TwinCAT PR 2 — IReadable + IWritable. ITwinCATClient + ITwinCATClientFactory abstraction — one client per AMS target, reused across reads/writes/probes. Shape differs from AbCip/AbLegacy where libplctag handles are per-tag — TwinCAT's AdsClient is a single connection with symbolic reads/writes issued against it, so the abstraction is coarser. AdsTwinCATClient is the default implementation wrapping Beckhoff.TwinCAT.Ads's AdsClient — ConnectAsync calls AdsClient.Connect(AmsNetId.Parse(netId), port) after setting Timeout in ms; ReadValueAsync dispatches TwinCATDataType to the CLR Type via MapToClrType (bool/sbyte/byte/short/ushort/int/uint/long/ulong/float/double/string/uint for time types) and calls AdsClient.ReadValueAsync(symbol, type, ct) which returns ResultAnyValue; unwraps .Value + .ErrorCode and maps non-NoError codes via TwinCATStatusMapper.MapAdsError. BOOL-within-word reads extract the bit after the underlying word read using ExtractBit over short/ushort/int/uint/long/ulong. WriteValueAsync converts the boxed value via ConvertForWrite (Convert.ToXxx per type) then calls AdsClient.WriteValueAsync returning ResultWrite; checks .ErrorCode for status mapping. BOOL-within-word writes throw NotSupportedException with a pointer to task #181 — same RMW gap as Modbus BitInRegister / AbCip BOOL-in-DINT / AbLegacy bit-within-N-file. ProbeAsync calls AdsClient.ReadStateAsync + checks AdsErrorCode.NoError. TwinCATDriver implements IReadable + IWritable — per-device ITwinCATClient cached in DeviceState.Client, lazy-connected on first read/write via EnsureConnectedAsync, connect-failure path disposes + clears the client so next call re-attempts cleanly. ReadAsync ordered-snapshot pattern matching AbCip/AbLegacy: unknown ref → BadNodeIdUnknown, unknown device → BadNodeIdUnknown, OperationCanceledException rethrow, any other exception → BadCommunicationError + Degraded health. WriteAsync similar — non-Writable tag → BadNotWritable upfront, NotSupportedException → BadNotSupported, FormatException/InvalidCastException (guard pattern) → BadTypeMismatch, OverflowException → BadOutOfRange, generic → BadCommunicationError. Symbol name resolution goes through TwinCATSymbolPath.TryParse(def.SymbolPath) with fallback to the raw def.SymbolPath if the path doesn't parse — the Beckhoff AdsClient handles the final validation at wire time. ShutdownAsync disposes each device's client. 14 new unit tests in TwinCATReadWriteTests using FakeTwinCATClient + FakeTwinCATClientFactory — unknown ref → BadNodeIdUnknown, successful DInt read with Good status + captured value + IsConnected=true after EnsureConnectedAsync, repeat reads reuse the connection (one Connect + multiple reads), ADS error code mapping via FakeTwinCATClient.ReadStatuses, read exception → BadCommunicationError + Degraded health, connect exception disposes the client, batched reads preserve order across DInt/Real/String types, non-Writable rejection, successful write logs symbol+type+value+bit for test inspection, write status-code mapping, write exception → BadCommunicationError, batch preserves order across success/non-writable/unknown, cancellation propagation, ShutdownAsync disposes the client. Total TwinCAT unit tests now 75/75 passing (+14 from PR 1's 61); full solution builds 0 errors; Modbus / AbCip / AbLegacy / other drivers untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ZB.MOM.WW.OtOpcUa.slnx

@@ -12,6 +12,7 @@
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.S7/ZB.MOM.WW.OtOpcUa.Driver.S7.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/ZB.MOM.WW.OtOpcUa.Driver.AbCip.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.csproj"/>
+        <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.Shared/ZB.MOM.WW.OtOpcUa.Client.Shared.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.CLI/ZB.MOM.WW.OtOpcUa.Client.CLI.csproj"/>
@@ -33,6 +34,7 @@
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests.csproj"/>
+        <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.IntegrationTests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.IntegrationTests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests.csproj"/>

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

@@ -8,15 +8,20 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
 ///     <see cref="IDriver"/> only at PR 1 time; read / write / discovery / subscribe / probe /
 ///     host-resolver capabilities ship in PRs 2 and 3.
 /// </summary>
-public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, IDisposable, IAsyncDisposable
+public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable,
+    IHostConnectivityProbe, IPerCallHostResolver, IDisposable, IAsyncDisposable
 {
     private readonly AbLegacyDriverOptions _options;
     private readonly string _driverInstanceId;
     private readonly IAbLegacyTagFactory _tagFactory;
+    private readonly PollGroupEngine _poll;
     private readonly Dictionary<string, DeviceState> _devices = new(StringComparer.OrdinalIgnoreCase);
     private readonly Dictionary<string, AbLegacyTagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
     private DriverHealth _health = new(DriverState.Unknown, null, null);
 
+    public event EventHandler<DataChangeEventArgs>? OnDataChange;
+    public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;
+
     public AbLegacyDriver(AbLegacyDriverOptions options, string driverInstanceId,
         IAbLegacyTagFactory? tagFactory = null)
     {
@@ -24,6 +29,10 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, IDisposable,
         _options = options;
         _driverInstanceId = driverInstanceId;
         _tagFactory = tagFactory ?? new LibplctagLegacyTagFactory();
+        _poll = new PollGroupEngine(
+            reader: ReadAsync,
+            onChange: (handle, tagRef, snapshot) =>
+                OnDataChange?.Invoke(this, new DataChangeEventArgs(handle, tagRef, snapshot)));
     }
 
     public string DriverInstanceId => _driverInstanceId;
@@ -43,6 +52,17 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, IDisposable,
                 _devices[device.HostAddress] = new DeviceState(addr, device, profile);
             }
             foreach (var tag in _options.Tags) _tagsByName[tag.Name] = tag;
+
+            // Probe loops — one per device when enabled + probe address configured.
+            if (_options.Probe.Enabled && !string.IsNullOrWhiteSpace(_options.Probe.ProbeAddress))
+            {
+                foreach (var state in _devices.Values)
+                {
+                    state.ProbeCts = new CancellationTokenSource();
+                    var ct = state.ProbeCts.Token;
+                    _ = Task.Run(() => ProbeLoopAsync(state, ct), ct);
+                }
+            }
             _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
         }
         catch (Exception ex)
@@ -59,13 +79,19 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, IDisposable,
         await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
     }
 
-    public Task ShutdownAsync(CancellationToken cancellationToken)
+    public async Task ShutdownAsync(CancellationToken cancellationToken)
     {
-        foreach (var state in _devices.Values) state.DisposeRuntimes();
+        await _poll.DisposeAsync().ConfigureAwait(false);
+        foreach (var state in _devices.Values)
+        {
+            try { state.ProbeCts?.Cancel(); } catch { }
+            state.ProbeCts?.Dispose();
+            state.ProbeCts = null;
+            state.DisposeRuntimes();
+        }
         _devices.Clear();
         _tagsByName.Clear();
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
-        return Task.CompletedTask;
     }
 
     public DriverHealth GetHealth() => _health;
@@ -194,6 +220,117 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, IDisposable,
         return results;
     }
 
