.
+        await WaitForAsync(() => Volatile.Read(ref inFlight) >= 3, TimeSpan.FromSeconds(2));
+        gate.SetResult(0);
+
+        var results = await writeTask;
+        results.Count.ShouldBe(3);
+        results[0].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        results[1].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        results[2].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        maxInFlight.ShouldBeGreaterThanOrEqualTo(2,
+            "ControlLogix supports request packing — packable writes should run concurrently within the device.");
+    }
+
+    [Fact]
+    public async Task Micro800_falls_back_to_sequential_writes()
+    {
+        // Micro800 has SupportsRequestPacking=false → writes go one-at-a-time; the gated fake
+        // never sees more than one in-flight at a time.
+        var gate = new TaskCompletionSource<int>(TaskCreationOptions.RunContinuationsAsynchronously);
+        gate.SetResult(0); // No need to gate — we just observe concurrency.
+        var inFlight = 0;
+        var maxInFlight = 0;
+        var factory = new FakeAbCipTagFactory
+        {
+            Customise = p => new GatedWriteFake(p, gate, () =>
+            {
+                var current = Interlocked.Increment(ref inFlight);
+                var observed = maxInFlight;
+                while (current > observed
+                    && Interlocked.CompareExchange(ref maxInFlight, current, observed) != observed)
+                    observed = maxInFlight;
+            }, () => Interlocked.Decrement(ref inFlight)),
+        };
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/", AbCipPlcFamily.Micro800)],
+            Tags =
+            [
+                new AbCipTagDefinition("A", "ab://10.0.0.5/", "A", AbCipDataType.DInt),
+                new AbCipTagDefinition("B", "ab://10.0.0.5/", "B", AbCipDataType.DInt),
+                new AbCipTagDefinition("C", "ab://10.0.0.5/", "C", AbCipDataType.DInt),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+        [
+            new WriteRequest("A", 1),
+            new WriteRequest("B", 2),
+            new WriteRequest("C", 3),
+        ], CancellationToken.None);
+
+        results.Count.ShouldBe(3);
+        results.ShouldAllBe(r => r.StatusCode == AbCipStatusMapper.Good);
+        maxInFlight.ShouldBe(1,
+            "Micro800 disables request packing — writes must execute sequentially.");
+    }
+
+    [Fact]
+    public async Task Bit_in_dint_writes_still_route_through_RMW_path()
+    {
+        // BOOL-with-bitIndex must hit the per-parent RMW semaphore — it must NOT go through
+        // the packable per-tag runtime path. We prove this by checking that:
+        //   (a) the per-tag "bit-selector" runtime is never created (it would throw via
+        //       LibplctagTagRuntime's NotSupportedException had the bypass happened);
+        //   (b) the parent-DINT runtime got both a Read and a Write.
+        var factory = new FakeAbCipTagFactory();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition("Flag3", "ab://10.0.0.5/1,0", "Flags.3", AbCipDataType.Bool),
+                new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+        [
+            new WriteRequest("Flag3", true),
+            new WriteRequest("Speed", 99),
+        ], CancellationToken.None);
+
+        results.Count.ShouldBe(2);
+        results[0].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        results[1].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+
+        // Parent runtime created lazily for Flags (no .3 suffix) — drove the RMW.
+        factory.Tags.ShouldContainKey("Flags");
+        factory.Tags["Flags"].ReadCount.ShouldBe(1);
+        factory.Tags["Flags"].WriteCount.ShouldBe(1);
+        // Speed went through the packable path.
+        factory.Tags["Speed"].WriteCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Per_tag_status_code_fan_out_works_on_partial_failure()
+    {
+        // Mix Good + BadTimeout + BadNotWritable + BadNodeIdUnknown across two devices to
+        // exercise the original-index preservation through the per-device plan + concurrent
+        // dispatch.
+        var factory = new FakeAbCipTagFactory
+        {
+            Customise = p => p.TagName == "B"
+                ? new FakeAbCipTag(p) { Status = -5 /* timeout */ }
+                : new FakeAbCipTag(p),
+        };
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices =
+            [
+                new AbCipDeviceOptions("ab://10.0.0.5/1,0"),
+                new AbCipDeviceOptions("ab://10.0.0.6/1,0"),
+            ],
+            Tags =
+            [
+                new AbCipTagDefinition("A", "ab://10.0.0.5/1,0", "A", AbCipDataType.DInt),
+                new AbCipTagDefinition("B", "ab://10.0.0.5/1,0", "B", AbCipDataType.DInt),
+                new AbCipTagDefinition("RO", "ab://10.0.0.5/1,0", "RO", AbCipDataType.DInt, Writable: false),
+                new AbCipTagDefinition("C", "ab://10.0.0.6/1,0", "C", AbCipDataType.DInt),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+        [
+            new WriteRequest("A", 1),
+            new WriteRequest("B", 2),
+            new WriteRequest("RO", 3),
+            new WriteRequest("UnknownTag", 4),
+            new WriteRequest("C", 5),
+        ], CancellationToken.None);
+
+        results.Count.ShouldBe(5);
+        results[0].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        results[1].StatusCode.ShouldBe(AbCipStatusMapper.BadTimeout);
+        results[2].StatusCode.ShouldBe(AbCipStatusMapper.BadNotWritable);
+        results[3].StatusCode.ShouldBe(AbCipStatusMapper.BadNodeIdUnknown);
+        results[4].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+    }
+
+    private static async Task WaitForAsync(Func<bool> predicate, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!predicate())
+        {
+            if (DateTime.UtcNow >= deadline)
+                throw new TimeoutException("predicate did not become true within timeout");
+            await Task.Delay(10).ConfigureAwait(false);
+        }
+    }
+
+    /// <summary>
+    ///     Test fake whose <see cref="WriteAsync"/> blocks on a shared
+    ///     <see cref="TaskCompletionSource"/> so the test can observe how many writes are
+    ///     simultaneously in flight inside the driver.
+    /// </summary>
+    private sealed class GatedWriteFake : FakeAbCipTag
+    {
+        private readonly TaskCompletionSource<int> _gate;
+        private readonly Action _onEnter;
+        private readonly Action _onExit;
+
+        public GatedWriteFake(AbCipTagCreateParams p, TaskCompletionSource<int> gate,
+            Action onEnter, Action onExit) : base(p)
+        {
+            _gate = gate;
+            _onEnter = onEnter;
+            _onExit = onExit;
+        }
+
+        public override async Task WriteAsync(CancellationToken ct)
+        {
+            _onEnter();
+            try
+            {
+                await _gate.Task.ConfigureAwait(false);
+                await base.WriteAsync(ct).ConfigureAwait(false);
+            }
+            finally
+            {
+                _onExit();
+            }
+        }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipTagPathTests.cs

@@ -123,6 +123,61 @@ public sealed class AbCipTagPathTests
         AbCipTagPath.TryParse("_private_tag")!.Segments.Single().Name.ShouldBe("_private_tag");
     }
 
+    [Fact]
+    public void Slice_basic_inclusive_range()
+    {
+        var p = AbCipTagPath.TryParse("Data[0..15]");
+        p.ShouldNotBeNull();
+        p.Slice.ShouldNotBeNull();
+        p.Slice!.Start.ShouldBe(0);
+        p.Slice.End.ShouldBe(15);
+        p.Slice.Count.ShouldBe(16);
+        p.BitIndex.ShouldBeNull();
+        p.Segments.Single().Name.ShouldBe("Data");
+        p.Segments.Single().Subscripts.ShouldBeEmpty();
+        p.ToLibplctagName().ShouldBe("Data[0..15]");
+        // Slice array name omits the `..End` so libplctag sees an anchored read at the start
+        // index; pair with ElementCount to cover the whole range.
+        p.ToLibplctagSliceArrayName().ShouldBe("Data[0]");
+    }
+
+    [Fact]
+    public void Slice_with_program_scope_and_member_chain()
+    {
+        var p = AbCipTagPath.TryParse("Program:MainProgram.Motors.Data[3..7]");
+        p.ShouldNotBeNull();
+        p.ProgramScope.ShouldBe("MainProgram");
+        p.Segments.Select(s => s.Name).ShouldBe(["Motors", "Data"]);
+        p.Slice!.Start.ShouldBe(3);
+        p.Slice.End.ShouldBe(7);
+        p.ToLibplctagName().ShouldBe("Program:MainProgram.Motors.Data[3..7]");
+        p.ToLibplctagSliceArrayName().ShouldBe("Program:MainProgram.Motors.Data[3]");
+    }
+
+    [Fact]
+    public void Slice_zero_length_single_element_allowed()
+    {
+        // [5..5] is a one-element slice — degenerate but legal (a single read of one element).
+        var p = AbCipTagPath.TryParse("Data[5..5]");
+        p.ShouldNotBeNull();
+        p.Slice!.Count.ShouldBe(1);
+    }
+
+    [Theory]
+    [InlineData("Data[5..3]")]              // M < N
+    [InlineData("Data[-1..5]")]             // negative start
+    [InlineData("Data[0..15].Member")]      // slice + sub-element
+    [InlineData("Data[0..15].3")]           // slice + bit index
+    [InlineData("Data[0..15,1]")]           // slice cannot be multi-dim
+    [InlineData("Data[0..15,2..3]")]        // multi-dim slice not supported
+    [InlineData("Data[..5]")]               // missing start
+    [InlineData("Data[5..]")]               // missing end
+    [InlineData("Data[a..5]")]              // non-numeric start
+    public void Invalid_slice_shapes_return_null(string input)
+    {
+        AbCipTagPath.TryParse(input).ShouldBeNull();
+    }
+
     [Fact]
     public void ToLibplctagName_recomposes_round_trip()
     {

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs

@@ -1,6 +1,7 @@
 using Shouldly;
 using Xunit;
 using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.PlcFamilies;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
 
