Phase 3 PR 25 — modbus-test-plan.md: integration-test playbook with per-device quirk catalog. ModbusPal is the chosen simulator; AutomationDirect DL205 is the first target device class with 6 pending quirks to document and cover with named tests (word order for 32-bit values, register-zero access policy, coil addressing base, maximum registers per FC03, response framing under sustained load, exception code on protected-bit coil write). Each quirk placeholder has a proposed test name so the user's validation work translates directly into integration tests. Test conventions section codifies the named-per-quirk pattern, skip-when-unreachable guard, real ModbusTcpTransport usage, and inter-test isolation. Sets up the harness-and-catalog structure future device families (Allen-Bradley Micrologix, Siemens S7-1200 Modbus gateway, Schneider M340, whatever the user hits) will slot into — same per-device catalog shape, cross-device patterns section for recurring quirks that can get promoted into driver defaults. Next concrete PRs proposed: PR 26 for the integration test project scaffold + DL205 profile + fixture with skip-guard + one smoke test, PR 27+ for the individual confirmed quirks one-per-PR.

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

ZB.MOM.WW.OtOpcUa.slnx

@@ -8,6 +8,7 @@
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.csproj"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.csproj"/>
+        <Project Path="src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ZB.MOM.WW.OtOpcUa.Driver.Modbus.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"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.UI/ZB.MOM.WW.OtOpcUa.Client.UI.csproj"/>
@@ -22,6 +23,7 @@
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E.csproj"/>
+        <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.CLI.Tests/ZB.MOM.WW.OtOpcUa.Client.CLI.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.UI.Tests/ZB.MOM.WW.OtOpcUa.Client.UI.Tests.csproj"/>

docs/v2/lmx-followups.md

@@ -0,0 +1,106 @@
+# LMX Galaxy bridge — remaining follow-ups
+
+State after PR 19: the Galaxy driver is functionally at v1 parity through the
+`IDriver` abstraction; the OPC UA server runs with LDAP-authenticated
+Basic256Sha256 endpoints and alarms are observable through
+`AlarmConditionState.ReportEvent`. The items below are what remains LMX-
+specific before the stack can fully replace the v1 deployment, in
+rough priority order.
+
+## 1. Proxy-side `IHistoryProvider` for `ReadAtTime` / `ReadEvents`
+
+**Status**: Host-side IPC shipped (PR 10 + PR 11). Proxy consumer not written.
+
+PR 10 added `HistoryReadAtTimeRequest/Response` on the IPC wire and
+`MxAccessGalaxyBackend.HistoryReadAtTimeAsync` delegates to
+`HistorianDataSource.ReadAtTimeAsync`. PR 11 did the same for events
+(`HistoryReadEventsRequest/Response` + `GalaxyHistoricalEvent`). The Proxy
+side (`GalaxyProxyDriver`) doesn't call those yet — `Core.Abstractions.IHistoryProvider`
+only exposes `ReadRawAsync` + `ReadProcessedAsync`.
+
+**To do**:
+- Extend `IHistoryProvider` with `ReadAtTimeAsync(string, DateTime[], …)` and
+  `ReadEventsAsync(string?, DateTime, DateTime, int, …)`.
+- `GalaxyProxyDriver` calls the new IPC message kinds.
+- `DriverNodeManager` wires the new capability methods onto `HistoryRead`
+  `AtTime` + `Events` service handlers.
+- Integration test: OPC UA client calls `HistoryReadAtTime` / `HistoryReadEvents`,
+  value flows through IPC to the Host's `HistorianDataSource`, back to the client.
+