+    // ---- ITagDiscovery ----
+
+    public Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+        var root = builder.Folder("AbLegacy", "AbLegacy");
+        foreach (var device in _options.Devices)
+        {
+            var label = device.DeviceName ?? device.HostAddress;
+            var deviceFolder = root.Folder(device.HostAddress, label);
+            var tagsForDevice = _options.Tags.Where(t =>
+                string.Equals(t.DeviceHostAddress, device.HostAddress, StringComparison.OrdinalIgnoreCase));
+            foreach (var tag in tagsForDevice)
+            {
+                deviceFolder.Variable(tag.Name, tag.Name, new DriverAttributeInfo(
+                    FullName: tag.Name,
+                    DriverDataType: tag.DataType.ToDriverDataType(),
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: tag.Writable
+                        ? SecurityClassification.Operate
+                        : SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false,
+                    WriteIdempotent: tag.WriteIdempotent));
+            }
+        }
+        return Task.CompletedTask;
+    }
+
+    // ---- ISubscribable (polling overlay via shared engine) ----
+
+    public Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken) =>
+        Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
+
+    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    {
+        _poll.Unsubscribe(handle);
+        return Task.CompletedTask;
+    }
+
+    // ---- IHostConnectivityProbe ----
+
+    public IReadOnlyList<HostConnectivityStatus> GetHostStatuses() =>
+        [.. _devices.Values.Select(s => new HostConnectivityStatus(s.Options.HostAddress, s.HostState, s.HostStateChangedUtc))];
+
+    private async Task ProbeLoopAsync(DeviceState state, CancellationToken ct)
+    {
+        var probeParams = new AbLegacyTagCreateParams(
+            Gateway: state.ParsedAddress.Gateway,
+            Port: state.ParsedAddress.Port,
+            CipPath: state.ParsedAddress.CipPath,
+            LibplctagPlcAttribute: state.Profile.LibplctagPlcAttribute,
+            TagName: _options.Probe.ProbeAddress!,
+            Timeout: _options.Probe.Timeout);
+
+        IAbLegacyTagRuntime? probeRuntime = null;
+        while (!ct.IsCancellationRequested)
+        {
+            var success = false;
+            try
+            {
+                probeRuntime ??= _tagFactory.Create(probeParams);
+                if (!state.ProbeInitialized)
+                {
+                    await probeRuntime.InitializeAsync(ct).ConfigureAwait(false);
+                    state.ProbeInitialized = true;
+                }
+                await probeRuntime.ReadAsync(ct).ConfigureAwait(false);
+                success = probeRuntime.GetStatus() == 0;
+            }
+            catch (OperationCanceledException) when (ct.IsCancellationRequested) { break; }
+            catch
+            {
+                try { probeRuntime?.Dispose(); } catch { }
+                probeRuntime = null;
+                state.ProbeInitialized = false;
+            }
+
+            TransitionDeviceState(state, success ? HostState.Running : HostState.Stopped);
+
+            try { await Task.Delay(_options.Probe.Interval, ct).ConfigureAwait(false); }
+            catch (OperationCanceledException) { break; }
+        }
+        try { probeRuntime?.Dispose(); } catch { }
+    }
+
+    private void TransitionDeviceState(DeviceState state, HostState newState)
+    {
+        HostState old;
+        lock (state.ProbeLock)
+        {
+            old = state.HostState;
+            if (old == newState) return;
+            state.HostState = newState;
+            state.HostStateChangedUtc = DateTime.UtcNow;
+        }
+        OnHostStatusChanged?.Invoke(this,
+            new HostStatusChangedEventArgs(state.Options.HostAddress, old, newState));
+    }
+
+    // ---- IPerCallHostResolver ----
+
+    public string ResolveHost(string fullReference)
+    {
+        if (_tagsByName.TryGetValue(fullReference, out var def))
+            return def.DeviceHostAddress;
+        return _options.Devices.FirstOrDefault()?.HostAddress ?? DriverInstanceId;
+    }
+
     private async Task<IAbLegacyTagRuntime> EnsureTagRuntimeAsync(
         DeviceState device, AbLegacyTagDefinition def, CancellationToken ct)
     {
@@ -237,6 +374,12 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, IDisposable,
         public Dictionary<string, IAbLegacyTagRuntime> Runtimes { get; } =
             new(StringComparer.OrdinalIgnoreCase);
 
+        public object ProbeLock { get; } = new();
+        public HostState HostState { get; set; } = HostState.Unknown;
+        public DateTime HostStateChangedUtc { get; set; } = DateTime.UtcNow;
+        public CancellationTokenSource? ProbeCts { get; set; }
+        public bool ProbeInitialized { get; set; }
+
         public void DisposeRuntimes()
         {
             foreach (var r in Runtimes.Values) r.Dispose();

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/AdsTwinCATClient.cs

@@ -0,0 +1,154 @@
+using TwinCAT.Ads;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     Default <see cref="ITwinCATClient"/> backed by Beckhoff's <see cref="AdsClient"/>.
+///     One instance per AMS target; reused across reads / writes / probes.
+/// </summary>
+/// <remarks>
+///     <para>Wire behavior depends on a reachable AMS router — on Windows the router comes
+///     from TwinCAT XAR; elsewhere from the <c>Beckhoff.TwinCAT.Ads.TcpRouter</c> package
+///     hosted by the server process. Neither is built-in here; deployment wires one in.</para>
+///
+///     <para>Error mapping — ADS error codes surface through <see cref="AdsErrorException"/>
+///     and get translated to OPC UA status codes via <see cref="TwinCATStatusMapper.MapAdsError"/>.</para>
+/// </remarks>
+internal sealed class AdsTwinCATClient : ITwinCATClient
+{
+    private readonly AdsClient _client = new();
+
+    public bool IsConnected => _client.IsConnected;
+
+    public Task ConnectAsync(TwinCATAmsAddress address, TimeSpan timeout, CancellationToken cancellationToken)
+    {
+        if (_client.IsConnected) return Task.CompletedTask;
+        _client.Timeout = (int)Math.Max(1_000, timeout.TotalMilliseconds);
+        var netId = AmsNetId.Parse(address.NetId);
+        _client.Connect(netId, address.Port);
+        return Task.CompletedTask;
+    }
+
+    public async Task<(object? value, uint status)> ReadValueAsync(
+        string symbolPath,
+        TwinCATDataType type,
+        int? bitIndex,
+        CancellationToken cancellationToken)
+    {
+        try
+        {
+            var clrType = MapToClrType(type);
+            var result = await _client.ReadValueAsync(symbolPath, clrType, cancellationToken)
+                .ConfigureAwait(false);
+
+            if (result.ErrorCode != AdsErrorCode.NoError)
+                return (null, TwinCATStatusMapper.MapAdsError((uint)result.ErrorCode));
+
+            var value = result.Value;
+            if (bitIndex is int bit && type == TwinCATDataType.Bool && value is not bool)
+                value = ExtractBit(value, bit);
+
+            return (value, TwinCATStatusMapper.Good);
+        }
+        catch (AdsErrorException ex)
+        {
+            return (null, TwinCATStatusMapper.MapAdsError((uint)ex.ErrorCode));
+        }
+    }
+
+    public async Task<uint> WriteValueAsync(
+        string symbolPath,
+        TwinCATDataType type,
+        int? bitIndex,
+        object? value,
+        CancellationToken cancellationToken)
+    {
+        if (bitIndex is int && type == TwinCATDataType.Bool)
+            throw new NotSupportedException(
+                "BOOL-within-word writes require read-modify-write; tracked in task #181.");
+
+        try
+        {
+            var converted = ConvertForWrite(type, value);
+            var result = await _client.WriteValueAsync(symbolPath, converted, cancellationToken)
+                .ConfigureAwait(false);
+            return result.ErrorCode == AdsErrorCode.NoError
+                ? TwinCATStatusMapper.Good
+                : TwinCATStatusMapper.MapAdsError((uint)result.ErrorCode);
+        }
+        catch (AdsErrorException ex)
+        {
+            return TwinCATStatusMapper.MapAdsError((uint)ex.ErrorCode);
+        }
+    }
+
+    public async Task<bool> ProbeAsync(CancellationToken cancellationToken)
+    {
+        try
+        {
+            var state = await _client.ReadStateAsync(cancellationToken).ConfigureAwait(false);
+            return state.ErrorCode == AdsErrorCode.NoError;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    public void Dispose() => _client.Dispose();
+
+    private static Type MapToClrType(TwinCATDataType type) => type switch
+    {
+        TwinCATDataType.Bool => typeof(bool),
+        TwinCATDataType.SInt => typeof(sbyte),
+        TwinCATDataType.USInt => typeof(byte),
+        TwinCATDataType.Int => typeof(short),
+        TwinCATDataType.UInt => typeof(ushort),
+        TwinCATDataType.DInt => typeof(int),
+        TwinCATDataType.UDInt => typeof(uint),
+        TwinCATDataType.LInt => typeof(long),
+        TwinCATDataType.ULInt => typeof(ulong),
+        TwinCATDataType.Real => typeof(float),
+        TwinCATDataType.LReal => typeof(double),
+        TwinCATDataType.String or TwinCATDataType.WString => typeof(string),
+        TwinCATDataType.Time or TwinCATDataType.Date
+            or TwinCATDataType.DateTime or TwinCATDataType.TimeOfDay => typeof(uint),
+        _ => typeof(int),
+    };
+
+    private static object ConvertForWrite(TwinCATDataType type, object? value) => type switch
+    {
+        TwinCATDataType.Bool => Convert.ToBoolean(value),
+        TwinCATDataType.SInt => Convert.ToSByte(value),
+        TwinCATDataType.USInt => Convert.ToByte(value),
+        TwinCATDataType.Int => Convert.ToInt16(value),
+        TwinCATDataType.UInt => Convert.ToUInt16(value),
+        TwinCATDataType.DInt => Convert.ToInt32(value),
+        TwinCATDataType.UDInt => Convert.ToUInt32(value),
+        TwinCATDataType.LInt => Convert.ToInt64(value),
+        TwinCATDataType.ULInt => Convert.ToUInt64(value),
+        TwinCATDataType.Real => Convert.ToSingle(value),
+        TwinCATDataType.LReal => Convert.ToDouble(value),
+        TwinCATDataType.String or TwinCATDataType.WString => Convert.ToString(value) ?? string.Empty,
+        TwinCATDataType.Time or TwinCATDataType.Date
+            or TwinCATDataType.DateTime or TwinCATDataType.TimeOfDay => Convert.ToUInt32(value),
+        _ => throw new NotSupportedException($"TwinCATDataType {type} not writable."),
+    };
+
+    private static bool ExtractBit(object? rawWord, int bit) => rawWord switch
+    {
+        short s => (s & (1 << bit)) != 0,
+        ushort us => (us & (1 << bit)) != 0,
+        int i => (i & (1 << bit)) != 0,
+        uint ui => (ui & (1u << bit)) != 0,
+        long l => (l & (1L << bit)) != 0,
+        ulong ul => (ul & (1UL << bit)) != 0,
+        _ => false,
+    };
+}
+
+/// <summary>Default <see cref="ITwinCATClientFactory"/> — one <see cref="AdsTwinCATClient"/> per call.</summary>
+internal sealed class AdsTwinCATClientFactory : ITwinCATClientFactory
+{
+    public ITwinCATClient Create() => new AdsTwinCATClient();
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/ITwinCATClient.cs