@@ -65,4 +66,271 @@ public sealed class AbLegacyAddressTests
         a.ShouldNotBeNull();
         a.ToLibplctagName().ShouldBe(input);
     }
+
+    // ---- PLC-5 octal I:/O: addressing (Issue #244) ----
+    //
+    // RSLogix 5 displays I:/O: word + bit indices as octal. `I:001/17` means rack 1, bit 15
+    // (octal 17). Other PCCC families (SLC500, MicroLogix, LogixPccc) keep decimal indices.
+    // Non-I/O file letters are always decimal regardless of family.
+
+    [Theory]
+    [InlineData("I:001/17", 1, 15)]   // octal 17 → bit 15
+    [InlineData("I:0/0", 0, 0)]       // boundary: octal 0
+    [InlineData("O:1/2", 1, 2)]       // octal 1, 2 happen to match decimal
+    [InlineData("I:010/10", 8, 8)]    // octal 10 → 8 (both word + bit)
+    [InlineData("I:007/7", 7, 7)]     // boundary: largest single octal digit
+    public void TryParse_Plc5_parses_io_indices_as_octal(string input, int expectedWord, int expectedBit)
+    {
+        var a = AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Plc5);
+        a.ShouldNotBeNull();
+        a.WordNumber.ShouldBe(expectedWord);
+        a.BitIndex.ShouldBe(expectedBit);
+    }
+
+    [Theory]
+    [InlineData("I:8/0")]    // word digit 8 illegal in octal
+    [InlineData("I:0/9")]    // bit digit 9 illegal in octal
+    [InlineData("O:128/0")]  // contains digit 8
+    [InlineData("I:0/18")]   // bit field octal-illegal because of '8'
+    public void TryParse_Plc5_rejects_octal_invalid_io_digits(string input)
+    {
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Plc5).ShouldBeNull();
+    }
+
+    [Theory]
+    // Non-I/O files stay decimal even on PLC-5 (e.g. N7:8 is integer 7, word 8).
+    [InlineData("N7:8", 7, 8)]
+    [InlineData("F8:9", 8, 9)]
+    public void TryParse_Plc5_keeps_non_io_indices_decimal(string input, int? expectedFile, int expectedWord)
+    {
+        var a = AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Plc5);
+        a.ShouldNotBeNull();
+        a.FileNumber.ShouldBe(expectedFile);
+        a.WordNumber.ShouldBe(expectedWord);
+    }
+
+    [Fact]
+    public void TryParse_Slc500_keeps_io_indices_decimal_back_compat()
+    {
+        // SLC500 has OctalIoAddressing=false — the digits are decimal as before.
+        var a = AbLegacyAddress.TryParse("I:10/15", AbLegacyPlcFamily.Slc500);
+        a.ShouldNotBeNull();
+        a.WordNumber.ShouldBe(10);
+        a.BitIndex.ShouldBe(15);
+
+        // Decimal '8' that PLC-5 would reject is fine on SLC500.
+        var b = AbLegacyAddress.TryParse("I:8/0", AbLegacyPlcFamily.Slc500);
+        b.ShouldNotBeNull();
+        b.WordNumber.ShouldBe(8);
+    }
+
+    [Fact]
+    public void TryParse_MicroLogix_and_LogixPccc_keep_io_indices_decimal()
+    {
+        AbLegacyAddress.TryParse("I:9/0", AbLegacyPlcFamily.MicroLogix).ShouldNotBeNull();
+        AbLegacyAddress.TryParse("I:9/0", AbLegacyPlcFamily.LogixPccc).ShouldNotBeNull();
+    }
+
+    [Fact]
+    public void Plc5Profile_advertises_octal_io_addressing()
+    {
+        AbLegacyPlcFamilyProfile.Plc5.OctalIoAddressing.ShouldBeTrue();
+        AbLegacyPlcFamilyProfile.Slc500.OctalIoAddressing.ShouldBeFalse();
+        AbLegacyPlcFamilyProfile.MicroLogix.OctalIoAddressing.ShouldBeFalse();
+        AbLegacyPlcFamilyProfile.LogixPccc.OctalIoAddressing.ShouldBeFalse();
+    }
+
+    // ---- MicroLogix function-file letters (Issue #245) ----
+    //
+    // MicroLogix 1100/1400 expose RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI function files. Other
+    // PCCC families (SLC500 / PLC-5 / LogixPccc) reject those file letters.
+
+    [Theory]
+    [InlineData("RTC:0.HR", "RTC", "HR")]
+    [InlineData("RTC:0.MIN", "RTC", "MIN")]
+    [InlineData("RTC:0.YR", "RTC", "YR")]
+    [InlineData("HSC:0.ACC", "HSC", "ACC")]
+    [InlineData("HSC:0.PRE", "HSC", "PRE")]
+    [InlineData("HSC:0.EN", "HSC", "EN")]
+    [InlineData("DLS:0.STR", "DLS", "STR")]
+    [InlineData("PTO:0.OF", "PTO", "OF")]
+    [InlineData("PWM:0.EN", "PWM", "EN")]
+    [InlineData("STI:0.SPM", "STI", "SPM")]
+    [InlineData("EII:0.PFN", "EII", "PFN")]
+    [InlineData("MMI:0.FT", "MMI", "FT")]
+    [InlineData("BHI:0.OS", "BHI", "OS")]
+    [InlineData("IOS:0.ID", "IOS", "ID")]
+    public void TryParse_MicroLogix_accepts_function_files(string input, string expectedLetter, string expectedSub)
+    {
+        var a = AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.MicroLogix);
+        a.ShouldNotBeNull();
+        a.FileLetter.ShouldBe(expectedLetter);
+        a.SubElement.ShouldBe(expectedSub);
+    }
+
+    [Theory]
+    [InlineData("RTC:0.HR")]
+    [InlineData("HSC:0.ACC")]
+    [InlineData("PTO:0.OF")]
+    [InlineData("BHI:0.OS")]
+    public void TryParse_Slc500_rejects_function_files(string input)
+    {
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Slc500).ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData("RTC:0.HR")]
+    [InlineData("HSC:0.ACC")]
+    public void TryParse_Plc5_and_LogixPccc_reject_function_files(string input)
+    {
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Plc5).ShouldBeNull();
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.LogixPccc).ShouldBeNull();
+    }
+
+    [Fact]
+    public void TryParse_Default_overload_rejects_function_files()
+    {
+        // Without a family the parser cannot allow MicroLogix-only letters — back-compat with
+        // the family-less overload from before #244.
+        AbLegacyAddress.TryParse("RTC:0.HR").ShouldBeNull();
+        AbLegacyAddress.TryParse("HSC:0.ACC").ShouldBeNull();
+    }
+
+    [Fact]
+    public void MicroLogixProfile_advertises_function_file_support()
+    {
+        AbLegacyPlcFamilyProfile.MicroLogix.SupportsFunctionFiles.ShouldBeTrue();
+        AbLegacyPlcFamilyProfile.Slc500.SupportsFunctionFiles.ShouldBeFalse();
+        AbLegacyPlcFamilyProfile.Plc5.SupportsFunctionFiles.ShouldBeFalse();
+        AbLegacyPlcFamilyProfile.LogixPccc.SupportsFunctionFiles.ShouldBeFalse();
+    }
+
+    // ---- Indirect / indexed addressing (Issue #247) ----
+    //
+    // PLC-5 / SLC permit `N7:[N7:0]` (word number sourced from another address) and
+    // `N[N7:0]:5` (file number sourced from another address). Recursion is capped at 1 — the
+    // inner address must itself be a plain direct PCCC reference.
+
+    [Fact]
+    public void TryParse_accepts_indirect_word_source()
+    {
+        var a = AbLegacyAddress.TryParse("N7:[N7:0]");
+        a.ShouldNotBeNull();
+        a.FileLetter.ShouldBe("N");
+        a.FileNumber.ShouldBe(7);
+        a.IndirectFileSource.ShouldBeNull();
+        a.IndirectWordSource.ShouldNotBeNull();
+        a.IndirectWordSource!.FileLetter.ShouldBe("N");
+        a.IndirectWordSource.FileNumber.ShouldBe(7);
+        a.IndirectWordSource.WordNumber.ShouldBe(0);
+        a.IsIndirect.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void TryParse_accepts_indirect_file_source()
+    {
+        var a = AbLegacyAddress.TryParse("N[N7:0]:5");
+        a.ShouldNotBeNull();
+        a.FileLetter.ShouldBe("N");
+        a.FileNumber.ShouldBeNull();
+        a.WordNumber.ShouldBe(5);
+        a.IndirectFileSource.ShouldNotBeNull();
+        a.IndirectFileSource!.FileLetter.ShouldBe("N");
+        a.IndirectFileSource.FileNumber.ShouldBe(7);
+        a.IndirectFileSource.WordNumber.ShouldBe(0);
+        a.IndirectWordSource.ShouldBeNull();
+        a.IsIndirect.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void TryParse_accepts_both_indirect_file_and_word()
+    {
+        var a = AbLegacyAddress.TryParse("N[N7:0]:[N7:1]");
+        a.ShouldNotBeNull();
+        a.IndirectFileSource.ShouldNotBeNull();
+        a.IndirectWordSource.ShouldNotBeNull();
+        a.IndirectWordSource!.WordNumber.ShouldBe(1);
+    }
+
+    [Theory]
+    [InlineData("N[N[N7:0]:0]:5")]   // depth-2 file source
+    [InlineData("N7:[N[N7:0]:0]")]   // depth-2 word source
+    [InlineData("N7:[N7:[N7:0]]")]   // depth-2 word source (nested word)
+    public void TryParse_rejects_depth_greater_than_one(string input)
+    {
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData("N7:[")]              // unbalanced bracket
+    [InlineData("N7:]")]              // unbalanced bracket
+    [InlineData("N[:5")]              // empty inner file source
+    [InlineData("N7:[]")]             // empty inner word source
+    [InlineData("N[X9:0]:5")]         // unknown file letter inside
+    public void TryParse_rejects_malformed_indirect(string input)
+    {
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+    }
+
+    [Fact]
+    public void ToLibplctagName_reemits_indirect_word_source()
+    {
+        var a = AbLegacyAddress.TryParse("N7:[N7:0]");
+        a.ShouldNotBeNull();
+        a.ToLibplctagName().ShouldBe("N7:[N7:0]");
+    }
+
+    [Fact]
+    public void ToLibplctagName_reemits_indirect_file_source()
+    {
+        var a = AbLegacyAddress.TryParse("N[N7:0]:5");
+        a.ShouldNotBeNull();
+        a.ToLibplctagName().ShouldBe("N[N7:0]:5");
+    }
+
+    [Fact]
+    public void TryParse_indirect_with_bit_outside_brackets()
+    {
+        // Outer bit applies to the resolved word; inner address is still depth-1.
+        var a = AbLegacyAddress.TryParse("N7:[N7:0]/3");
+        a.ShouldNotBeNull();
+        a.BitIndex.ShouldBe(3);
+        a.IndirectWordSource.ShouldNotBeNull();
+        a.ToLibplctagName().ShouldBe("N7:[N7:0]/3");
+    }
+
+    [Fact]
+    public void TryParse_Plc5_indirect_inner_address_obeys_octal()
+    {
+        // Inner I:/O: indices on PLC-5 must obey octal rules even when nested in brackets.
+        var a = AbLegacyAddress.TryParse("N7:[I:010/10]", AbLegacyPlcFamily.Plc5);
+        a.ShouldNotBeNull();
+        a.IndirectWordSource.ShouldNotBeNull();
+        a.IndirectWordSource!.WordNumber.ShouldBe(8);   // octal 010 → 8
+        a.IndirectWordSource.BitIndex.ShouldBe(8);      // octal 10 → 8
+
+        // Octal-illegal digit '8' inside an inner I: address is rejected on PLC-5.
+        AbLegacyAddress.TryParse("N7:[I:8/0]", AbLegacyPlcFamily.Plc5).ShouldBeNull();
+    }
+
+    [Fact]
+    public void TryParse_indirect_inner_cannot_itself_be_indirect()
+    {
+        AbLegacyAddress.TryParse("N7:[N7:[N7:0]]").ShouldBeNull();
+        AbLegacyAddress.TryParse("N[N[N7:0]:5]:5").ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData("RTC", "HR", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Int32)]
+    [InlineData("RTC", "EN", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    [InlineData("HSC", "ACC", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Int32)]
+    [InlineData("HSC", "EN", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    [InlineData("DLS", "STR", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Int32)]
+    [InlineData("DLS", "EN", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    [InlineData("PWM", "OUT", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    public void FunctionFile_subelement_catalogue_maps_to_expected_driver_type(
+        string letter, string sub, ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType expected)
+    {
+        AbLegacyFunctionFile.SubElementType(letter, sub).ShouldBe(expected);
+    }
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDriverTests.cs