+## 2. Write-gating by role
+
+**Status**: `RoleBasedIdentity.Roles` populated on the session (PR 19) but
+`DriverNodeManager.OnWriteValue` doesn't consult it.
+
+CLAUDE.md defines the role set: `ReadOnly` / `WriteOperate` / `WriteTune` /
+`WriteConfigure` / `AlarmAck`. Each `DriverAttributeInfo.SecurityClassification`
+maps to a required role for writes.
+
+**To do**:
+- Add a `RoleRequirements` table: `SecurityClassification` → required role.
+- `OnWriteValue` reads `context.UserIdentity` → cast to `RoleBasedIdentity`
+  → check role membership before calling `IWritable.WriteAsync`. Return
+  `BadUserAccessDenied` on miss.
+- Unit test against a fake `ISystemContext` with varying role sets.
+
+## 3. Admin UI client-cert trust management
+
+**Status**: Server side auto-accepts untrusted client certs when the
+`AutoAcceptUntrustedClientCertificates` option is true (dev default).
+Production deployments want operator-controlled trust via the Admin UI.
+
+**To do**:
+- Surface the server's rejected-certificate store in the Admin UI.
+- Page to move certs between `rejected` / `trusted`.
+- Flip `AutoAcceptUntrustedClientCertificates` to false once Admin UI is the
+  trust gate.
+
+## 4. Live-LDAP integration test
+
+**Status**: PR 19 unit-tested the auth-flow shape; the live bind path is
+exercised only by the pre-existing `Admin.Tests/LdapLiveBindTests.cs` which
+uses the same Novell library against a running GLAuth at `localhost:3893`.
+
+**To do**:
+- Add `OpcUaServerIntegrationTests.Valid_username_authenticates_against_live_ldap`
+  with the same skip-when-unreachable guard.
+- Assert `session.Identity` on the server side carries the expected role
+  after bind — requires exposing a test hook or reading identity from a
+  new `IHostConnectivityProbe`-style "whoami" variable in the address space.
+
+## 5. Full Galaxy live-service smoke test against the merged v2 stack
+
+**Status**: Individual pieces have live smoke tests (PR 5 MXAccess, PR 13
+probe manager, PR 14 alarm tracker), but the full loop — OPC UA client →
+`OtOpcUaServer` → `GalaxyProxyDriver` (in-process) → named-pipe to
+Galaxy.Host subprocess → live MXAccess runtime → real Galaxy objects — has
+no single end-to-end smoke test.
+
+**To do**:
+- Test that spawns the full topology, discovers a deployed Galaxy object,
+  subscribes to one of its attributes, writes a value back, and asserts the
+  write round-tripped through MXAccess. Skip when ArchestrA isn't running.
+
+## 6. Second driver instance on the same server
+
+**Status**: `DriverHost.RegisterAsync` supports multiple drivers; the OPC UA
+server creates one `DriverNodeManager` per driver and isolates their
+subtrees under distinct namespace URIs. Not proven with two active
+`GalaxyProxyDriver` instances pointing at different Galaxies.
+
+**To do**:
+- Integration test that registers two driver instances, each with a distinct
+  `DriverInstanceId` + endpoint in its own session, asserts nodes from both
+  appear under the correct subtrees, alarm events land on the correct
+  instance's condition nodes.
+
+## 7. Host-status per-AppEngine granularity → Admin UI dashboard
+
+**Status**: PR 13 ships per-platform/per-AppEngine `ScanState` probing; PR 17
+surfaces the resulting `OnHostStatusChanged` events through OPC UA. Admin
+UI doesn't render a per-host dashboard yet.
+
+**To do**:
+- SignalR hub push of `HostStatusChangedEventArgs` to the Admin UI.
+- Dashboard page showing each tracked host, current state, last transition
+  time, failure count.