@@ -0,0 +1,55 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     Wire-layer abstraction over one connection to a TwinCAT AMS target. One instance per
+///     <see cref="TwinCATAmsAddress"/>; reused across reads / writes / probes for the device.
+///     Tests swap in a fake via <see cref="ITwinCATClientFactory"/>.
+/// </summary>
+/// <remarks>
+///     Unlike libplctag-backed drivers where one native handle exists per tag, TwinCAT's
+///     AdsClient is one connection per target with symbolic reads / writes issued against it.
+///     The abstraction reflects that — single <see cref="ConnectAsync"/>, many
+///     <see cref="ReadValueAsync"/> / <see cref="WriteValueAsync"/> calls.
+/// </remarks>
+public interface ITwinCATClient : IDisposable
+{
+    /// <summary>Establish the AMS connection. Idempotent — subsequent calls are no-ops when already connected.</summary>
+    Task ConnectAsync(TwinCATAmsAddress address, TimeSpan timeout, CancellationToken cancellationToken);
+
+    /// <summary>True when the AMS router + target both accept commands.</summary>
+    bool IsConnected { get; }
+
+    /// <summary>
+    ///     Read a symbolic value. Returns a boxed .NET value matching the requested
+    ///     <paramref name="type"/>, or <c>null</c> when the read produced no data; the
+    ///     <c>status</c> tuple member carries the mapped OPC UA status (0 = Good).
+    /// </summary>
+    Task<(object? value, uint status)> ReadValueAsync(
+        string symbolPath,
+        TwinCATDataType type,
+        int? bitIndex,
+        CancellationToken cancellationToken);
+
+    /// <summary>
+    ///     Write a symbolic value. Returns the mapped OPC UA status for the operation
+    ///     (0 = Good, non-zero = error mapped via <see cref="TwinCATStatusMapper"/>).
+    /// </summary>
+    Task<uint> WriteValueAsync(
+        string symbolPath,
+        TwinCATDataType type,
+        int? bitIndex,
+        object? value,
+        CancellationToken cancellationToken);
+
+    /// <summary>
+    ///     Cheap health probe — returns <c>true</c> when the target's AMS state is reachable.
+    ///     Used by <see cref="Core.Abstractions.IHostConnectivityProbe"/>'s probe loop.
+    /// </summary>
+    Task<bool> ProbeAsync(CancellationToken cancellationToken);
+}
+
+/// <summary>Factory for <see cref="ITwinCATClient"/>s. One client per device.</summary>
+public interface ITwinCATClientFactory
+{
+    ITwinCATClient Create();
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATAmsAddress.cs

@@ -0,0 +1,64 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     Parsed TwinCAT AMS address — six-octet AMS Net ID + port. Canonical form
+///     <c>ads://{netId}:{port}</c> where <c>netId</c> is five-dot-separated octets (six of them)
+///     and <c>port</c> is the AMS service port (851 = TC3 PLC runtime 1, 852 = runtime 2, 801 /
+///     811 / 821 = TC2 PLC runtimes, 10000 = system service, etc.).
+/// </summary>
+/// <remarks>
+///     Format examples:
+///     <list type="bullet">
+///         <item><c>ads://5.23.91.23.1.1:851</c> — remote TC3 runtime</item>
+///         <item><c>ads://5.23.91.23.1.1</c> — defaults to port 851 (TC3 PLC runtime 1)</item>
+///         <item><c>ads://127.0.0.1.1.1:851</c> — local loopback (when the router is local)</item>
+///     </list>
+///     <para>AMS Net ID is NOT an IP — it's a Beckhoff-specific identifier that the router
+///     translates to an IP route. Typically the first four octets match the host's IPv4 and
+///     the last two are <c>.1.1</c>, but the router can be configured otherwise.</para>
+/// </remarks>
+public sealed record TwinCATAmsAddress(string NetId, int Port)
+{
+    /// <summary>Default AMS port — TC3 PLC runtime 1.</summary>
+    public const int DefaultPlcPort = 851;
+
+    public override string ToString() => Port == DefaultPlcPort
+        ? $"ads://{NetId}"
+        : $"ads://{NetId}:{Port}";
+
+    public static TwinCATAmsAddress? TryParse(string? value)
+    {
+        if (string.IsNullOrWhiteSpace(value)) return null;
+        const string prefix = "ads://";
+        if (!value.StartsWith(prefix, StringComparison.OrdinalIgnoreCase)) return null;
+
+        var body = value[prefix.Length..];
+        if (string.IsNullOrEmpty(body)) return null;
+
+        var colonIdx = body.LastIndexOf(':');
+        string netId;
+        var port = DefaultPlcPort;
+        if (colonIdx >= 0)
+        {
+            netId = body[..colonIdx];
+            if (!int.TryParse(body[(colonIdx + 1)..], out port) || port is <= 0 or > 65535)
+                return null;
+        }
+        else
+        {
+            netId = body;
+        }
+
+        if (!IsValidNetId(netId)) return null;
+        return new TwinCATAmsAddress(netId, port);
+    }
+
+    private static bool IsValidNetId(string netId)
+    {
+        var parts = netId.Split('.');
+        if (parts.Length != 6) return false;
+        foreach (var p in parts)
+            if (!byte.TryParse(p, out _)) return false;
+        return true;
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDataType.cs