@@ -102,4 +102,96 @@ public sealed class AbLegacyDriverTests
         AbLegacyDataType.String.ToDriverDataType().ShouldBe(DriverDataType.String);
         AbLegacyDataType.TimerElement.ToDriverDataType().ShouldBe(DriverDataType.Int32);
     }
+
+    [Theory]
+    [InlineData(AbLegacyDataType.TimerElement, "EN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.TimerElement, "TT", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.TimerElement, "DN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.TimerElement, "PRE", DriverDataType.Int32)]
+    [InlineData(AbLegacyDataType.TimerElement, "ACC", DriverDataType.Int32)]
+    [InlineData(AbLegacyDataType.CounterElement, "CU", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.CounterElement, "CD", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.CounterElement, "DN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.CounterElement, "OV", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.CounterElement, "UN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.CounterElement, "PRE", DriverDataType.Int32)]
+    [InlineData(AbLegacyDataType.CounterElement, "ACC", DriverDataType.Int32)]
+    [InlineData(AbLegacyDataType.ControlElement, "EN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "EU", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "DN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "EM", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "ER", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "UL", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "IN", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "FD", DriverDataType.Boolean)]
+    [InlineData(AbLegacyDataType.ControlElement, "LEN", DriverDataType.Int32)]
+    [InlineData(AbLegacyDataType.ControlElement, "POS", DriverDataType.Int32)]
+    public void EffectiveDriverDataType_resolves_subelements(
+        AbLegacyDataType dataType, string subElement, DriverDataType expected)
+    {
+        AbLegacyDataTypeExtensions.EffectiveDriverDataType(dataType, subElement).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void EffectiveDriverDataType_unknown_subelement_falls_back_to_base()
+    {
+        // Permissive — keeps the driver from refusing tags whose sub-element we don't catalogue.
+        AbLegacyDataTypeExtensions.EffectiveDriverDataType(AbLegacyDataType.TimerElement, "BOGUS")
+            .ShouldBe(DriverDataType.Int32);
+        AbLegacyDataTypeExtensions.EffectiveDriverDataType(AbLegacyDataType.TimerElement, null)
+            .ShouldBe(DriverDataType.Int32);
+        AbLegacyDataTypeExtensions.EffectiveDriverDataType(AbLegacyDataType.Int, "DN")
+            .ShouldBe(DriverDataType.Int32);
+    }
+
+    [Theory]
+    [InlineData(AbLegacyDataType.TimerElement, "DN", 13)]
+    [InlineData(AbLegacyDataType.TimerElement, "TT", 14)]
+    [InlineData(AbLegacyDataType.TimerElement, "EN", 15)]
+    [InlineData(AbLegacyDataType.CounterElement, "UN", 10)]
+    [InlineData(AbLegacyDataType.CounterElement, "OV", 11)]
+    [InlineData(AbLegacyDataType.CounterElement, "DN", 12)]
+    [InlineData(AbLegacyDataType.CounterElement, "CD", 13)]
+    [InlineData(AbLegacyDataType.CounterElement, "CU", 14)]
+    [InlineData(AbLegacyDataType.ControlElement, "FD", 8)]
+    [InlineData(AbLegacyDataType.ControlElement, "IN", 9)]
+    [InlineData(AbLegacyDataType.ControlElement, "UL", 10)]
+    [InlineData(AbLegacyDataType.ControlElement, "ER", 11)]
+    [InlineData(AbLegacyDataType.ControlElement, "EM", 12)]
+    [InlineData(AbLegacyDataType.ControlElement, "DN", 13)]
+    [InlineData(AbLegacyDataType.ControlElement, "EU", 14)]
+    [InlineData(AbLegacyDataType.ControlElement, "EN", 15)]
+    public void StatusBitIndex_maps_to_standard_pccc_positions(
+        AbLegacyDataType dataType, string subElement, int expectedBit)
+    {
+        AbLegacyDataTypeExtensions.StatusBitIndex(dataType, subElement).ShouldBe(expectedBit);
+    }
+
+    [Fact]
+    public void StatusBitIndex_for_word_subelements_is_null()
+    {
+        AbLegacyDataTypeExtensions.StatusBitIndex(AbLegacyDataType.TimerElement, "PRE").ShouldBeNull();
+        AbLegacyDataTypeExtensions.StatusBitIndex(AbLegacyDataType.CounterElement, "ACC").ShouldBeNull();
+        AbLegacyDataTypeExtensions.StatusBitIndex(AbLegacyDataType.ControlElement, "LEN").ShouldBeNull();
+        AbLegacyDataTypeExtensions.StatusBitIndex(AbLegacyDataType.TimerElement, null).ShouldBeNull();
+        AbLegacyDataTypeExtensions.StatusBitIndex(AbLegacyDataType.Int, "DN").ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData(AbLegacyDataType.TimerElement, "DN", true)]
+    [InlineData(AbLegacyDataType.TimerElement, "TT", true)]
+    [InlineData(AbLegacyDataType.TimerElement, "EN", false)] // operator-controllable
+    [InlineData(AbLegacyDataType.CounterElement, "DN", true)]
+    [InlineData(AbLegacyDataType.CounterElement, "OV", true)]
+    [InlineData(AbLegacyDataType.CounterElement, "UN", true)]
+    [InlineData(AbLegacyDataType.CounterElement, "CU", false)]
+    [InlineData(AbLegacyDataType.ControlElement, "DN", true)]
+    [InlineData(AbLegacyDataType.ControlElement, "ER", true)]
+    [InlineData(AbLegacyDataType.ControlElement, "EM", true)]
+    [InlineData(AbLegacyDataType.ControlElement, "EN", false)]
+    public void IsPlcSetStatusBit_classifies_writable_vs_status_bits(
+        AbLegacyDataType dataType, string subElement, bool expected)
+    {
+        AbLegacyDataTypeExtensions.IsPlcSetStatusBit(dataType, subElement).ShouldBe(expected);
+    }
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyReadWriteTests.cs

@@ -256,4 +256,113 @@ public sealed class AbLegacyReadWriteTests
             Value = value;
         }
     }
+
+    // ---- Timer / Counter / Control sub-element bit semantics (issue #246) ----
+
+    [Theory]
+    [InlineData("T4:0.DN", 13)]
+    [InlineData("T4:0.TT", 14)]
+    [InlineData("T4:0.EN", 15)]
+    public async Task Timer_status_bit_decodes_correct_position(string address, int bitPos)
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", address, AbLegacyDataType.TimerElement));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        // Seed a parent-word with only the target bit set.
+        factory.Customise = p => new FakeAbLegacyTag(p) { Value = 1 << bitPos };
+
+        var snapshots = await drv.ReadAsync(["X"], CancellationToken.None);
+
+        snapshots.Single().Value.ShouldBe(true);
+        // The driver must have asked the runtime for the right bit position.
+        factory.Tags[address].LastDecodeBitIndex.ShouldBe(bitPos);
+    }
+
+    [Fact]
+    public async Task Timer_PRE_subelement_decodes_as_int_word()
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyTagDefinition("Pre", "ab://10.0.0.5/1,0", "T4:0.PRE", AbLegacyDataType.TimerElement));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbLegacyTag(p) { Value = 5000 };
+
+        var snapshots = await drv.ReadAsync(["Pre"], CancellationToken.None);
+
+        snapshots.Single().Value.ShouldBe(5000);
+        factory.Tags["T4:0.PRE"].LastDecodeBitIndex.ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData("C5:0.UN", 10)]
+    [InlineData("C5:0.OV", 11)]
+    [InlineData("C5:0.DN", 12)]
+    [InlineData("C5:0.CD", 13)]
+    [InlineData("C5:0.CU", 14)]
+    public async Task Counter_status_bit_decodes_correct_position(string address, int bitPos)
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", address, AbLegacyDataType.CounterElement));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbLegacyTag(p) { Value = 1 << bitPos };
+
+        var snapshots = await drv.ReadAsync(["X"], CancellationToken.None);
+        snapshots.Single().Value.ShouldBe(true);
+        factory.Tags[address].LastDecodeBitIndex.ShouldBe(bitPos);
+    }
+
+    [Theory]
+    [InlineData("R6:0.FD", 8)]
+    [InlineData("R6:0.IN", 9)]
+    [InlineData("R6:0.UL", 10)]
+    [InlineData("R6:0.ER", 11)]
+    [InlineData("R6:0.EM", 12)]
+    [InlineData("R6:0.DN", 13)]
+    [InlineData("R6:0.EU", 14)]
+    [InlineData("R6:0.EN", 15)]
+    public async Task Control_status_bit_decodes_correct_position(string address, int bitPos)
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", address, AbLegacyDataType.ControlElement));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbLegacyTag(p) { Value = 1 << bitPos };
+
+        var snapshots = await drv.ReadAsync(["X"], CancellationToken.None);
+        snapshots.Single().Value.ShouldBe(true);
+        factory.Tags[address].LastDecodeBitIndex.ShouldBe(bitPos);
+    }
+
+    [Fact]
+    public async Task Status_bit_returns_false_when_parent_word_bit_is_clear()
+    {
+        var (drv, factory) = NewDriver(
+            new AbLegacyTagDefinition("Done", "ab://10.0.0.5/1,0", "T4:0.DN", AbLegacyDataType.TimerElement));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        // Bit 14 (TT) set, bit 13 (DN) clear.
+        factory.Customise = p => new FakeAbLegacyTag(p) { Value = 1 << 14 };
+
+        var snapshots = await drv.ReadAsync(["Done"], CancellationToken.None);
+        snapshots.Single().Value.ShouldBe(false);
+    }
+
+    [Theory]
+    [InlineData("T4:0.DN", AbLegacyDataType.TimerElement)]
+    [InlineData("T4:0.TT", AbLegacyDataType.TimerElement)]
+    [InlineData("C5:0.DN", AbLegacyDataType.CounterElement)]
+    [InlineData("C5:0.OV", AbLegacyDataType.CounterElement)]
+    [InlineData("C5:0.UN", AbLegacyDataType.CounterElement)]
+    [InlineData("R6:0.ER", AbLegacyDataType.ControlElement)]
+    [InlineData("R6:0.EM", AbLegacyDataType.ControlElement)]
+    [InlineData("R6:0.DN", AbLegacyDataType.ControlElement)]
+    [InlineData("R6:0.FD", AbLegacyDataType.ControlElement)]
+    public async Task Writes_to_PLC_set_status_bits_return_BadNotWritable(
+        string address, AbLegacyDataType dataType)
+    {
+        var (drv, _) = NewDriver(
+            new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", address, dataType));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("X", true)], CancellationToken.None);
+        results.Single().StatusCode.ShouldBe(AbLegacyStatusMapper.BadNotWritable);
+    }
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs

@@ -40,7 +40,25 @@ internal class FakeAbLegacyTag : IAbLegacyTagRuntime
     }
 
     public virtual int GetStatus() => Status;
-    public virtual object? DecodeValue(AbLegacyDataType type, int? bitIndex) => Value;
+    public int? LastDecodeBitIndex { get; private set; }
+    public AbLegacyDataType? LastDecodeType { get; private set; }
+    public virtual object? DecodeValue(AbLegacyDataType type, int? bitIndex)
+    {
+        LastDecodeType = type;
+        LastDecodeBitIndex = bitIndex;
+        // If the test seeded a parent-word value (ushort/short/int) and the driver asked for a
+        // specific status bit, mask it out so we can assert the correct bit reaches the client.
+        if (bitIndex is int bit && Value is not null and not bool)
+        {
+            try
+            {
+                var word = Convert.ToInt32(Value);
+                return ((word >> bit) & 1) != 0;
+            }
+            catch (Exception ex) when (ex is FormatException or InvalidCastException) { }
+        }
+        return Value;
+    }
     public virtual void EncodeValue(AbLegacyDataType type, int? bitIndex, object? value) => Value = value;
     public virtual void Dispose() => Disposed = true;
 }

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasCapabilityTests.cs

@@ -31,8 +31,10 @@ public sealed class FocasCapabilityTests
 
         builder.Folders.ShouldContain(f => f.BrowseName == "FOCAS");
         builder.Folders.ShouldContain(f => f.BrowseName == "focas://10.0.0.5:8193" && f.DisplayName == "Lathe-1");
-        builder.Variables.Single(v => v.BrowseName == "Run").Info.SecurityClass.ShouldBe(SecurityClassification.Operate);
-        builder.Variables.Single(v => v.BrowseName == "Alarm").Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+        // Per-tag and Status/ fields can share a BrowseName ("Run", "Alarm") under different
+        // parent folders — disambiguate by FullName, which is unique per node.
+        builder.Variables.Single(v => v.Info.FullName == "Run").Info.SecurityClass.ShouldBe(SecurityClassification.Operate);
+        builder.Variables.Single(v => v.Info.FullName == "Alarm").Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
     }
 
     // ---- ISubscribable ----

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasFigureScalingDiagnosticsTests.cs

@@ -0,0 +1,273 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class FocasFigureScalingDiagnosticsTests
+{
+    private const string Host = "focas://10.0.0.7:8193";
+
+    /// <summary>
+    ///     Variant of <see cref="FakeFocasClient"/> that returns configurable
+    ///     per-axis figure scaling for the F1-f cache + diagnostics surface
+    ///     (issue #262).
+    /// </summary>
+    private sealed class FigureAwareFakeFocasClient : FakeFocasClient, IFocasClient
+    {
+        public IReadOnlyDictionary<string, int>? Scaling { get; set; }
+
+        Task<IReadOnlyDictionary<string, int>?> IFocasClient.GetFigureScalingAsync(CancellationToken ct) =>
+            Task.FromResult(Scaling);
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Diagnostics_subtree_with_five_counters()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Mill-1")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-diag", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Diagnostics" && f.DisplayName == "Diagnostics");
+        var diagVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Diagnostics/")).ToList();
+        diagVars.Count.ShouldBe(5);
+
+        // Verify per-field types match the documented surface (Int64 counters,
+        // String error message, DateTime last-success timestamp).
+        diagVars.Single(v => v.BrowseName == "ReadCount")
+            .Info.DriverDataType.ShouldBe(DriverDataType.Int64);
+        diagVars.Single(v => v.BrowseName == "ReadFailureCount")
+            .Info.DriverDataType.ShouldBe(DriverDataType.Int64);
+        diagVars.Single(v => v.BrowseName == "ReconnectCount")
+            .Info.DriverDataType.ShouldBe(DriverDataType.Int64);
+        diagVars.Single(v => v.BrowseName == "LastErrorMessage")
+            .Info.DriverDataType.ShouldBe(DriverDataType.String);
+        diagVars.Single(v => v.BrowseName == "LastSuccessfulRead")
+            .Info.DriverDataType.ShouldBe(DriverDataType.DateTime);
+
+        foreach (var v in diagVars)
+            v.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+    }
+
+    [Fact]
+    public async Task ReadAsync_publishes_diagnostics_counters_after_probe_ticks()
+    {
+        // Probe enabled — successful ticks bump ReadCount + LastSuccessfulRead;
+        // ReconnectCount bumps once on the initial connect (issue #262).
+        var fake = new FakeFocasClient { ProbeResult = true };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(30) },
+        }, "drv-diag-read", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for at least 2 successful probe ticks so ReadCount > 0 deterministically.
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Diagnostics/ReadCount"], CancellationToken.None)).Single();
+            return snap.Value is long n && n >= 2;
+        }, TimeSpan.FromSeconds(3));
+
+        var refs = new[]
+        {
+            $"{Host}::Diagnostics/ReadCount",
+            $"{Host}::Diagnostics/ReadFailureCount",
+            $"{Host}::Diagnostics/ReconnectCount",
+            $"{Host}::Diagnostics/LastErrorMessage",
+            $"{Host}::Diagnostics/LastSuccessfulRead",
+        };
+        var snaps = await drv.ReadAsync(refs, CancellationToken.None);
+
+        ((long)snaps[0].Value!).ShouldBeGreaterThanOrEqualTo(2);
+        ((long)snaps[1].Value!).ShouldBe(0); // no failures on a healthy probe
+        ((long)snaps[2].Value!).ShouldBe(1); // one initial connect
+        snaps[3].Value.ShouldBe(string.Empty);
+        ((DateTime)snaps[4].Value!).ShouldBeGreaterThan(DateTime.MinValue);
+
+        foreach (var s in snaps) s.StatusCode.ShouldBe(FocasStatusMapper.Good);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ReadAsync_increments_ReadFailureCount_when_probe_returns_false()
+    {
+        // ProbeResult=false → success branch is skipped, ReadFailureCount bumps each
+        // tick. The connect itself succeeded so ReconnectCount is 1.
+        var fake = new FakeFocasClient { ProbeResult = false };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(30) },
+        }, "drv-diag-fail", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Diagnostics/ReadFailureCount"], CancellationToken.None)).Single();
+            return snap.Value is long n && n >= 2;
+        }, TimeSpan.FromSeconds(3));
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Diagnostics/ReadCount", $"{Host}::Diagnostics/ReadFailureCount"],
+            CancellationToken.None);
+        ((long)snaps[0].Value!).ShouldBe(0);
+        ((long)snaps[1].Value!).ShouldBeGreaterThanOrEqualTo(2);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ApplyFigureScaling_divides_raw_position_by_ten_to_the_decimal_places()
+    {
+        // Cache populated via probe-tick GetFigureScalingAsync. ApplyFigureScaling
+        // default is true → rawValue / 10^dec for the named axis (issue #262).
+        var fake = new FigureAwareFakeFocasClient
+        {
+            Scaling = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase)
+            {
+                ["axis1"] = 3,  // X-axis: 3 decimal places (mm * 1000)
+                ["axis2"] = 4,  // Y-axis: 4 decimal places
+            },
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(30) },
+        }, "drv-fig", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for the probe-tick path to populate the cache (one successful tick is
+        // enough — the figure-scaling read happens whenever the cache is null).
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Diagnostics/ReadCount"], CancellationToken.None)).Single();
+            return snap.Value is long n && n >= 1;
+        }, TimeSpan.FromSeconds(3));
+
+        // 100000 / 10^3 = 100.0 mm
+        drv.ApplyFigureScaling(Host, "axis1", 100000).ShouldBe(100.0);
+        // 250000 / 10^4 = 25.0 mm
+        drv.ApplyFigureScaling(Host, "axis2", 250000).ShouldBe(25.0);
+        // Unknown axis → raw value passes through.
+        drv.ApplyFigureScaling(Host, "axis3", 42).ShouldBe(42.0);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ApplyFigureScaling_returns_raw_when_FixedTreeApplyFigureScaling_is_false()
+    {
+        // ApplyFigureScaling=false short-circuits before the cache lookup so the raw
+        // integer is published unchanged. Migration parity for deployments that already
+        // surfaced raw values from older drivers (issue #262).
+        var fake = new FigureAwareFakeFocasClient
+        {
+            Scaling = new Dictionary<string, int> { ["axis1"] = 3 },
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(30) },
+            FixedTree = new FocasFixedTreeOptions { ApplyFigureScaling = false },
+        }, "drv-fig-off", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Diagnostics/ReadCount"], CancellationToken.None)).Single();
+            return snap.Value is long n && n >= 1;
+        }, TimeSpan.FromSeconds(3));
+
+        // Even though the cache has axis1 → 3 decimal places, ApplyFigureScaling=false
+        // means the raw value passes through unchanged.
+        drv.ApplyFigureScaling(Host, "axis1", 100000).ShouldBe(100000.0);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task FwlibFocasClient_GetFigureScaling_returns_null_when_disconnected()
+    {
+        // Construction is licence-safe (no DLL load); the unconnected client must
+        // short-circuit before P/Invoke so the driver leaves the cache untouched.
+        var client = new FwlibFocasClient();
+        (await client.GetFigureScalingAsync(CancellationToken.None)).ShouldBeNull();
+    }
+
+    [Fact]
+    public void DecodeFigureScaling_extracts_per_axis_decimal_places_from_buffer()
+    {
+        // Build an IODBAXIS-shaped buffer: 3 axes, decimal places = 3, 4, 0. Per
+        // fwlib32.h each axis entry is { short dec, short unit, short reserved,
+        // short reserved2 } = 8 bytes; we only read dec.
+        var buf = new byte[FwlibNative.MAX_AXIS * 8];
+        // Axis 1: dec=3
+        buf[0] = 3; buf[1] = 0;
+        // Axis 2: dec=4
+        buf[8] = 4; buf[9] = 0;
+        // Axis 3: dec=0 (already zero)
+
+        var map = FwlibFocasClient.DecodeFigureScaling(buf, count: 3);
+        map.Count.ShouldBe(3);
+        map["axis1"].ShouldBe(3);
+        map["axis2"].ShouldBe(4);
+        map["axis3"].ShouldBe(0);
+
+        // Out-of-range count clamps to MAX_AXIS so a malformed CNC reply doesn't
+        // overrun the buffer.
+        var clamped = FwlibFocasClient.DecodeFigureScaling(buf, count: 99);
+        clamped.Count.ShouldBe(FwlibNative.MAX_AXIS);
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!await condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasMessagesBlockTextFixedTreeTests.cs