docs/v2/modbus-test-plan.md

@@ -0,0 +1,103 @@
+# Modbus driver — test plan + device-quirk catalog
+
+The Modbus TCP driver unit tests (PRs 21–24) cover the protocol surface against an
+in-memory fake transport. They validate the codec, state machine, and function-code
+routing against a textbook Modbus server. That's necessary but not sufficient: real PLC
+populations disagree with the spec in small, device-specific ways, and a driver that
+passes textbook tests can still misbehave against actual equipment.
+
+This doc is the harness-and-quirks playbook. It's what gets wired up in the
+`tests/Driver.Modbus.IntegrationTests` project when we ship that (PR 26 candidate).
+
+## Harness
+
+**Chosen simulator: ModbusPal** (Java, scriptable). Rationale:
+- Scriptable enough to mimic device-specific behaviors (non-standard register
+  layouts, custom exception codes, intentional response delays).
+- Runs locally, no CI dependency. Tests skip when `localhost:502` (or the configured
+  simulator endpoint) isn't reachable.
+- Free + long-maintained — physical PLC bench is unavailable in most dev
+  environments, and renting cloud PLCs isn't worth the per-test cost.
+
+**Setup pattern** (not yet codified in a script — will land alongside the integration
+test project):
+1. Install ModbusPal, load the per-device `.xmpp` profile from
+   `tests/Driver.Modbus.IntegrationTests/ModbusPal/` (TBD directory).
+2. Start the simulator listening on `localhost:502` (or override via
+   `MODBUS_SIM_ENDPOINT` env var).
+3. `dotnet test` the integration project — tests auto-skip when the endpoint is
+   unreachable, so forgetting to start the simulator doesn't wedge CI.
+
+## Per-device quirk catalog
+
+### AutomationDirect DL205
+
+First known target device. Quirks to document and cover with named tests (to be
+filled in when user validates each behavior in ModbusPal with a DL205 profile):
+
+- **Word order for 32-bit values**: _pending_ — confirm whether DL205 uses ABCD
+  (Modbus TCP standard) or CDAB (Siemens-style word-swap) for Int32/UInt32/Float32.
+  Test name: `DL205_Float32_word_order_is_CDAB` (or `ABCD`, whichever proves out).
+- **Register-zero access**: _pending_ — some DL205 configurations reject FC03 at
+  register 0 with exception code 02 (illegal data address). If confirmed, the
+  integration test suite verifies `ModbusProbeOptions.ProbeAddress` default of 0
+  triggers the rejection and operators must override; test name:
+  `DL205_FC03_at_register_0_returns_IllegalDataAddress`.
+- **Coil addressing base**: _pending_ — DL205 documentation sometimes uses 1-based
+  coil addresses; verify the driver's zero-based addressing matches the physical
+  PLC without an off-by-one adjustment.
+- **Maximum registers per FC03**: _pending_ — Modbus spec caps at 125; some DL205
+  models enforce a lower limit (e.g., 64). Test name:
+  `DL205_FC03_beyond_max_registers_returns_IllegalDataValue`.
+- **Response framing under sustained load**: _pending_ — the driver's
+  single-flight semaphore assumes the server pairs requests/responses by
+  transaction id; at least one DL205 firmware revision is reported to drop the
+  TxId under load. If reproduced in ModbusPal we add a retry + log-and-continue
+  path to `ModbusTcpTransport`.
+- **Exception code on coil write to a protected bit**: _pending_ — some DL205
+  setups protect internal coils; the driver should surface the PLC's exception
+  PDU as `BadNotWritable` rather than `BadInternalError`.
+
+_User action item_: as each quirk is validated in ModbusPal, replace the _pending_
+marker with the confirmed behavior and file a named test in the integration suite.
+
+### Future devices
+
+One section per device class, same shape as DL205. Quirks that apply across
+multiple devices (e.g., "all AB PLCs use CDAB") can be noted in the cross-device
+patterns section below once we have enough data points.
+
+## Cross-device patterns
+
+Once multiple device catalogs accumulate, quirks that recur across two or more
+vendors get promoted into driver defaults or opt-in options:
+
+- _(empty — filled in as catalogs grow)_
+
+## Test conventions
+
+- **One named test per quirk.** `DL205_word_order_is_CDAB_for_Float32` is easier to
+  diagnose on failure than a generic `Float32_roundtrip`. The `DL205_` prefix makes
+  filtering by device class trivial (`--filter "DisplayName~DL205"`).
+- **Skip with a clear SkipReason.** Follow the pattern from
+  `GalaxyRepositoryLiveSmokeTests`: check reachability in the fixture, capture
+  a `SkipReason` string, and have each test call `Assert.Skip(SkipReason)` when
+  it's set. Don't throw — skipped tests read cleanly in CI logs.
+- **Use the real `ModbusTcpTransport`.** Integration tests exercise the wire
+  protocol end-to-end. The in-memory `FakeTransport` from the unit test suite is
+  deliberately not used here — its value is speed + determinism, which doesn't
+  help reproduce device-specific issues.
+- **Don't depend on ModbusPal state between tests.** Each test resets the
+  simulator's register bank or uses a unique address range. Avoid relying on
+  "previous test left value at register 10" setups that flake when tests run in
+  parallel or re-order.
+
+## Next concrete PRs
+
+- **PR 26 — Integration test project + DL205 profile scaffold**: creates
+  `tests/Driver.Modbus.IntegrationTests`, imports the ModbusPal profile (or
+  generates it from JSON), adds the fixture with skip-when-unreachable, plus
+  one smoke test that reads a register. No DL205-specific assertions yet — that
+  waits for the user to validate each quirk.
+- **PR 27+**: one PR per confirmed DL205 quirk, landing the named test + any
+  driver-side adjustment (e.g., retry on dropped TxId) needed to pass it.