@@ -0,0 +1,49 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     TwinCAT / IEC 61131-3 atomic data types. Wider type surface than Logix because IEC adds
+///     <c>WSTRING</c> (UTF-16) and <c>TIME</c>/<c>DATE</c>/<c>DT</c>/<c>TOD</c> variants.
+/// </summary>
+public enum TwinCATDataType
+{
+    Bool,
+    SInt,   // signed 8-bit
+    USInt,  // unsigned 8-bit
+    Int,    // signed 16-bit
+    UInt,   // unsigned 16-bit
+    DInt,   // signed 32-bit
+    UDInt,  // unsigned 32-bit
+    LInt,   // signed 64-bit
+    ULInt,  // unsigned 64-bit
+    Real,   // 32-bit IEEE-754
+    LReal,  // 64-bit IEEE-754
+    String, // ASCII string
+    WString,// UTF-16 string
+    Time,   // TIME — ms since epoch of day, stored as UDINT
+    Date,   // DATE — days since 1970-01-01, stored as UDINT
+    DateTime, // DT — seconds since 1970-01-01, stored as UDINT
+    TimeOfDay,// TOD — ms since midnight, stored as UDINT
+    /// <summary>UDT / FB instance. Resolved per member at discovery time.</summary>
+    Structure,
+}
+
+public static class TwinCATDataTypeExtensions
+{
+    public static DriverDataType ToDriverDataType(this TwinCATDataType t) => t switch
+    {
+        TwinCATDataType.Bool => DriverDataType.Boolean,
+        TwinCATDataType.SInt or TwinCATDataType.USInt
+            or TwinCATDataType.Int or TwinCATDataType.UInt
+            or TwinCATDataType.DInt or TwinCATDataType.UDInt => DriverDataType.Int32,
+        TwinCATDataType.LInt or TwinCATDataType.ULInt => DriverDataType.Int32, // matches Int64 gap
+        TwinCATDataType.Real => DriverDataType.Float32,
+        TwinCATDataType.LReal => DriverDataType.Float64,
+        TwinCATDataType.String or TwinCATDataType.WString => DriverDataType.String,
+        TwinCATDataType.Time or TwinCATDataType.Date
+            or TwinCATDataType.DateTime or TwinCATDataType.TimeOfDay => DriverDataType.Int32,
+        TwinCATDataType.Structure => DriverDataType.String,
+        _ => DriverDataType.Int32,
+    };
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -0,0 +1,219 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     TwinCAT ADS driver — talks to Beckhoff PLC runtimes (TC2 + TC3) via AMS / ADS. PR 1 ships
+///     the <see cref="IDriver"/> skeleton; read / write / discover / subscribe / probe / host-
+///     resolver land in PRs 2 and 3.
+/// </summary>
+public sealed class TwinCATDriver : IDriver, IReadable, IWritable, IDisposable, IAsyncDisposable
+{
+    private readonly TwinCATDriverOptions _options;
+    private readonly string _driverInstanceId;
+    private readonly ITwinCATClientFactory _clientFactory;
+    private readonly Dictionary<string, DeviceState> _devices = new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, TwinCATTagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
+    private DriverHealth _health = new(DriverState.Unknown, null, null);
+
+    public TwinCATDriver(TwinCATDriverOptions options, string driverInstanceId,
+        ITwinCATClientFactory? clientFactory = null)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        _options = options;
+        _driverInstanceId = driverInstanceId;
+        _clientFactory = clientFactory ?? new AdsTwinCATClientFactory();
+    }
+
+    public string DriverInstanceId => _driverInstanceId;
+    public string DriverType => "TwinCAT";
+
+    public Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        _health = new DriverHealth(DriverState.Initializing, null, null);
+        try
+        {
+            foreach (var device in _options.Devices)
+            {
+                var addr = TwinCATAmsAddress.TryParse(device.HostAddress)
+                    ?? throw new InvalidOperationException(
+                        $"TwinCAT device has invalid HostAddress '{device.HostAddress}' — expected 'ads://{{netId}}:{{port}}'.");
+                _devices[device.HostAddress] = new DeviceState(addr, device);
+            }
+            foreach (var tag in _options.Tags) _tagsByName[tag.Name] = tag;
+            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
+        }
+        catch (Exception ex)
+        {
+            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
+            throw;
+        }
+        return Task.CompletedTask;
+    }
+
+    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
+        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
+    }
+
+    public Task ShutdownAsync(CancellationToken cancellationToken)
+    {
+        foreach (var state in _devices.Values) state.DisposeClient();
+        _devices.Clear();
+        _tagsByName.Clear();
+        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
+        return Task.CompletedTask;
+    }
+
+    public DriverHealth GetHealth() => _health;
+    public long GetMemoryFootprint() => 0;
+    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    internal int DeviceCount => _devices.Count;
+    internal DeviceState? GetDeviceState(string hostAddress) =>
+        _devices.TryGetValue(hostAddress, out var s) ? s : null;
+
+    // ---- IReadable ----
+
+    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(fullReferences);
+        var now = DateTime.UtcNow;
+        var results = new DataValueSnapshot[fullReferences.Count];
+
+        for (var i = 0; i < fullReferences.Count; i++)
+        {
+            var reference = fullReferences[i];
+            if (!_tagsByName.TryGetValue(reference, out var def))
+            {
+                results[i] = new DataValueSnapshot(null, TwinCATStatusMapper.BadNodeIdUnknown, null, now);
+                continue;
+            }
+            if (!_devices.TryGetValue(def.DeviceHostAddress, out var device))
+            {
+                results[i] = new DataValueSnapshot(null, TwinCATStatusMapper.BadNodeIdUnknown, null, now);
+                continue;
+            }
+
+            try
+            {
+                var client = await EnsureConnectedAsync(device, cancellationToken).ConfigureAwait(false);
+                var parsed = TwinCATSymbolPath.TryParse(def.SymbolPath);
+                var symbolName = parsed?.ToAdsSymbolName() ?? def.SymbolPath;
+                var (value, status) = await client.ReadValueAsync(
+                    symbolName, def.DataType, parsed?.BitIndex, cancellationToken).ConfigureAwait(false);
+
+                results[i] = new DataValueSnapshot(value, status, now, now);
+                if (status == TwinCATStatusMapper.Good)
+                    _health = new DriverHealth(DriverState.Healthy, now, null);
+                else
+                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead,
+                        $"ADS status {status:X8} reading {reference}");
+            }
+            catch (OperationCanceledException) { throw; }
+            catch (Exception ex)
+            {
+                results[i] = new DataValueSnapshot(null, TwinCATStatusMapper.BadCommunicationError, null, now);
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            }
+        }
+
+        return results;
+    }
+
+    // ---- IWritable ----
+
+    public async Task<IReadOnlyList<WriteResult>> WriteAsync(
+        IReadOnlyList<WriteRequest> writes, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(writes);
+        var results = new WriteResult[writes.Count];
+
+        for (var i = 0; i < writes.Count; i++)
+        {
+            var w = writes[i];
+            if (!_tagsByName.TryGetValue(w.FullReference, out var def))
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadNodeIdUnknown);
+                continue;
+            }
+            if (!def.Writable)
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadNotWritable);
+                continue;
+            }
+            if (!_devices.TryGetValue(def.DeviceHostAddress, out var device))
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadNodeIdUnknown);
+                continue;
+            }
+
+            try
+            {
+                var client = await EnsureConnectedAsync(device, cancellationToken).ConfigureAwait(false);
+                var parsed = TwinCATSymbolPath.TryParse(def.SymbolPath);
+                var symbolName = parsed?.ToAdsSymbolName() ?? def.SymbolPath;
+                var status = await client.WriteValueAsync(
+                    symbolName, def.DataType, parsed?.BitIndex, w.Value, cancellationToken).ConfigureAwait(false);
+                results[i] = new WriteResult(status);
+            }
+            catch (OperationCanceledException) { throw; }
+            catch (NotSupportedException nse)
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadNotSupported);
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, nse.Message);
+            }
+            catch (Exception ex) when (ex is FormatException or InvalidCastException)
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadTypeMismatch);
+            }
+            catch (OverflowException)
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadOutOfRange);
+            }
+            catch (Exception ex)
+            {
+                results[i] = new WriteResult(TwinCATStatusMapper.BadCommunicationError);
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            }
+        }
+
+        return results;
+    }
+
+    private async Task<ITwinCATClient> EnsureConnectedAsync(DeviceState device, CancellationToken ct)
+    {
+        if (device.Client is { IsConnected: true } c) return c;
+        device.Client ??= _clientFactory.Create();
+        try
+        {
+            await device.Client.ConnectAsync(device.ParsedAddress, _options.Timeout, ct)
+                .ConfigureAwait(false);
+        }
+        catch
+        {
+            device.Client.Dispose();
+            device.Client = null;
+            throw;
+        }
+        return device.Client;
+    }
+
+    public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();
+    public async ValueTask DisposeAsync() => await ShutdownAsync(CancellationToken.None).ConfigureAwait(false);
+
+    internal sealed class DeviceState(TwinCATAmsAddress parsedAddress, TwinCATDeviceOptions options)
+    {
+        public TwinCATAmsAddress ParsedAddress { get; } = parsedAddress;
+        public TwinCATDeviceOptions Options { get; } = options;
+        public ITwinCATClient? Client { get; set; }
+
+        public void DisposeClient()
+        {
+            Client?.Dispose();
+            Client = null;
+        }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriverOptions.cs

@@ -0,0 +1,41 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     TwinCAT ADS driver configuration. One instance supports N targets (each identified by
+///     an AMS Net ID + port). Compiles + runs without a local AMS router but every wire call
+///     fails with <c>BadCommunicationError</c> until a router is reachable.
+/// </summary>
+public sealed class TwinCATDriverOptions
+{
+    public IReadOnlyList<TwinCATDeviceOptions> Devices { get; init; } = [];
+    public IReadOnlyList<TwinCATTagDefinition> Tags { get; init; } = [];
+    public TwinCATProbeOptions Probe { get; init; } = new();
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+}
+
+/// <summary>
+///     One TwinCAT target. <paramref name="HostAddress"/> must parse via
+///     <see cref="TwinCATAmsAddress.TryParse"/>; misconfigured devices fail driver initialisation.
+/// </summary>
+public sealed record TwinCATDeviceOptions(
+    string HostAddress,
+    string? DeviceName = null);
+
+/// <summary>
+///     One TwinCAT-backed OPC UA variable. <paramref name="SymbolPath"/> is the full TwinCAT
+///     symbolic name (e.g. <c>MAIN.bStart</c>, <c>GVL.Counter</c>, <c>Motor1.Status.Running</c>).
+/// </summary>
+public sealed record TwinCATTagDefinition(
+    string Name,
+    string DeviceHostAddress,
+    string SymbolPath,
+    TwinCATDataType DataType,
+    bool Writable = true,
+    bool WriteIdempotent = false);
+
+public sealed class TwinCATProbeOptions
+{
+    public bool Enabled { get; init; } = true;
+    public TimeSpan Interval { get; init; } = TimeSpan.FromSeconds(5);
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATStatusMapper.cs