@@ -0,0 +1,231 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class FocasMessagesBlockTextFixedTreeTests
+{
+    private const string Host = "focas://10.0.0.7:8193";
+
+    /// <summary>
+    ///     Variant of <see cref="FakeFocasClient"/> that returns configurable
+    ///     <see cref="FocasOperatorMessagesInfo"/> + <see cref="FocasCurrentBlockInfo"/>
+    ///     snapshots for the F1-e Messages/External/Latest + Program/CurrentBlock
+    ///     fixed-tree (issue #261).
+    /// </summary>
+    private sealed class MessagesAwareFakeFocasClient : FakeFocasClient, IFocasClient
+    {
+        public FocasOperatorMessagesInfo? Messages { get; set; }
+        public FocasCurrentBlockInfo? CurrentBlock { get; set; }
+
+        Task<FocasOperatorMessagesInfo?> IFocasClient.GetOperatorMessagesAsync(CancellationToken ct) =>
+            Task.FromResult(Messages);
+
+        Task<FocasCurrentBlockInfo?> IFocasClient.GetCurrentBlockAsync(CancellationToken ct) =>
+            Task.FromResult(CurrentBlock);
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Messages_External_Latest_and_Program_CurrentBlock_nodes()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Mill-1")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-msg", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Messages" && f.DisplayName == "Messages");
+        builder.Folders.ShouldContain(f => f.BrowseName == "External" && f.DisplayName == "External");
+        builder.Folders.ShouldContain(f => f.BrowseName == "Program" && f.DisplayName == "Program");
+
+        var latest = builder.Variables.SingleOrDefault(v =>
+            v.Info.FullName == $"{Host}::Messages/External/Latest");
+        latest.BrowseName.ShouldBe("Latest");
+        latest.Info.DriverDataType.ShouldBe(DriverDataType.String);
+        latest.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+
+        var block = builder.Variables.SingleOrDefault(v =>
+            v.Info.FullName == $"{Host}::Program/CurrentBlock");
+        block.BrowseName.ShouldBe("CurrentBlock");
+        block.Info.DriverDataType.ShouldBe(DriverDataType.String);
+        block.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+    }
+
+    [Fact]
+    public async Task ReadAsync_serves_Messages_Latest_and_CurrentBlock_from_cached_snapshot()
+    {
+        var fake = new MessagesAwareFakeFocasClient
+        {
+            Messages = new FocasOperatorMessagesInfo(
+            [
+                new FocasOperatorMessage(2001, "OPMSG", "TOOL CHANGE READY"),
+                new FocasOperatorMessage(3010, "EXTERN", "DOOR OPEN"),
+            ]),
+            CurrentBlock = new FocasCurrentBlockInfo("G01 X100. Y200. F500."),
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-msg-read", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Program/CurrentBlock"], CancellationToken.None)).Single();
+            return snap.StatusCode == FocasStatusMapper.Good;
+        }, TimeSpan.FromSeconds(3));
+
+        var refs = new[]
+        {
+            $"{Host}::Messages/External/Latest",
+            $"{Host}::Program/CurrentBlock",
+        };
+        var snaps = await drv.ReadAsync(refs, CancellationToken.None);
+
+        // "Latest" surfaces the last entry in the message snapshot — issue #261 permits
+        // this minimal "latest message" surface in lieu of full ring-buffer coverage.
+        snaps[0].Value.ShouldBe("DOOR OPEN");
+        snaps[1].Value.ShouldBe("G01 X100. Y200. F500.");
+        foreach (var s in snaps) s.StatusCode.ShouldBe(FocasStatusMapper.Good);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ReadAsync_returns_BadCommunicationError_when_caches_are_empty()
+    {
+        // Probe disabled — neither cache populates; the nodes still resolve as known
+        // references but report Bad until the first poll. Mirrors the f1a/f1b/f1c/f1d
+        // policy.
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-msg-empty", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Messages/External/Latest", $"{Host}::Program/CurrentBlock"],
+            CancellationToken.None);
+        snaps[0].StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+        snaps[1].StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+    }
+
+    [Fact]
+    public async Task ReadAsync_publishes_empty_string_when_message_snapshot_is_empty()
+    {
+        // Empty snapshot (CNC reported no active messages) still publishes Good +
+        // empty string — operators distinguish "no messages" from "Bad" without
+        // having to read separate availability nodes.
+        var fake = new MessagesAwareFakeFocasClient
+        {
+            Messages = new FocasOperatorMessagesInfo([]),
+            CurrentBlock = new FocasCurrentBlockInfo(""),
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-msg-empty-snap", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Messages/External/Latest"], CancellationToken.None)).Single();
+            return snap.StatusCode == FocasStatusMapper.Good;
+        }, TimeSpan.FromSeconds(3));
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Messages/External/Latest", $"{Host}::Program/CurrentBlock"],
+            CancellationToken.None);
+        snaps[0].Value.ShouldBe(string.Empty);
+        snaps[0].StatusCode.ShouldBe(FocasStatusMapper.Good);
+        snaps[1].Value.ShouldBe(string.Empty);
+        snaps[1].StatusCode.ShouldBe(FocasStatusMapper.Good);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task FwlibFocasClient_GetOperatorMessages_and_GetCurrentBlock_return_null_when_disconnected()
+    {
+        // Construction is licence-safe (no DLL load); the unconnected client must
+        // short-circuit before P/Invoke. Returns null → driver leaves the cache
+        // untouched, matching the policy in f1a/f1b/f1c/f1d.
+        var client = new FwlibFocasClient();
+        (await client.GetOperatorMessagesAsync(CancellationToken.None)).ShouldBeNull();
+        (await client.GetCurrentBlockAsync(CancellationToken.None)).ShouldBeNull();
+    }
+
+    [Fact]
+    public void TrimAnsiPadding_strips_trailing_nulls_and_spaces_for_round_trip()
+    {
+        // The CNC right-pads block text + opmsg bodies with NULs or spaces; the
+        // managed side trims them so the same message round-trips with stable text
+        // (issue #261). Stops at the first NUL so reused buffers don't leak old bytes.
+        var buf = new byte[16];
+        var bytes = System.Text.Encoding.ASCII.GetBytes("G01 X10 ");
+        Array.Copy(bytes, buf, bytes.Length);
+        FwlibFocasClient.TrimAnsiPadding(buf).ShouldBe("G01 X10");
+
+        // NUL-terminated mid-buffer with trailing spaces beyond the NUL — trim stops
+        // at the NUL so leftover bytes in the rest of the buffer are ignored.
+        var buf2 = new byte[32];
+        var bytes2 = System.Text.Encoding.ASCII.GetBytes("OPMSG TEXT");
+        Array.Copy(bytes2, buf2, bytes2.Length);
+        // After NUL the buffer has zeros — already invisible — but explicit space
+        // padding before the NUL should be trimmed.
+        var buf3 = new byte[32];
+        var bytes3 = System.Text.Encoding.ASCII.GetBytes("HELLO     ");
+        Array.Copy(bytes3, buf3, bytes3.Length);
+        FwlibFocasClient.TrimAnsiPadding(buf2).ShouldBe("OPMSG TEXT");
+        FwlibFocasClient.TrimAnsiPadding(buf3).ShouldBe("HELLO");
+
+        // Empty buffer → empty string (no exception).
+        FwlibFocasClient.TrimAnsiPadding(new byte[8]).ShouldBe(string.Empty);
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!await condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasModalOverrideFixedTreeTests.cs

@@ -0,0 +1,231 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class FocasModalOverrideFixedTreeTests
+{
+    private const string Host = "focas://10.0.0.6:8193";
+
+    /// <summary>
+    ///     Variant of <see cref="FakeFocasClient"/> that returns configurable
+    ///     <see cref="FocasModalInfo"/> + <see cref="FocasOverrideInfo"/> snapshots.
+    /// </summary>
+    private sealed class ModalAwareFakeFocasClient : FakeFocasClient, IFocasClient
+    {
+        public FocasModalInfo? Modal { get; set; }
+        public FocasOverrideInfo? Override { get; set; }
+        public FocasOverrideParameters? LastOverrideParams { get; private set; }
+
+        Task<FocasModalInfo?> IFocasClient.GetModalAsync(CancellationToken ct) =>
+            Task.FromResult(Modal);
+
+        Task<FocasOverrideInfo?> IFocasClient.GetOverrideAsync(
+            FocasOverrideParameters parameters, CancellationToken ct)
+        {
+            LastOverrideParams = parameters;
+            return Task.FromResult(Override);
+        }
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Modal_folder_with_4_Int16_codes_per_device()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Lathe-2")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-modal", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Modal" && f.DisplayName == "Modal");
+        var modalVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Modal/")).ToList();
+        modalVars.Count.ShouldBe(4);
+        string[] expected = ["MCode", "SCode", "TCode", "BCode"];
+        foreach (var name in expected)
+        {
+            var node = modalVars.SingleOrDefault(v => v.BrowseName == name);
+            node.BrowseName.ShouldBe(name);
+            node.Info.DriverDataType.ShouldBe(DriverDataType.Int16);
+            node.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+            node.Info.FullName.ShouldBe($"{Host}::Modal/{name}");
+        }
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_omits_Override_folder_when_no_parameters_configured()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)], // OverrideParameters defaults to null
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-no-overrides", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldNotContain(f => f.BrowseName == "Override");
+        builder.Variables.ShouldNotContain(v => v.Info.FullName.Contains("::Override/"));
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_only_configured_Override_fields()
+    {
+        // Spindle + Jog suppressed (null parameters) — only Feed + Rapid show up.
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices =
+            [
+                new FocasDeviceOptions(Host,
+                    OverrideParameters: new FocasOverrideParameters(6010, 6011, null, null)),
+            ],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-partial-overrides", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Override");
+        var overrideVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Override/")).ToList();
+        overrideVars.Count.ShouldBe(2);
+        overrideVars.ShouldContain(v => v.BrowseName == "Feed");
+        overrideVars.ShouldContain(v => v.BrowseName == "Rapid");
+        overrideVars.ShouldNotContain(v => v.BrowseName == "Spindle");
+        overrideVars.ShouldNotContain(v => v.BrowseName == "Jog");
+    }
+
+    [Fact]
+    public async Task ReadAsync_serves_Modal_and_Override_fields_from_cached_snapshot()
+    {
+        var fake = new ModalAwareFakeFocasClient
+        {
+            Modal = new FocasModalInfo(MCode: 8, SCode: 1200, TCode: 101, BCode: 0),
+            Override = new FocasOverrideInfo(Feed: 100, Rapid: 50, Spindle: 110, Jog: 25),
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices =
+            [
+                new FocasDeviceOptions(Host,
+                    OverrideParameters: FocasOverrideParameters.Default),
+            ],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-modal-read", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for at least one probe tick to populate both caches.
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Modal/MCode"], CancellationToken.None)).Single();
+            return snap.StatusCode == FocasStatusMapper.Good;
+        }, TimeSpan.FromSeconds(3));
+
+        var refs = new[]
+        {
+            $"{Host}::Modal/MCode",
+            $"{Host}::Modal/SCode",
+            $"{Host}::Modal/TCode",
+            $"{Host}::Modal/BCode",
+            $"{Host}::Override/Feed",
+            $"{Host}::Override/Rapid",
+            $"{Host}::Override/Spindle",
+            $"{Host}::Override/Jog",
+        };
+        var snaps = await drv.ReadAsync(refs, CancellationToken.None);
+
+        snaps[0].Value.ShouldBe((short)8);
+        snaps[1].Value.ShouldBe((short)1200);
+        snaps[2].Value.ShouldBe((short)101);
+        snaps[3].Value.ShouldBe((short)0);
+        snaps[4].Value.ShouldBe((short)100);
+        snaps[5].Value.ShouldBe((short)50);
+        snaps[6].Value.ShouldBe((short)110);
+        snaps[7].Value.ShouldBe((short)25);
+        foreach (var s in snaps) s.StatusCode.ShouldBe(FocasStatusMapper.Good);
+        // The driver hands the device's configured override parameters to the wire client
+        // verbatim — defaulting to 30i numbers.
+        fake.LastOverrideParams.ShouldNotBeNull();
+        fake.LastOverrideParams!.FeedParam.ShouldBe<ushort?>(6010);
+        fake.LastOverrideParams.RapidParam.ShouldBe<ushort?>(6011);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ReadAsync_returns_BadCommunicationError_when_caches_are_empty()
+    {
+        // Probe disabled — neither modal nor override caches populate; the nodes still
+        // resolve as known references but report Bad until the first successful poll.
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices =
+            [
+                new FocasDeviceOptions(Host,
+                    OverrideParameters: FocasOverrideParameters.Default),
+            ],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-empty-cache", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Modal/MCode", $"{Host}::Override/Feed"], CancellationToken.None);
+        snaps[0].StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+        snaps[1].StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+    }
+
+    [Fact]
+    public async Task FwlibFocasClient_GetModal_and_GetOverride_return_null_when_disconnected()
+    {
+        // Construction is licence-safe (no DLL load); the unconnected client must short-
+        // circuit before P/Invoke. Returns null → driver leaves the cache untouched.
+        var client = new FwlibFocasClient();
+        (await client.GetModalAsync(CancellationToken.None)).ShouldBeNull();
+        (await client.GetOverrideAsync(
+            FocasOverrideParameters.Default, CancellationToken.None)).ShouldBeNull();
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!await condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasProductionFixedTreeTests.cs