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/IModbusTransport.cs

@@ -0,0 +1,25 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Abstraction over the Modbus TCP socket. Takes a <c>PDU</c> (function code + data, excluding
+///     the 7-byte MBAP header) and returns the response PDU — the transport owns transaction-id
+///     pairing, framing, and socket I/O. Tests supply in-memory fakes.
+/// </summary>
+public interface IModbusTransport : IAsyncDisposable
+{
+    Task ConnectAsync(CancellationToken ct);
+
+    /// <summary>
+    ///     Send a Modbus PDU (function code + function-specific data) and read the response PDU.
+    ///     Throws <see cref="ModbusException"/> when the server returns an exception PDU
+    ///     (function code + 0x80 + exception code).
+    /// </summary>
+    Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct);
+}
+
+public sealed class ModbusException(byte functionCode, byte exceptionCode, string message)
+    : Exception(message)
+{
+    public byte FunctionCode { get; } = functionCode;
+    public byte ExceptionCode { get; } = exceptionCode;
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -0,0 +1,583 @@
+using System.Buffers.Binary;
+using System.Text.Json;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Modbus TCP implementation of <see cref="IDriver"/> + <see cref="ITagDiscovery"/> +
+///     <see cref="IReadable"/> + <see cref="IWritable"/>. First native-protocol greenfield
+///     driver for the v2 stack — validates the driver-agnostic <c>IAddressSpaceBuilder</c> +
+///     <c>IReadable</c>/<c>IWritable</c> abstractions generalize beyond Galaxy.
+/// </summary>
+/// <remarks>
+///     Scope limits: synchronous Read/Write only, no subscriptions (Modbus has no push model;
+///     subscriptions would need a polling loop over the declared tags — additive PR). Historian
+///     + alarm capabilities are out of scope (the protocol doesn't express them).
+/// </remarks>
+public sealed class ModbusDriver(ModbusDriverOptions options, string driverInstanceId,
+    Func<ModbusDriverOptions, IModbusTransport>? transportFactory = null)
+    : IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IDisposable, IAsyncDisposable
+{
+    // Active polling subscriptions. Each subscription owns a background Task that polls the
+    // tags at its configured interval, diffs against _lastKnownValues, and fires OnDataChange
+    // per changed tag. UnsubscribeAsync cancels the task via the CTS stored on the handle.
+    private readonly System.Collections.Concurrent.ConcurrentDictionary<long, SubscriptionState> _subscriptions = new();
+    private long _nextSubscriptionId;
+
+    public event EventHandler<DataChangeEventArgs>? OnDataChange;
+    public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;
+
+    // Single-host probe state — Modbus driver talks to exactly one endpoint so the "hosts"
+    // collection has at most one entry. HostName is the Host:Port string so the Admin UI can
+    // display the PLC endpoint uniformly with Galaxy platforms/engines.
+    private readonly object _probeLock = new();
+    private HostState _hostState = HostState.Unknown;
+    private DateTime _hostStateChangedUtc = DateTime.UtcNow;
+    private CancellationTokenSource? _probeCts;
+    private readonly ModbusDriverOptions _options = options;
+    private readonly Func<ModbusDriverOptions, IModbusTransport> _transportFactory =
+        transportFactory ?? (o => new ModbusTcpTransport(o.Host, o.Port, o.Timeout));
+
+    private IModbusTransport? _transport;
+    private DriverHealth _health = new(DriverState.Unknown, null, null);
+    private readonly Dictionary<string, ModbusTagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
+
+    public string DriverInstanceId => driverInstanceId;
+    public string DriverType => "Modbus";
+
+    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        _health = new DriverHealth(DriverState.Initializing, null, null);
+        try
+        {
+            _transport = _transportFactory(_options);
+            await _transport.ConnectAsync(cancellationToken).ConfigureAwait(false);
+            foreach (var t in _options.Tags) _tagsByName[t.Name] = t;
+            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
+
+            // PR 23: kick off the probe loop once the transport is up. Initial state stays
+            // Unknown until the first probe tick succeeds — avoids broadcasting a premature
+            // Running transition before any register round-trip has happened.
+            if (_options.Probe.Enabled)
+            {
+                _probeCts = new CancellationTokenSource();
+                _ = Task.Run(() => ProbeLoopAsync(_probeCts.Token), _probeCts.Token);
+            }
+        }
+        catch (Exception ex)
+        {
+            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
+            throw;
+        }
+    }
+
+    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        await ShutdownAsync(cancellationToken);
+        await InitializeAsync(driverConfigJson, cancellationToken);
+    }
+
+    public async Task ShutdownAsync(CancellationToken cancellationToken)
+    {
+        try { _probeCts?.Cancel(); } catch { }
+        _probeCts?.Dispose();
+        _probeCts = null;
+
+        foreach (var state in _subscriptions.Values)