@@ -0,0 +1,43 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     Maps AMS / ADS error codes to OPC UA StatusCodes. ADS error codes are defined in
+///     <c>AdsErrorCode</c> from <c>Beckhoff.TwinCAT.Ads</c> — this mapper covers the ones a
+///     driver actually encounters during normal operation (symbol-not-found, access-denied,
+///     timeout, router-not-initialized, invalid-group/offset, etc.).
+/// </summary>
+public static class TwinCATStatusMapper
+{
+    public const uint Good = 0u;
+    public const uint BadInternalError = 0x80020000u;
+    public const uint BadNodeIdUnknown = 0x80340000u;
+    public const uint BadNotWritable = 0x803B0000u;
+    public const uint BadOutOfRange = 0x803C0000u;
+    public const uint BadNotSupported = 0x803D0000u;
+    public const uint BadDeviceFailure = 0x80550000u;
+    public const uint BadCommunicationError = 0x80050000u;
+    public const uint BadTimeout = 0x800A0000u;
+    public const uint BadTypeMismatch = 0x80730000u;
+
+    /// <summary>
+    ///     Map an AMS / ADS error code (uint from AdsErrorCode enum). 0 = success; non-zero
+    ///     codes follow Beckhoff's AMS error table (7 = target port not found, 1792 =
+    ///     ADSERR_DEVICE_SRVNOTSUPP, 1793 = ADSERR_DEVICE_INVALIDGRP, 1794 =
+    ///     ADSERR_DEVICE_INVALIDOFFSET, 1798 = ADSERR_DEVICE_SYMBOLNOTFOUND, 1808 =
+    ///     ADSERR_DEVICE_ACCESSDENIED, 1861 = ADSERR_CLIENT_SYNCTIMEOUT).
+    /// </summary>
+    public static uint MapAdsError(uint adsError) => adsError switch
+    {
+        0 => Good,
+        6 or 7 => BadCommunicationError,           // target port unreachable
+        1792 => BadNotSupported,                    // service not supported
+        1793 => BadOutOfRange,                      // invalid index group
+        1794 => BadOutOfRange,                      // invalid index offset
+        1798 => BadNodeIdUnknown,                   // symbol not found
+        1807 => BadDeviceFailure,                   // device in invalid state
+        1808 => BadNotWritable,                     // access denied
+        1811 or 1812 => BadOutOfRange,              // size mismatch
+        1861 => BadTimeout,                          // sync timeout
+        _ => BadCommunicationError,
+    };
+}

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATSymbolPath.cs

@@ -0,0 +1,103 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+/// <summary>
+///     Parsed TwinCAT symbolic tag path. Handles global-variable-list (<c>GVL.Counter</c>),
+///     program-variable (<c>MAIN.bStart</c>), structured member access
+///     (<c>Motor1.Status.Running</c>), array subscripts (<c>Data[5]</c>), multi-dim arrays
+///     (<c>Matrix[1,2]</c>), and bit-access (<c>Flags.0</c>).
+/// </summary>
+/// <remarks>
+///     <para>TwinCAT's symbolic syntax mirrors IEC 61131-3 structured-text identifiers — so the
+///     grammar maps cleanly onto the AbCip Logix path parser, but without Logix's
+///     <c>Program:</c> scope prefix. The leading segment is the namespace (POU name /
+///     GVL name) and subsequent segments walk into struct/array members.</para>
+/// </remarks>
+public sealed record TwinCATSymbolPath(
+    IReadOnlyList<TwinCATSymbolSegment> Segments,
+    int? BitIndex)
+{
+    public string ToAdsSymbolName()
+    {
+        var buf = new System.Text.StringBuilder();
+        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(']');
+        }
+        if (BitIndex is not null) buf.Append('.').Append(BitIndex.Value);
+        return buf.ToString();
+    }
+
+    public static TwinCATSymbolPath? TryParse(string? value)
+    {
+        if (string.IsNullOrWhiteSpace(value)) return null;
+        var src = value.Trim();
+
+        var parts = new List<string>();
+        var depth = 0;
+        var start = 0;
+        for (var i = 0; i < src.Length; i++)
+        {
+            var c = src[i];
+            if (c == '[') depth++;
+            else if (c == ']') depth--;
+            else if (c == '.' && depth == 0)
+            {
+                parts.Add(src[start..i]);
+                start = i + 1;
+            }
+        }
+        parts.Add(src[start..]);
+        if (depth != 0 || parts.Any(string.IsNullOrEmpty)) return null;
+
+        int? bitIndex = null;
+        if (parts.Count >= 2 && int.TryParse(parts[^1], out var maybeBit)
+            && maybeBit is >= 0 and <= 31
+            && !parts[^1].Contains('['))
+        {
+            bitIndex = maybeBit;
+            parts.RemoveAt(parts.Count - 1);
+        }
+
+        var segments = new List<TwinCATSymbolSegment>(parts.Count);
+        foreach (var part in parts)
+        {
+            var bracketIdx = part.IndexOf('[');
+            if (bracketIdx < 0)
+            {
+                if (!IsValidIdent(part)) return null;
+                segments.Add(new TwinCATSymbolSegment(part, []));
+                continue;
+            }
+            if (!part.EndsWith(']')) return null;
+            var name = part[..bracketIdx];
+            if (!IsValidIdent(name)) return null;
+            var inner = part[(bracketIdx + 1)..^1];
+            var subs = new List<int>();
+            foreach (var tok in inner.Split(','))
+            {
+                if (!int.TryParse(tok, out var n) || n < 0) return null;
+                subs.Add(n);
+            }
+            if (subs.Count == 0) return null;
+            segments.Add(new TwinCATSymbolSegment(name, subs));
+        }
+        if (segments.Count == 0) return null;
+
+        return new TwinCATSymbolPath(segments, bitIndex);
+    }
+
+    private static bool IsValidIdent(string s)
+    {
+        if (string.IsNullOrEmpty(s)) return false;
+        if (!char.IsLetter(s[0]) && s[0] != '_') return false;
+        for (var i = 1; i < s.Length; i++)
+            if (!char.IsLetterOrDigit(s[i]) && s[i] != '_') return false;
+        return true;
+    }
+}
+
+public sealed record TwinCATSymbolSegment(string Name, IReadOnlyList<int> Subscripts);

src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.csproj

@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <LangVersion>latest</LangVersion>
+        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <RootNamespace>ZB.MOM.WW.OtOpcUa.Driver.TwinCAT</RootNamespace>
+        <AssemblyName>ZB.MOM.WW.OtOpcUa.Driver.TwinCAT</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <!-- Official Beckhoff ADS client. Requires a running AMS router (TwinCAT XAR, TwinCAT HMI
+             Server, or the standalone Beckhoff.TwinCAT.Ads.TcpRouter package) to reach remote
+             systems. The router is a runtime concern, not a build concern — the library compiles
+             + runs fine without one; ADS calls just fail with transport errors. -->
+        <PackageReference Include="Beckhoff.TwinCAT.Ads" Version="7.0.172"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests"/>
+    </ItemGroup>
+
+</Project>