@@ -0,0 +1,156 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class FocasProductionFixedTreeTests
+{
+    private const string Host = "focas://10.0.0.5:8193";
+
+    /// <summary>
+    ///     Variant of <see cref="FakeFocasClient"/> that returns a configurable
+    ///     <see cref="FocasProductionInfo"/> snapshot from <c>GetProductionAsync</c>.
+    /// </summary>
+    private sealed class ProductionAwareFakeFocasClient : FakeFocasClient, IFocasClient
+    {
+        public FocasProductionInfo? Production { get; set; }
+
+        Task<FocasProductionInfo?> IFocasClient.GetProductionAsync(CancellationToken ct) =>
+            Task.FromResult(Production);
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Production_folder_with_4_Int32_nodes_per_device()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Lathe-1")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-1", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Production" && f.DisplayName == "Production");
+        var prodVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Production/")).ToList();
+        prodVars.Count.ShouldBe(4);
+        string[] expected = ["PartsProduced", "PartsRequired", "PartsTotal", "CycleTimeSeconds"];
+        foreach (var name in expected)
+        {
+            var node = prodVars.SingleOrDefault(v => v.BrowseName == name);
+            node.BrowseName.ShouldBe(name);
+            node.Info.DriverDataType.ShouldBe(DriverDataType.Int32);
+            node.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+            node.Info.FullName.ShouldBe($"{Host}::Production/{name}");
+        }
+    }
+
+    [Fact]
+    public async Task ReadAsync_serves_each_Production_field_from_cached_snapshot()
+    {
+        var fake = new ProductionAwareFakeFocasClient
+        {
+            Production = new FocasProductionInfo(
+                PartsProduced: 17,
+                PartsRequired: 100,
+                PartsTotal: 4242,
+                CycleTimeSeconds: 73),
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for at least one probe tick to populate the cache.
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Production/PartsProduced"], CancellationToken.None)).Single();
+            return snap.StatusCode == FocasStatusMapper.Good;
+        }, TimeSpan.FromSeconds(3));
+
+        var refs = new[]
+        {
+            $"{Host}::Production/PartsProduced",
+            $"{Host}::Production/PartsRequired",
+            $"{Host}::Production/PartsTotal",
+            $"{Host}::Production/CycleTimeSeconds",
+        };
+        var snaps = await drv.ReadAsync(refs, CancellationToken.None);
+
+        snaps[0].Value.ShouldBe(17);
+        snaps[1].Value.ShouldBe(100);
+        snaps[2].Value.ShouldBe(4242);
+        snaps[3].Value.ShouldBe(73);
+        foreach (var s in snaps) s.StatusCode.ShouldBe(FocasStatusMapper.Good);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ReadAsync_returns_BadCommunicationError_when_production_cache_is_empty()
+    {
+        // Probe disabled — cache never populates; the production nodes still resolve as
+        // known references but report Bad until the first successful poll lands.
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-1", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Production/PartsProduced"], CancellationToken.None);
+        snaps.Single().StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+    }
+
+    [Fact]
+    public async Task FwlibFocasClient_GetProductionAsync_returns_null_when_disconnected()
+    {
+        // Construction is licence-safe (no DLL load); calling GetProductionAsync on the
+        // unconnected client must not P/Invoke. Returns null → driver leaves the cache
+        // in its current state.
+        var client = new FwlibFocasClient();
+        var result = await client.GetProductionAsync(CancellationToken.None);
+        result.ShouldBeNull();
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!await condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasStatusFixedTreeTests.cs

@@ -0,0 +1,197 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class FocasStatusFixedTreeTests
+{
+    private const string Host = "focas://10.0.0.5:8193";
+
+    /// <summary>
+    ///     Variant of <see cref="FakeFocasClient"/> that returns a configurable
+    ///     <see cref="FocasStatusInfo"/> snapshot from <see cref="GetStatusAsync"/>. Probe
+    ///     keeps its existing boolean semantic so the back-compat path stays exercised.
+    /// </summary>
+    private sealed class StatusAwareFakeFocasClient : FakeFocasClient, IFocasClient
+    {
+        public FocasStatusInfo? Status { get; set; }
+
+        // Shadow the default interface implementation with a real one. Explicit interface
+        // form so callers via IFocasClient hit this override; FakeFocasClient itself
+        // doesn't declare a virtual GetStatusAsync (the contract has a default impl).
+        Task<FocasStatusInfo?> IFocasClient.GetStatusAsync(CancellationToken ct) =>
+            Task.FromResult(Status);
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Status_folder_with_9_Int16_nodes_per_device()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Lathe-1")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-1", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Status" && f.DisplayName == "Status");
+        var statusVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Status/")).ToList();
+        statusVars.Count.ShouldBe(9);
+        string[] expected = ["Tmmode", "Aut", "Run", "Motion", "Mstb", "EmergencyStop", "Alarm", "Edit", "Dummy"];
+        foreach (var name in expected)
+        {
+            var node = statusVars.SingleOrDefault(v => v.BrowseName == name);
+            node.BrowseName.ShouldBe(name);
+            node.Info.DriverDataType.ShouldBe(DriverDataType.Int16);
+            node.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+            node.Info.FullName.ShouldBe($"{Host}::Status/{name}");
+        }
+    }
+
+    [Fact]
+    public async Task ReadAsync_serves_each_Status_field_from_cached_ODBST_snapshot()
+    {
+        var fake = new StatusAwareFakeFocasClient
+        {
+            Status = new FocasStatusInfo(
+                Dummy: 0, Tmmode: 1, Aut: 2, Run: 3, Motion: 4,
+                Mstb: 5, EmergencyStop: 1, Alarm: 7, Edit: 6),
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for at least one probe tick to populate the cache.
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Status/Tmmode"], CancellationToken.None)).Single();
+            return snap.StatusCode == FocasStatusMapper.Good;
+        }, TimeSpan.FromSeconds(3));
+
+        var refs = new[]
+        {
+            $"{Host}::Status/Tmmode",
+            $"{Host}::Status/Aut",
+            $"{Host}::Status/Run",
+            $"{Host}::Status/Motion",
+            $"{Host}::Status/Mstb",
+            $"{Host}::Status/EmergencyStop",
+            $"{Host}::Status/Alarm",
+            $"{Host}::Status/Edit",
+            $"{Host}::Status/Dummy",
+        };
+        var snaps = await drv.ReadAsync(refs, CancellationToken.None);
+
+        snaps[0].Value.ShouldBe((short)1);  // Tmmode
+        snaps[1].Value.ShouldBe((short)2);  // Aut
+        snaps[2].Value.ShouldBe((short)3);  // Run
+        snaps[3].Value.ShouldBe((short)4);  // Motion
+        snaps[4].Value.ShouldBe((short)5);  // Mstb
+        snaps[5].Value.ShouldBe((short)1);  // EmergencyStop
+        snaps[6].Value.ShouldBe((short)7);  // Alarm
+        snaps[7].Value.ShouldBe((short)6);  // Edit
+        snaps[8].Value.ShouldBe((short)0);  // Dummy
+        foreach (var s in snaps) s.StatusCode.ShouldBe(FocasStatusMapper.Good);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ReadAsync_returns_BadCommunicationError_when_status_cache_is_empty()
+    {
+        // Probe disabled — cache never populates; the status nodes still resolve as
+        // known references but report Bad until the first successful poll lands.
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-1", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Status/Tmmode"], CancellationToken.None);
+        snaps.Single().StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+    }
+
+    [Fact]
+    public async Task Existing_boolean_probe_path_still_works_alongside_GetStatusAsync()
+    {
+        // Back-compat guard: ProbeAsync's existing boolean contract is preserved. A client
+        // that doesn't override GetStatusAsync (default null) leaves the cache untouched
+        // but the probe still flips host state to Running.
+        var fake = new FakeFocasClient { ProbeResult = true };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await WaitForAsync(() => Task.FromResult(
+            drv.GetHostStatuses().Any(h => h.State == HostState.Running)),
+            TimeSpan.FromSeconds(3));
+
+        // No GetStatusAsync override → cache stays empty → status nodes report Bad,
+        // but the rest of the driver keeps functioning.
+        var snap = (await drv.ReadAsync(
+            [$"{Host}::Status/Tmmode"], CancellationToken.None)).Single();
+        snap.StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task FwlibFocasClient_GetStatusAsync_returns_null_when_disconnected()
+    {
+        // Construction is licence-safe (no DLL load); calling GetStatusAsync on the
+        // unconnected client must not P/Invoke. Returns null → driver leaves the cache
+        // in its current state.
+        var client = new FwlibFocasClient();
+        var result = await client.GetStatusAsync(CancellationToken.None);
+        result.ShouldBeNull();
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!await condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasToolingOffsetsFixedTreeTests.cs