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

@@ -0,0 +1,249 @@
+using System.Collections.Concurrent;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class AbLegacyCapabilityTests
+{
+    // ---- ITagDiscovery ----
+
+    [Fact]
+    public async Task DiscoverAsync_emits_pre_declared_tags_under_device_folder()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0", DeviceName: "Press-SLC-1")],
+            Tags =
+            [
+                new AbLegacyTagDefinition("Speed", "ab://10.0.0.5/1,0", "N7:0", AbLegacyDataType.Int),
+                new AbLegacyTagDefinition("Temperature", "ab://10.0.0.5/1,0", "F8:0", AbLegacyDataType.Float, Writable: false),
+            ],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "AbLegacy");
+        builder.Folders.ShouldContain(f => f.BrowseName == "ab://10.0.0.5/1,0" && f.DisplayName == "Press-SLC-1");
+        builder.Variables.Count.ShouldBe(2);
+        builder.Variables.Single(v => v.BrowseName == "Speed").Info.SecurityClass.ShouldBe(SecurityClassification.Operate);
+        builder.Variables.Single(v => v.BrowseName == "Temperature").Info.SecurityClass.ShouldBe(SecurityClassification.ViewOnly);
+    }
+
+    // ---- ISubscribable ----
+
+    [Fact]
+    public async Task Subscribe_initial_poll_raises_OnDataChange()
+    {
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new FakeAbLegacyTag(p) { Value = 42 },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", "N7:0", AbLegacyDataType.Int)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["X"], TimeSpan.FromMilliseconds(200), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 1, TimeSpan.FromSeconds(2));
+
+        events.First().Snapshot.Value.ShouldBe(42);
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Unsubscribe_halts_polling()
+    {
+        var tagRef = new FakeAbLegacyTag(
+            new AbLegacyTagCreateParams("10.0.0.5", 44818, "1,0", "slc500", "N7:0", TimeSpan.FromSeconds(2))) { Value = 1 };
+        var factory = new FakeAbLegacyTagFactory { Customise = _ => tagRef };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("X", "ab://10.0.0.5/1,0", "N7:0", AbLegacyDataType.Int)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["X"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 1, TimeSpan.FromSeconds(1));
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        var afterUnsub = events.Count;
+        tagRef.Value = 999;
+        await Task.Delay(300);
+        events.Count.ShouldBe(afterUnsub);
+    }
+
+    // ---- IHostConnectivityProbe ----
+
+    [Fact]
+    public async Task GetHostStatuses_returns_one_per_device()
+    {
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices =
+            [
+                new AbLegacyDeviceOptions("ab://10.0.0.5/1,0"),
+                new AbLegacyDeviceOptions("ab://10.0.0.6/1,0"),
+            ],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        drv.GetHostStatuses().Count.ShouldBe(2);
+    }
+
+    [Fact]
+    public async Task Probe_transitions_to_Running_on_successful_read()
+    {
+        var factory = new FakeAbLegacyTagFactory { Customise = p => new FakeAbLegacyTag(p) };
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Probe = new AbLegacyProbeOptions
+            {
+                Enabled = true, Interval = TimeSpan.FromMilliseconds(100),
+                Timeout = TimeSpan.FromMilliseconds(50), ProbeAddress = "S:0",
+            },
+        }, "drv-1", factory);
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await WaitForAsync(() => transitions.Any(t => t.NewState == HostState.Running), TimeSpan.FromSeconds(2));
+
+        drv.GetHostStatuses().Single().State.ShouldBe(HostState.Running);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Probe_transitions_to_Stopped_on_read_failure()
+    {
+        var factory = new FakeAbLegacyTagFactory { Customise = p => new FakeAbLegacyTag(p) { ThrowOnRead = true } };
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Probe = new AbLegacyProbeOptions
+            {
+                Enabled = true, Interval = TimeSpan.FromMilliseconds(100),
+                Timeout = TimeSpan.FromMilliseconds(50), ProbeAddress = "S:0",
+            },
+        }, "drv-1", factory);
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await WaitForAsync(() => transitions.Any(t => t.NewState == HostState.Stopped), TimeSpan.FromSeconds(2));
+
+        drv.GetHostStatuses().Single().State.ShouldBe(HostState.Stopped);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Probe_disabled_when_ProbeAddress_is_null()
+    {
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Probe = new AbLegacyProbeOptions { Enabled = true, ProbeAddress = null },
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await Task.Delay(200);
+
+        drv.GetHostStatuses().Single().State.ShouldBe(HostState.Unknown);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    // ---- IPerCallHostResolver ----
+
+    [Fact]
+    public async Task ResolveHost_returns_declared_device_for_known_tag()
+    {
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices =
+            [
+                new AbLegacyDeviceOptions("ab://10.0.0.5/1,0"),
+                new AbLegacyDeviceOptions("ab://10.0.0.6/1,0"),
+            ],
+            Tags =
+            [
+                new AbLegacyTagDefinition("A", "ab://10.0.0.5/1,0", "N7:0", AbLegacyDataType.Int),
+                new AbLegacyTagDefinition("B", "ab://10.0.0.6/1,0", "N7:0", AbLegacyDataType.Int),
+            ],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        drv.ResolveHost("A").ShouldBe("ab://10.0.0.5/1,0");
+        drv.ResolveHost("B").ShouldBe("ab://10.0.0.6/1,0");
+    }
+
+    [Fact]
+    public async Task ResolveHost_falls_back_to_first_device_for_unknown()
+    {
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        drv.ResolveHost("missing").ShouldBe("ab://10.0.0.5/1,0");
+    }
+
+    [Fact]
+    public async Task ResolveHost_falls_back_to_DriverInstanceId_when_no_devices()
+    {
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions(), "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        drv.ResolveHost("anything").ShouldBe("drv-1");
+    }
+
+    // ---- helpers ----
+
+    private static async Task WaitForAsync(Func<bool> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!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.TwinCAT.Tests/FakeTwinCATClient.cs

@@ -0,0 +1,72 @@
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+internal class FakeTwinCATClient : ITwinCATClient
+{
+    public bool IsConnected { get; private set; }
+    public int ConnectCount { get; private set; }
+    public int DisposeCount { get; private set; }
+    public bool ThrowOnConnect { get; set; }
+    public bool ThrowOnRead { get; set; }
+    public bool ThrowOnWrite { get; set; }
+    public bool ThrowOnProbe { get; set; }
+    public Exception? Exception { get; set; }
+    public Dictionary<string, object?> Values { get; } = new(StringComparer.OrdinalIgnoreCase);
+    public Dictionary<string, uint> ReadStatuses { get; } = new(StringComparer.OrdinalIgnoreCase);
+    public Dictionary<string, uint> WriteStatuses { get; } = new(StringComparer.OrdinalIgnoreCase);
+    public List<(string symbol, TwinCATDataType type, int? bit, object? value)> WriteLog { get; } = new();
+    public bool ProbeResult { get; set; } = true;
+
+    public virtual Task ConnectAsync(TwinCATAmsAddress address, TimeSpan timeout, CancellationToken ct)
+    {
+        ConnectCount++;
+        if (ThrowOnConnect) throw Exception ?? new InvalidOperationException();
+        IsConnected = true;
+        return Task.CompletedTask;
+    }
+
+    public virtual Task<(object? value, uint status)> ReadValueAsync(
+        string symbolPath, TwinCATDataType type, int? bitIndex, CancellationToken ct)
+    {
+        if (ThrowOnRead) throw Exception ?? new InvalidOperationException();
+        var status = ReadStatuses.TryGetValue(symbolPath, out var s) ? s : TwinCATStatusMapper.Good;
+        var value = Values.TryGetValue(symbolPath, out var v) ? v : null;
+        return Task.FromResult((value, status));
+    }
+
+    public virtual Task<uint> WriteValueAsync(
+        string symbolPath, TwinCATDataType type, int? bitIndex, object? value, CancellationToken ct)
+    {
+        if (ThrowOnWrite) throw Exception ?? new InvalidOperationException();
+        WriteLog.Add((symbolPath, type, bitIndex, value));
+        Values[symbolPath] = value;
+        var status = WriteStatuses.TryGetValue(symbolPath, out var s) ? s : TwinCATStatusMapper.Good;
+        return Task.FromResult(status);
+    }
+
+    public virtual Task<bool> ProbeAsync(CancellationToken ct)
+    {
+        if (ThrowOnProbe) return Task.FromResult(false);
+        return Task.FromResult(ProbeResult);
+    }
+
+    public virtual void Dispose()
+    {
+        DisposeCount++;
+        IsConnected = false;
+    }
+}
+
+internal sealed class FakeTwinCATClientFactory : ITwinCATClientFactory
+{
+    public List<FakeTwinCATClient> Clients { get; } = new();
+    public Func<FakeTwinCATClient>? Customise { get; set; }
+
+    public ITwinCATClient Create()
+    {
+        var client = Customise?.Invoke() ?? new FakeTwinCATClient();
+        Clients.Add(client);
+        return client;
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATAmsAddressTests.cs

@@ -0,0 +1,59 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class TwinCATAmsAddressTests
+{
+    [Theory]
+    [InlineData("ads://5.23.91.23.1.1:851", "5.23.91.23.1.1", 851)]
+    [InlineData("ads://5.23.91.23.1.1:852", "5.23.91.23.1.1", 852)]
+    [InlineData("ads://5.23.91.23.1.1", "5.23.91.23.1.1", 851)]                  // default port
+    [InlineData("ads://127.0.0.1.1.1:851", "127.0.0.1.1.1", 851)]
+    [InlineData("ADS://5.23.91.23.1.1:851", "5.23.91.23.1.1", 851)]              // case-insensitive scheme
+    [InlineData("ads://10.0.0.1.1.1:10000", "10.0.0.1.1.1", 10000)]              // system service port
+    public void TryParse_accepts_valid_ams_addresses(string input, string netId, int port)
+    {
+        var parsed = TwinCATAmsAddress.TryParse(input);
+        parsed.ShouldNotBeNull();
+        parsed.NetId.ShouldBe(netId);
+        parsed.Port.ShouldBe(port);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData("tcp://5.23.91.23.1.1:851")]        // wrong scheme
+    [InlineData("ads:5.23.91.23.1.1:851")]          // missing //
+    [InlineData("ads://")]                           // empty body
+    [InlineData("ads://5.23.91.23.1:851")]           // only 5 octets
+    [InlineData("ads://5.23.91.23.1.1.1:851")]       // 7 octets
+    [InlineData("ads://5.23.91.256.1.1:851")]        // octet > 255
+    [InlineData("ads://5.23.91.23.1.1:0")]           // port 0
+    [InlineData("ads://5.23.91.23.1.1:65536")]       // port out of range
+    [InlineData("ads://5.23.91.23.1.1:abc")]         // non-numeric port
+    [InlineData("ads://a.b.c.d.e.f:851")]            // non-numeric octets
+    public void TryParse_rejects_invalid_forms(string? input)
+    {
+        TwinCATAmsAddress.TryParse(input).ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData("5.23.91.23.1.1", 851, "ads://5.23.91.23.1.1")]        // default port stripped
+    [InlineData("5.23.91.23.1.1", 852, "ads://5.23.91.23.1.1:852")]
+    public void ToString_canonicalises(string netId, int port, string expected)
+    {
+        new TwinCATAmsAddress(netId, port).ToString().ShouldBe(expected);
+    }
+
+    [Fact]
+    public void RoundTrip_is_stable()
+    {
+        const string input = "ads://5.23.91.23.1.1:852";
+        var parsed = TwinCATAmsAddress.TryParse(input)!;
+        TwinCATAmsAddress.TryParse(parsed.ToString()).ShouldBe(parsed);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATDriverTests.cs

@@ -0,0 +1,105 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class TwinCATDriverTests
+{
+    [Fact]
+    public void DriverType_is_TwinCAT()
+    {
+        var drv = new TwinCATDriver(new TwinCATDriverOptions(), "drv-1");
+        drv.DriverType.ShouldBe("TwinCAT");
+        drv.DriverInstanceId.ShouldBe("drv-1");
+    }
+
+    [Fact]
+    public async Task InitializeAsync_parses_device_addresses()
+    {
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices =
+            [
+                new TwinCATDeviceOptions("ads://5.23.91.23.1.1:851"),
+                new TwinCATDeviceOptions("ads://10.0.0.1.1.1:852", DeviceName: "Machine2"),
+            ],
+        }, "drv-1");
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        drv.DeviceCount.ShouldBe(2);
+        drv.GetDeviceState("ads://5.23.91.23.1.1:851")!.ParsedAddress.Port.ShouldBe(851);
+        drv.GetDeviceState("ads://10.0.0.1.1.1:852")!.Options.DeviceName.ShouldBe("Machine2");
+    }
+
+    [Fact]
+    public async Task InitializeAsync_malformed_address_faults()
+    {
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions("not-an-address")],
+        }, "drv-1");
+
+        await Should.ThrowAsync<InvalidOperationException>(
+            () => drv.InitializeAsync("{}", CancellationToken.None));
+        drv.GetHealth().State.ShouldBe(DriverState.Faulted);
+    }
+
+    [Fact]
+    public async Task ShutdownAsync_clears_devices()
+    {
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions("ads://5.23.91.23.1.1:851")],
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+        drv.DeviceCount.ShouldBe(0);
+        drv.GetHealth().State.ShouldBe(DriverState.Unknown);
+    }
+
+    [Fact]
+    public async Task ReinitializeAsync_cycles_devices()
+    {
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions("ads://5.23.91.23.1.1:851")],
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await drv.ReinitializeAsync("{}", CancellationToken.None);
+
+        drv.DeviceCount.ShouldBe(1);
+        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
+    }
+
+    [Fact]
+    public void DataType_mapping_covers_atomic_iec_types()
+    {
+        TwinCATDataType.Bool.ToDriverDataType().ShouldBe(DriverDataType.Boolean);
+        TwinCATDataType.DInt.ToDriverDataType().ShouldBe(DriverDataType.Int32);
+        TwinCATDataType.Real.ToDriverDataType().ShouldBe(DriverDataType.Float32);
+        TwinCATDataType.LReal.ToDriverDataType().ShouldBe(DriverDataType.Float64);
+        TwinCATDataType.String.ToDriverDataType().ShouldBe(DriverDataType.String);
+        TwinCATDataType.WString.ToDriverDataType().ShouldBe(DriverDataType.String);
+        TwinCATDataType.Time.ToDriverDataType().ShouldBe(DriverDataType.Int32);
+    }
+
+    [Theory]
+    [InlineData(0u, TwinCATStatusMapper.Good)]
+    [InlineData(1798u, TwinCATStatusMapper.BadNodeIdUnknown)]   // symbol not found
+    [InlineData(1808u, TwinCATStatusMapper.BadNotWritable)]     // access denied
+    [InlineData(1861u, TwinCATStatusMapper.BadTimeout)]         // sync timeout
+    [InlineData(1793u, TwinCATStatusMapper.BadOutOfRange)]      // invalid index group
+    [InlineData(1794u, TwinCATStatusMapper.BadOutOfRange)]      // invalid index offset
+    [InlineData(1792u, TwinCATStatusMapper.BadNotSupported)]    // service not supported
+    [InlineData(7u, TwinCATStatusMapper.BadCommunicationError)] // port unreachable
+    [InlineData(99999u, TwinCATStatusMapper.BadCommunicationError)] // unknown → generic comm fail
+    public void StatusMapper_covers_known_ads_error_codes(uint adsError, uint expected)
+    {
+        TwinCATStatusMapper.MapAdsError(adsError).ShouldBe(expected);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATReadWriteTests.cs

@@ -0,0 +1,255 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class TwinCATReadWriteTests
+{
+    private static (TwinCATDriver drv, FakeTwinCATClientFactory factory) NewDriver(params TwinCATTagDefinition[] tags)
+    {
+        var factory = new FakeTwinCATClientFactory();
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions("ads://5.23.91.23.1.1:851")],
+            Tags = tags,
+            Probe = new TwinCATProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        return (drv, factory);
+    }
+
+    // ---- Read ----
+
+    [Fact]
+    public async Task Unknown_reference_maps_to_BadNodeIdUnknown()
+    {
+        var (drv, _) = NewDriver();
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snapshots = await drv.ReadAsync(["missing"], CancellationToken.None);
+        snapshots.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadNodeIdUnknown);
+    }
+
+    [Fact]
+    public async Task Successful_DInt_read_returns_Good_value()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("Speed", "ads://5.23.91.23.1.1:851", "MAIN.Speed", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { Values = { ["MAIN.Speed"] = 4200 } };
+
+        var snapshots = await drv.ReadAsync(["Speed"], CancellationToken.None);
+
+        snapshots.Single().StatusCode.ShouldBe(TwinCATStatusMapper.Good);
+        snapshots.Single().Value.ShouldBe(4200);
+        factory.Clients[0].ConnectCount.ShouldBe(1);
+        factory.Clients[0].IsConnected.ShouldBeTrue();
+    }
+
+    [Fact]
+    public async Task Repeat_read_reuses_connection()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "GVL.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { Values = { ["GVL.X"] = 1 } };
+
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ReadAsync(["X"], CancellationToken.None);
+
+        // One client, one connect — subsequent calls reuse the connected client.
+        factory.Clients.Count.ShouldBe(1);
+        factory.Clients[0].ConnectCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Read_with_ADS_error_maps_via_status_mapper()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("Ghost", "ads://5.23.91.23.1.1:851", "MAIN.Missing", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () =>
+        {
+            var c = new FakeTwinCATClient();
+            c.ReadStatuses["MAIN.Missing"] = TwinCATStatusMapper.BadNodeIdUnknown;
+            return c;
+        };
+
+        var snapshots = await drv.ReadAsync(["Ghost"], CancellationToken.None);
+        snapshots.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadNodeIdUnknown);
+    }
+
+    [Fact]
+    public async Task Read_exception_surfaces_BadCommunicationError()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "MAIN.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { ThrowOnRead = true };
+
+        var snapshots = await drv.ReadAsync(["X"], CancellationToken.None);
+        snapshots.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadCommunicationError);
+        drv.GetHealth().State.ShouldBe(DriverState.Degraded);
+    }
+
+    [Fact]
+    public async Task Connect_failure_surfaces_BadCommunicationError_and_disposes_client()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "MAIN.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { ThrowOnConnect = true };
+
+        var snapshots = await drv.ReadAsync(["X"], CancellationToken.None);
+        snapshots.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadCommunicationError);
+        factory.Clients[0].DisposeCount.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Batched_reads_preserve_order()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("A", "ads://5.23.91.23.1.1:851", "MAIN.A", TwinCATDataType.DInt),
+            new TwinCATTagDefinition("B", "ads://5.23.91.23.1.1:851", "MAIN.B", TwinCATDataType.Real),
+            new TwinCATTagDefinition("C", "ads://5.23.91.23.1.1:851", "MAIN.C", TwinCATDataType.String));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient
+        {
+            Values =
+            {
+                ["MAIN.A"] = 1,
+                ["MAIN.B"] = 3.14f,
+                ["MAIN.C"] = "hello",
+            },
+        };
+
+        var snapshots = await drv.ReadAsync(["A", "B", "C"], CancellationToken.None);
+        snapshots[0].Value.ShouldBe(1);
+        snapshots[1].Value.ShouldBe(3.14f);
+        snapshots[2].Value.ShouldBe("hello");
+    }
+
+    // ---- Write ----
+
+    [Fact]
+    public async Task Non_writable_tag_rejected_with_BadNotWritable()
+    {
+        var (drv, _) = NewDriver(
+            new TwinCATTagDefinition("RO", "ads://5.23.91.23.1.1:851", "MAIN.RO", TwinCATDataType.DInt, Writable: false));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("RO", 1)], CancellationToken.None);
+        results.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadNotWritable);
+    }
+
+    [Fact]
+    public async Task Successful_write_logs_symbol_type_value()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("Speed", "ads://5.23.91.23.1.1:851", "MAIN.Speed", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("Speed", 4200)], CancellationToken.None);
+
+        results.Single().StatusCode.ShouldBe(TwinCATStatusMapper.Good);
+        var write = factory.Clients[0].WriteLog.Single();
+        write.symbol.ShouldBe("MAIN.Speed");
+        write.type.ShouldBe(TwinCATDataType.DInt);
+        write.value.ShouldBe(4200);
+    }
+
+    [Fact]
+    public async Task Write_with_ADS_error_surfaces_mapped_status()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "MAIN.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () =>
+        {
+            var c = new FakeTwinCATClient();
+            c.WriteStatuses["MAIN.X"] = TwinCATStatusMapper.BadNotWritable;
+            return c;
+        };
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("X", 1)], CancellationToken.None);
+        results.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadNotWritable);
+    }
+
+    [Fact]
+    public async Task Write_exception_surfaces_BadCommunicationError()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "MAIN.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { ThrowOnWrite = true };
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("X", 1)], CancellationToken.None);
+        results.Single().StatusCode.ShouldBe(TwinCATStatusMapper.BadCommunicationError);
+    }
+
+    [Fact]
+    public async Task Batch_write_preserves_order_across_outcomes()
+    {
+        var factory = new FakeTwinCATClientFactory();
+        var drv = new TwinCATDriver(new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions("ads://5.23.91.23.1.1:851")],
+            Tags =
+            [
+                new TwinCATTagDefinition("A", "ads://5.23.91.23.1.1:851", "MAIN.A", TwinCATDataType.DInt),
+                new TwinCATTagDefinition("B", "ads://5.23.91.23.1.1:851", "MAIN.B", TwinCATDataType.DInt, Writable: false),
+            ],
+            Probe = new TwinCATProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+        [
+            new WriteRequest("A", 1),
+            new WriteRequest("B", 2),
+            new WriteRequest("Unknown", 3),
+        ], CancellationToken.None);
+
+        results.Count.ShouldBe(3);
+        results[0].StatusCode.ShouldBe(TwinCATStatusMapper.Good);
+        results[1].StatusCode.ShouldBe(TwinCATStatusMapper.BadNotWritable);
+        results[2].StatusCode.ShouldBe(TwinCATStatusMapper.BadNodeIdUnknown);
+    }
+
+    [Fact]
+    public async Task Cancellation_propagates()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "MAIN.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient
+        {
+            ThrowOnRead = true,
+            Exception = new OperationCanceledException(),
+        };
+
+        await Should.ThrowAsync<OperationCanceledException>(
+            () => drv.ReadAsync(["X"], CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task ShutdownAsync_disposes_client()
+    {
+        var (drv, factory) = NewDriver(
+            new TwinCATTagDefinition("X", "ads://5.23.91.23.1.1:851", "MAIN.X", TwinCATDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = () => new FakeTwinCATClient { Values = { ["MAIN.X"] = 1 } };
+
+        await drv.ReadAsync(["X"], CancellationToken.None);
+        await drv.ShutdownAsync(CancellationToken.None);
+
+        factory.Clients[0].DisposeCount.ShouldBe(1);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATSymbolPathTests.cs