@@ -0,0 +1,220 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class FocasToolingOffsetsFixedTreeTests
+{
+    private const string Host = "focas://10.0.0.7:8193";
+
+    /// <summary>
+    ///     Variant of <see cref="FakeFocasClient"/> that returns configurable
+    ///     <see cref="FocasToolingInfo"/> + <see cref="FocasWorkOffsetsInfo"/> snapshots
+    ///     for the F1-d Tooling/CurrentTool + Offsets/ fixed-tree (issue #260).
+    /// </summary>
+    private sealed class ToolingAwareFakeFocasClient : FakeFocasClient, IFocasClient
+    {
+        public FocasToolingInfo? Tooling { get; set; }
+        public FocasWorkOffsetsInfo? WorkOffsets { get; set; }
+
+        Task<FocasToolingInfo?> IFocasClient.GetToolingAsync(CancellationToken ct) =>
+            Task.FromResult(Tooling);
+
+        Task<FocasWorkOffsetsInfo?> IFocasClient.GetWorkOffsetsAsync(CancellationToken ct) =>
+            Task.FromResult(WorkOffsets);
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Tooling_folder_with_CurrentTool_node()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Mill-1")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-tooling", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Tooling" && f.DisplayName == "Tooling");
+        var toolingVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Tooling/")).ToList();
+        toolingVars.Count.ShouldBe(1);
+        var node = toolingVars.Single();
+        node.BrowseName.ShouldBe("CurrentTool");
+        node.Info.DriverDataType.ShouldBe(DriverDataType.Int16);
+        node.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+        node.Info.FullName.ShouldBe($"{Host}::Tooling/CurrentTool");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_emits_Offsets_folder_with_G54_to_G59_each_with_3_axes()
+    {
+        // Six standard slots (G54..G59) * three axes (X/Y/Z) = 18 Float64 nodes per
+        // device. Extended G54.1 P1..P48 deferred per the F1-d plan.
+        var builder = new RecordingBuilder();
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host, DeviceName: "Mill-1")],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-offsets", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Offsets");
+        string[] expectedSlots = ["G54", "G55", "G56", "G57", "G58", "G59"];
+        foreach (var slot in expectedSlots)
+            builder.Folders.ShouldContain(f => f.BrowseName == slot);
+        var offsetVars = builder.Variables.Where(v =>
+            v.Info.FullName.Contains("::Offsets/")).ToList();
+        offsetVars.Count.ShouldBe(6 * 3);
+        foreach (var slot in expectedSlots)
+        foreach (var axis in new[] { "X", "Y", "Z" })
+        {
+            var fullRef = $"{Host}::Offsets/{slot}/{axis}";
+            var node = offsetVars.SingleOrDefault(v => v.Info.FullName == fullRef);
+            node.Info.DriverDataType.ShouldBe(DriverDataType.Float64);
+            node.Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+        }
+    }
+
+    [Fact]
+    public async Task ReadAsync_serves_Tooling_and_Offsets_fields_from_cached_snapshot()
+    {
+        var fake = new ToolingAwareFakeFocasClient
+        {
+            Tooling = new FocasToolingInfo(CurrentTool: 17),
+            WorkOffsets = new FocasWorkOffsetsInfo(
+            [
+                new FocasWorkOffset("G54", X: 100.5, Y: 200.25, Z: -50.0),
+                new FocasWorkOffset("G55", X: 0,     Y: 0,      Z: 0),
+                new FocasWorkOffset("G56", X: 0,     Y: 0,      Z: 0),
+                new FocasWorkOffset("G57", X: 0,     Y: 0,      Z: 0),
+                new FocasWorkOffset("G58", X: 0,     Y: 0,      Z: 0),
+                new FocasWorkOffset("G59", X: 1,     Y: 2,      Z: 3),
+            ]),
+        };
+        var factory = new FakeFocasClientFactory { Customise = () => fake };
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = true, Interval = TimeSpan.FromMilliseconds(50) },
+        }, "drv-tooling-read", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for at least one probe tick to populate both caches.
+        await WaitForAsync(async () =>
+        {
+            var snap = (await drv.ReadAsync(
+                [$"{Host}::Tooling/CurrentTool"], CancellationToken.None)).Single();
+            return snap.StatusCode == FocasStatusMapper.Good;
+        }, TimeSpan.FromSeconds(3));
+
+        var refs = new[]
+        {
+            $"{Host}::Tooling/CurrentTool",
+            $"{Host}::Offsets/G54/X",
+            $"{Host}::Offsets/G54/Y",
+            $"{Host}::Offsets/G54/Z",
+            $"{Host}::Offsets/G59/X",
+        };
+        var snaps = await drv.ReadAsync(refs, CancellationToken.None);
+
+        snaps[0].Value.ShouldBe((short)17);
+        snaps[1].Value.ShouldBe(100.5);
+        snaps[2].Value.ShouldBe(200.25);
+        snaps[3].Value.ShouldBe(-50.0);
+        snaps[4].Value.ShouldBe(1.0);
+        foreach (var s in snaps) s.StatusCode.ShouldBe(FocasStatusMapper.Good);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task ReadAsync_returns_BadCommunicationError_when_caches_are_empty()
+    {
+        // Probe disabled — neither tooling nor offsets caches populate; the nodes
+        // still resolve as known references but report Bad until the first poll.
+        var drv = new FocasDriver(new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(Host)],
+            Tags = [],
+            Probe = new FocasProbeOptions { Enabled = false },
+        }, "drv-empty-tooling", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snaps = await drv.ReadAsync(
+            [$"{Host}::Tooling/CurrentTool", $"{Host}::Offsets/G54/X"], CancellationToken.None);
+        snaps[0].StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+        snaps[1].StatusCode.ShouldBe(FocasStatusMapper.BadCommunicationError);
+    }
+
+    [Fact]
+    public async Task FwlibFocasClient_GetTooling_and_GetWorkOffsets_return_null_when_disconnected()
+    {
+        // Construction is licence-safe (no DLL load); the unconnected client must
+        // short-circuit before P/Invoke. Returns null → driver leaves the cache
+        // untouched, matching the policy in f1a/f1b/f1c.
+        var client = new FwlibFocasClient();
+        (await client.GetToolingAsync(CancellationToken.None)).ShouldBeNull();
+        (await client.GetWorkOffsetsAsync(CancellationToken.None)).ShouldBeNull();
+    }
+
+    [Fact]
+    public void DecodeOfsbAxis_applies_decimal_point_count_like_macro_decode()
+    {
+        // Layout per fwlib32.h: int data, short dec, short unit, short disp = 10 bytes.
+        // Three axes (X=12345 / dec=3 = 12.345; Y=-500 / dec=2 = -5.00; Z=0 / dec=0 = 0).
+        var buf = new byte[80];
+        WriteAxis(buf, 0, raw: 12345, dec: 3);
+        WriteAxis(buf, 1, raw: -500,  dec: 2);
+        WriteAxis(buf, 2, raw: 0,     dec: 0);
+
+        FwlibFocasClient.DecodeOfsbAxis(buf, 0).ShouldBe(12.345, tolerance: 1e-9);
+        FwlibFocasClient.DecodeOfsbAxis(buf, 1).ShouldBe(-5.0, tolerance: 1e-9);
+        FwlibFocasClient.DecodeOfsbAxis(buf, 2).ShouldBe(0.0, tolerance: 1e-9);
+    }
+
+    private static void WriteAxis(byte[] buf, int axisIndex, int raw, short dec)
+    {
+        var offset = axisIndex * 10;
+        System.Buffers.Binary.BinaryPrimitives.WriteInt32LittleEndian(buf.AsSpan(offset, 4), raw);
+        System.Buffers.Binary.BinaryPrimitives.WriteInt16LittleEndian(buf.AsSpan(offset + 4, 2), dec);
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!await condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientDriverScaffoldTests.cs

@@ -67,6 +67,45 @@ public sealed class OpcUaClientDriverScaffoldTests
         health.LastError.ShouldNotBeNull();
     }
 
+    [Fact]
+    public void Default_subscription_tuning_matches_prior_hard_coded_values()
+    {
+        // PR #273: lifted hard-coded Subscription parameters into options; defaults MUST
+        // remain wire-identical so existing deployments see no behaviour change.
+        var subs = new OpcUaClientDriverOptions().Subscriptions;
+        subs.KeepAliveCount.ShouldBe(10);
+        subs.LifetimeCount.ShouldBe(1000u);
+        subs.MaxNotificationsPerPublish.ShouldBe(0u, "0 = unlimited per OPC UA spec");
+        subs.Priority.ShouldBe((byte)0);
+        subs.MinPublishingIntervalMs.ShouldBe(50);
+        subs.AlarmsPriority.ShouldBe((byte)1, "alarms get a higher priority than data tags so they aren't starved during bursts");
+    }
+
+    [Fact]
+    public void Subscription_defaults_are_overridable_via_options()
+    {
+        // Operators tuning a flaky-network deployment should be able to bump LifetimeCount /
+        // lower MaxNotificationsPerPublish without recompiling the driver. Verify the record
+        // is overridable end-to-end.
+        var opts = new OpcUaClientDriverOptions
+        {
+            Subscriptions = new OpcUaSubscriptionDefaults(
+                KeepAliveCount: 25,
+                LifetimeCount: 5000u,
+                MaxNotificationsPerPublish: 200u,
+                Priority: 7,
+                MinPublishingIntervalMs: 100,
+                AlarmsPriority: 9),
+        };
+
+        opts.Subscriptions.KeepAliveCount.ShouldBe(25);
+        opts.Subscriptions.LifetimeCount.ShouldBe(5000u);
+        opts.Subscriptions.MaxNotificationsPerPublish.ShouldBe(200u);
+        opts.Subscriptions.Priority.ShouldBe((byte)7);
+        opts.Subscriptions.MinPublishingIntervalMs.ShouldBe(100);
+        opts.Subscriptions.AlarmsPriority.ShouldBe((byte)9);
+    }
+
     [Fact]
     public async Task Reinitialize_against_unreachable_endpoint_re_throws()
     {

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientMonitoredTagSpecTests.cs

@@ -0,0 +1,175 @@
+using Opc.Ua;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Unit tests for <see cref="MonitoredTagSpec"/> -> SDK <c>MonitoredItem</c> mapping.
+///     Assertion-only — no live SDK session required, so the tests run on every CI without
+///     a real OPC UA server fixture.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientMonitoredTagSpecTests
+{
+    private static readonly NodeId SampleNodeId = new("Demo", 2);
+
+    [Fact]
+    public void BuildMonitoredItem_with_all_defaults_matches_legacy_hard_coded_values()
+    {
+        // Spec with every per-tag knob null should behave identically to the legacy
+        // string-only SubscribeAsync path: Reporting / SamplingInterval=publishInterval /
+        // QueueSize=1 / DiscardOldest=true / no filter.
+        var spec = new MonitoredTagSpec("ns=2;s=Demo");
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+
+        item.SamplingInterval.ShouldBe(250);
+        item.QueueSize.ShouldBe(1u);
+        item.DiscardOldest.ShouldBeTrue();
+        item.MonitoringMode.ShouldBe(MonitoringMode.Reporting);
+        item.Filter.ShouldBeNull();
+        item.Handle.ShouldBe("ns=2;s=Demo",
+            "the tag string is routed through Handle so the Notification callback can identify the changed tag without re-parsing DisplayName");
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_applies_per_tag_sampling_interval_independent_of_publish_interval()
+    {
+        // Per-tag SamplingInterval lets the server sample faster than it publishes — useful
+        // for events that change between publish ticks. If the spec sets it explicitly, the
+        // mapping uses that value, not the publish-interval default.
+        var spec = new MonitoredTagSpec("ns=2;s=Fast", SamplingIntervalMs: 50);
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 1000);
+        item.SamplingInterval.ShouldBe(50);
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_applies_queue_size_and_discard_oldest_overrides()
+    {
+        var spec = new MonitoredTagSpec("ns=2;s=DeepQueue", QueueSize: 100, DiscardOldest: false);
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        item.QueueSize.ShouldBe(100u);
+        item.DiscardOldest.ShouldBeFalse(
+            "discard-oldest=false preserves earliest values — useful for audit-trail subscriptions where the first overflow sample is the most diagnostic");
+    }
+
+    [Theory]
+    [InlineData(SubscriptionMonitoringMode.Disabled, MonitoringMode.Disabled)]
+    [InlineData(SubscriptionMonitoringMode.Sampling, MonitoringMode.Sampling)]
+    [InlineData(SubscriptionMonitoringMode.Reporting, MonitoringMode.Reporting)]
+    public void BuildMonitoredItem_maps_each_monitoring_mode(SubscriptionMonitoringMode input, MonitoringMode expected)
+    {
+        var spec = new MonitoredTagSpec("ns=2;s=Mode", MonitoringMode: input);
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        item.MonitoringMode.ShouldBe(expected);
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_with_absolute_deadband_emits_DataChangeFilter()
+    {
+        var spec = new MonitoredTagSpec(
+            "ns=2;s=Analog",
+            DataChangeFilter: new DataChangeFilterSpec(
+                Core.Abstractions.DataChangeTrigger.StatusValue,
+                Core.Abstractions.DeadbandType.Absolute,
+                DeadbandValue: 0.5));
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        var filter = item.Filter.ShouldBeOfType<DataChangeFilter>();
+        filter.Trigger.ShouldBe(Opc.Ua.DataChangeTrigger.StatusValue);
+        filter.DeadbandType.ShouldBe((uint)Opc.Ua.DeadbandType.Absolute);
+        filter.DeadbandValue.ShouldBe(0.5);
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_with_percent_deadband_emits_percent_filter()
+    {
+        // PercentDeadband is calculated server-side as a fraction of EURange; the driver
+        // emits the filter unconditionally and lets the server return BadFilterNotAllowed
+        // if EURange isn't set on the variable. SubscribeAsync's catch-block swallows that
+        // status so other items in the batch still get created.
+        var spec = new MonitoredTagSpec(
+            "ns=2;s=Pct",
+            DataChangeFilter: new DataChangeFilterSpec(
+                Core.Abstractions.DataChangeTrigger.StatusValueTimestamp,
+                Core.Abstractions.DeadbandType.Percent,
+                DeadbandValue: 5.0));
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        var filter = item.Filter.ShouldBeOfType<DataChangeFilter>();
+        filter.Trigger.ShouldBe(Opc.Ua.DataChangeTrigger.StatusValueTimestamp);
+        filter.DeadbandType.ShouldBe((uint)Opc.Ua.DeadbandType.Percent);
+        filter.DeadbandValue.ShouldBe(5.0);
+    }
+
+    [Theory]
+    [InlineData(Core.Abstractions.DataChangeTrigger.Status, Opc.Ua.DataChangeTrigger.Status)]
+    [InlineData(Core.Abstractions.DataChangeTrigger.StatusValue, Opc.Ua.DataChangeTrigger.StatusValue)]
+    [InlineData(Core.Abstractions.DataChangeTrigger.StatusValueTimestamp, Opc.Ua.DataChangeTrigger.StatusValueTimestamp)]
+    public void MapTrigger_round_trips_each_enum_value(
+        Core.Abstractions.DataChangeTrigger input, Opc.Ua.DataChangeTrigger expected)
+        => OpcUaClientDriver.MapTrigger(input).ShouldBe(expected);
+
+    [Theory]
+    [InlineData(Core.Abstractions.DeadbandType.None, Opc.Ua.DeadbandType.None)]
+    [InlineData(Core.Abstractions.DeadbandType.Absolute, Opc.Ua.DeadbandType.Absolute)]
+    [InlineData(Core.Abstractions.DeadbandType.Percent, Opc.Ua.DeadbandType.Percent)]
+    public void MapDeadbandType_round_trips_each_enum_value(
+        Core.Abstractions.DeadbandType input, Opc.Ua.DeadbandType expected)
+        => OpcUaClientDriver.MapDeadbandType(input).ShouldBe(expected);
+
+    [Fact]
+    public async Task DefaultInterfaceImplementation_routes_through_legacy_overload()
+    {
+        // ISubscribable's default interface impl of the per-tag overload delegates to the
+        // simple-string overload, ignoring per-tag knobs. Drivers that DON'T override the
+        // new overload (Modbus / S7 / Galaxy / TwinCAT / FOCAS / AbCip / AbLegacy) still
+        // accept MonitoredTagSpec lists and just pass through the tag names — back-compat
+        // for ISubscribable consumers.
+        var stub = new StubSubscribableDriver();
+        var specs = new[]
+        {
+            new MonitoredTagSpec("Tag1", SamplingIntervalMs: 50, QueueSize: 5),
+            new MonitoredTagSpec("Tag2", DataChangeFilter: new DataChangeFilterSpec(
+                Core.Abstractions.DataChangeTrigger.StatusValue,
+                Core.Abstractions.DeadbandType.Absolute,
+                1.0)),
+        };
+
+        ISubscribable iface = stub;
+        _ = await iface.SubscribeAsync(specs, TimeSpan.FromMilliseconds(250), TestContext.Current.CancellationToken);
+
+        stub.LastTagNames.ShouldBe(["Tag1", "Tag2"]);
+        stub.LastPublishingInterval.ShouldBe(TimeSpan.FromMilliseconds(250));
+    }
+
+    /// <summary>
+    ///     Test-double <see cref="ISubscribable"/> that records whatever the legacy
+    ///     <c>SubscribeAsync(IReadOnlyList&lt;string&gt;, ...)</c> overload was called with.
+    ///     Used to verify the default-impl per-tag overload routes correctly without needing
+    ///     a real OPC UA session.
+    /// </summary>
+    private sealed class StubSubscribableDriver : ISubscribable
+    {
+        public IReadOnlyList<string>? LastTagNames { get; private set; }
+        public TimeSpan LastPublishingInterval { get; private set; }
+
+        public Task<ISubscriptionHandle> SubscribeAsync(
+            IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+        {
+            LastTagNames = fullReferences;
+            LastPublishingInterval = publishingInterval;
+            return Task.FromResult<ISubscriptionHandle>(new StubHandle());
+        }
+
+        public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken) => Task.CompletedTask;
+
+#pragma warning disable CS0067 // event never used — the test only asserts the SubscribeAsync call routing
+        public event EventHandler<DataChangeEventArgs>? OnDataChange;
+#pragma warning restore CS0067
+    }
+
+    private sealed record StubHandle() : ISubscriptionHandle
+    {
+        public string DiagnosticId => "stub";
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientOperationLimitsTests.cs

@@ -0,0 +1,139 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Unit tests for the OperationLimits chunking surface (PR #275 / opcuaclient-3). Focused
+///     on the static <see cref="OpcUaClientDriver.ChunkBy{T}"/> helper + the
+///     <see cref="OpcUaClientDriver.OperationLimitsCache"/> sentinel semantics. Live
+///     end-to-end tests against an in-process server land in the integration suite.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientOperationLimitsTests
+{
+    [Fact]
+    public void ChunkBy_with_cap_5_splits_12_items_into_3_slices_of_5_5_2()
+    {
+        // The PR-3 acceptance scenario: server advertises MaxNodesPerRead=5, client batches a
+        // 12-tag read; driver must issue exactly 3 wire calls of sizes 5/5/2 in order.
+        var input = Enumerable.Range(0, 12).ToArray();
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: 5).ToArray();
+
+        slices.Length.ShouldBe(3);
+        slices[0].Count.ShouldBe(5);
+        slices[1].Count.ShouldBe(5);
+        slices[2].Count.ShouldBe(2);
+        // Order + offsets must reflect the original sequence — chunking must not reorder
+        // tags, otherwise the indexMap ↔ result-index alignment breaks.
+        slices[0].ShouldBe(new[] { 0, 1, 2, 3, 4 });
+        slices[1].ShouldBe(new[] { 5, 6, 7, 8, 9 });
+        slices[2].ShouldBe(new[] { 10, 11 });
+    }
+
+    [Fact]
+    public void ChunkBy_with_null_cap_yields_single_slice_no_chunking()
+    {
+        // cap=null is the "fetch hasn't completed" / "server reports 0 = no limit" sentinel.
+        // Both must collapse to a single SDK call so the wire path doesn't change when the
+        // server doesn't impose a cap.
+        var input = Enumerable.Range(0, 12).ToArray();
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: null).ToArray();
+
+        slices.Length.ShouldBe(1, "null cap means no chunking — single SDK call");
+        slices[0].Count.ShouldBe(12);
+    }
+
+    [Fact]
+    public void ChunkBy_with_zero_cap_yields_single_slice_no_chunking()
+    {
+        // OPC UA Part 5: 0 is the wire-level "no limit" sentinel. NormalizeLimit folds it
+        // into null upstream of ChunkBy, but the chunker itself must also treat 0 as
+        // no-chunking — defence in depth in case a caller bypasses NormalizeLimit.
+        var input = Enumerable.Range(0, 7).ToArray();
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: 0).ToArray();
+
+        slices.Length.ShouldBe(1);
+        slices[0].Count.ShouldBe(7);
+    }
+
+    [Fact]
+    public void ChunkBy_with_cap_larger_than_input_yields_single_slice()
+    {
+        var input = new[] { 1, 2, 3 };
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: 100).ToArray();
+
+        slices.Length.ShouldBe(1);
+        slices[0].Count.ShouldBe(3);
+    }
+
+    [Fact]
+    public void ChunkBy_with_empty_input_yields_no_slices()
+    {
+        // Empty batch must short-circuit before the wire call — saves a round-trip and
+        // matches the !toSend.Count == 0 guard in the driver.
+        var input = Array.Empty<int>();
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: 5).ToArray();
+
+        slices.Length.ShouldBe(0);
+    }
+
+    [Fact]
+    public void ChunkBy_with_cap_equal_to_input_size_yields_single_slice()
+    {
+        // Edge case: exactly N items at cap N. Must NOT produce an extra empty slice.
+        var input = Enumerable.Range(0, 5).ToArray();
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: 5).ToArray();
+
+        slices.Length.ShouldBe(1);
+        slices[0].Count.ShouldBe(5);
+    }
+
+    [Fact]
+    public void ChunkBy_with_cap_1_splits_each_item_into_its_own_slice()
+    {
+        // Pathological cap — degrades to N wire calls. Verifies the chunker handles the
+        // boundary cleanly without off-by-one.
+        var input = new[] { 10, 20, 30 };
+
+        var slices = OpcUaClientDriver.ChunkBy<int>(input, cap: 1).ToArray();
+
+        slices.Length.ShouldBe(3);
+        slices[0].ShouldBe(new[] { 10 });
+        slices[1].ShouldBe(new[] { 20 });
+        slices[2].ShouldBe(new[] { 30 });
+    }
+
+    [Fact]
+    public void OperationLimitsCache_records_all_four_caps_as_nullable_uint()
+    {
+        // The cache surfaces the four limits the driver chunks against. Storing as uint?
+        // lets the chunker distinguish "not yet fetched" / "no limit" (null) from "limit=N".
+        var cache = new OpcUaClientDriver.OperationLimitsCache(
+            MaxNodesPerRead: 100u,
+            MaxNodesPerWrite: 50u,
+            MaxNodesPerBrowse: null,
+            MaxNodesPerHistoryReadData: 10u);
+
+        cache.MaxNodesPerRead.ShouldBe(100u);
+        cache.MaxNodesPerWrite.ShouldBe(50u);
+        cache.MaxNodesPerBrowse.ShouldBeNull();
+        cache.MaxNodesPerHistoryReadData.ShouldBe(10u);
+    }
+
+    [Fact]
+    public void Driver_starts_with_no_cached_OperationLimits()
+    {
+        // Pre-init / pre-first-batch state: cache is null so callers fall through to
+        // single-call behaviour. Lazy fetch happens on the first ReadAsync/WriteAsync.
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-cache-init");
+
+        drv.OperationLimitsForTest.ShouldBeNull();
+    }
+}