@@ -0,0 +1,138 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class TwinCATSymbolPathTests
+{
+    [Fact]
+    public void Single_segment_global_variable_parses()
+    {
+        var p = TwinCATSymbolPath.TryParse("Counter");
+        p.ShouldNotBeNull();
+        p.Segments.Single().Name.ShouldBe("Counter");
+        p.ToAdsSymbolName().ShouldBe("Counter");
+    }
+
+    [Fact]
+    public void POU_dot_variable_parses()
+    {
+        var p = TwinCATSymbolPath.TryParse("MAIN.bStart");
+        p.ShouldNotBeNull();
+        p.Segments.Select(s => s.Name).ShouldBe(["MAIN", "bStart"]);
+        p.ToAdsSymbolName().ShouldBe("MAIN.bStart");
+    }
+
+    [Fact]
+    public void GVL_reference_parses()
+    {
+        var p = TwinCATSymbolPath.TryParse("GVL.Counter");
+        p.ShouldNotBeNull();
+        p.Segments.Select(s => s.Name).ShouldBe(["GVL", "Counter"]);
+        p.ToAdsSymbolName().ShouldBe("GVL.Counter");
+    }
+
+    [Fact]
+    public void Structured_member_access_splits()
+    {
+        var p = TwinCATSymbolPath.TryParse("Motor1.Status.Running");
+        p.ShouldNotBeNull();
+        p.Segments.Select(s => s.Name).ShouldBe(["Motor1", "Status", "Running"]);
+    }
+
+    [Fact]
+    public void Array_subscript_parses()
+    {
+        var p = TwinCATSymbolPath.TryParse("Data[5]");
+        p.ShouldNotBeNull();
+        p.Segments.Single().Subscripts.ShouldBe([5]);
+        p.ToAdsSymbolName().ShouldBe("Data[5]");
+    }
+
+    [Fact]
+    public void Multi_dim_array_subscript_parses()
+    {
+        var p = TwinCATSymbolPath.TryParse("Matrix[1,2]");
+        p.ShouldNotBeNull();
+        p.Segments.Single().Subscripts.ShouldBe([1, 2]);
+    }
+
+    [Fact]
+    public void Bit_access_captured_as_bit_index()
+    {
+        var p = TwinCATSymbolPath.TryParse("Flags.3");
+        p.ShouldNotBeNull();
+        p.Segments.Single().Name.ShouldBe("Flags");
+        p.BitIndex.ShouldBe(3);
+        p.ToAdsSymbolName().ShouldBe("Flags.3");
+    }
+
+    [Fact]
+    public void Bit_access_after_member_path()
+    {
+        var p = TwinCATSymbolPath.TryParse("GVL.Status.7");
+        p.ShouldNotBeNull();
+        p.Segments.Select(s => s.Name).ShouldBe(["GVL", "Status"]);
+        p.BitIndex.ShouldBe(7);
+    }
+
+    [Fact]
+    public void Combined_scope_member_subscript_bit()
+    {
+        var p = TwinCATSymbolPath.TryParse("MAIN.Motors[0].Status.5");
+        p.ShouldNotBeNull();
+        p.Segments.Select(s => s.Name).ShouldBe(["MAIN", "Motors", "Status"]);
+        p.Segments[1].Subscripts.ShouldBe([0]);
+        p.BitIndex.ShouldBe(5);
+        p.ToAdsSymbolName().ShouldBe("MAIN.Motors[0].Status.5");
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData(".Motor")]              // leading dot
+    [InlineData("Motor.")]              // trailing dot
+    [InlineData("Motor.[0]")]           // empty segment
+    [InlineData("1bad")]                // ident starts with digit
+    [InlineData("Bad Name")]            // space in ident
+    [InlineData("Motor[]")]             // empty subscript
+    [InlineData("Motor[-1]")]           // negative subscript
+    [InlineData("Motor[a]")]            // non-numeric subscript
+    [InlineData("Motor[")]              // unbalanced bracket
+    [InlineData("Flags.32")]            // bit out of range (treated as ident → invalid shape)
+    public void Invalid_shapes_return_null(string? input)
+    {
+        TwinCATSymbolPath.TryParse(input).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Underscore_prefix_idents_accepted()
+    {
+        TwinCATSymbolPath.TryParse("_internal_var")!.Segments.Single().Name.ShouldBe("_internal_var");
+    }
+
+    [Fact]
+    public void ToAdsSymbolName_roundtrips()
+    {
+        var cases = new[]
+        {
+            "Counter",
+            "MAIN.bStart",
+            "GVL.Counter",
+            "Motor1.Status.Running",
+            "Data[5]",
+            "Matrix[1,2]",
+            "Flags.3",
+            "MAIN.Motors[0].Status.5",
+        };
+        foreach (var c in cases)
+        {
+            var parsed = TwinCATSymbolPath.TryParse(c);
+            parsed.ShouldNotBeNull(c);
+            parsed.ToAdsSymbolName().ShouldBe(c);
+        }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests.csproj

@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <RootNamespace>ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="xunit.v3" Version="1.1.0"/>
+        <PackageReference Include="Shouldly" Version="4.3.0"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0"/>
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.0.2">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\src\ZB.MOM.WW.OtOpcUa.Driver.TwinCAT\ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-37gx-xxp4-5rgx"/>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-w3x6-4m5h-cxqf"/>
+    </ItemGroup>
+
+</Project>