+        {
+            try { state.Cts.Cancel(); } catch { }
+            state.Cts.Dispose();
+        }
+        _subscriptions.Clear();
+
+        if (_transport is not null) await _transport.DisposeAsync().ConfigureAwait(false);
+        _transport = null;
+        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
+    }
+
+    public DriverHealth GetHealth() => _health;
+    public long GetMemoryFootprint() => 0;
+    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    // ---- ITagDiscovery ----
+
+    public Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+        var folder = builder.Folder("Modbus", "Modbus");
+        foreach (var t in _options.Tags)
+        {
+            folder.Variable(t.Name, t.Name, new DriverAttributeInfo(
+                FullName: t.Name,
+                DriverDataType: MapDataType(t.DataType),
+                IsArray: false,
+                ArrayDim: null,
+                SecurityClass: t.Writable ? SecurityClassification.Operate : SecurityClassification.ViewOnly,
+                IsHistorized: false,
+                IsAlarm: false));
+        }
+        return Task.CompletedTask;
+    }
+
+    // ---- IReadable ----
+
+    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        var transport = RequireTransport();
+        var now = DateTime.UtcNow;
+        var results = new DataValueSnapshot[fullReferences.Count];
+        for (var i = 0; i < fullReferences.Count; i++)
+        {
+            if (!_tagsByName.TryGetValue(fullReferences[i], out var tag))
+            {
+                results[i] = new DataValueSnapshot(null, StatusBadNodeIdUnknown, null, now);
+                continue;
+            }
+            try
+            {
+                var value = await ReadOneAsync(transport, tag, cancellationToken).ConfigureAwait(false);
+                results[i] = new DataValueSnapshot(value, 0u, now, now);
+                _health = new DriverHealth(DriverState.Healthy, now, null);
+            }
+            catch (Exception ex)
+            {
+                results[i] = new DataValueSnapshot(null, StatusBadInternalError, null, now);
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            }
+        }
+        return results;
+    }
+
+    private async Task<object> ReadOneAsync(IModbusTransport transport, ModbusTagDefinition tag, CancellationToken ct)
+    {
+        switch (tag.Region)
+        {
+            case ModbusRegion.Coils:
+            {
+                var pdu = new byte[] { 0x01, (byte)(tag.Address >> 8), (byte)(tag.Address & 0xFF), 0x00, 0x01 };
+                var resp = await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+                return (resp[2] & 0x01) == 1;
+            }
+            case ModbusRegion.DiscreteInputs:
+            {
+                var pdu = new byte[] { 0x02, (byte)(tag.Address >> 8), (byte)(tag.Address & 0xFF), 0x00, 0x01 };
+                var resp = await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+                return (resp[2] & 0x01) == 1;
+            }
+            case ModbusRegion.HoldingRegisters:
+            case ModbusRegion.InputRegisters:
+            {
+                var quantity = RegisterCount(tag);
+                var fc = tag.Region == ModbusRegion.HoldingRegisters ? (byte)0x03 : (byte)0x04;
+                var pdu = new byte[] { fc, (byte)(tag.Address >> 8), (byte)(tag.Address & 0xFF),
+                                       (byte)(quantity >> 8), (byte)(quantity & 0xFF) };
+                var resp = await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+                // resp = [fc][byte-count][data...]
+                var data = new ReadOnlySpan<byte>(resp, 2, resp[1]);
+                return DecodeRegister(data, tag);
+            }
+            default:
+                throw new InvalidOperationException($"Unknown region {tag.Region}");
+        }
+    }
+
+    // ---- IWritable ----
+
+    public async Task<IReadOnlyList<WriteResult>> WriteAsync(
+        IReadOnlyList<WriteRequest> writes, CancellationToken cancellationToken)
+    {
+        var transport = RequireTransport();
+        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 tag))
+            {
+                results[i] = new WriteResult(StatusBadNodeIdUnknown);
+                continue;
+            }
+            if (!tag.Writable || tag.Region is ModbusRegion.DiscreteInputs or ModbusRegion.InputRegisters)
+            {
+                results[i] = new WriteResult(StatusBadNotWritable);
+                continue;
+            }
+            try
+            {
+                await WriteOneAsync(transport, tag, w.Value, cancellationToken).ConfigureAwait(false);
+                results[i] = new WriteResult(0u);
+            }
+            catch (Exception)
+            {
+                results[i] = new WriteResult(StatusBadInternalError);
+            }
+        }
+        return results;
+    }
+
+    private async Task WriteOneAsync(IModbusTransport transport, ModbusTagDefinition tag, object? value, CancellationToken ct)
+    {
+        switch (tag.Region)
+        {
+            case ModbusRegion.Coils:
+            {
+                var on = Convert.ToBoolean(value);
+                var pdu = new byte[] { 0x05, (byte)(tag.Address >> 8), (byte)(tag.Address & 0xFF),
+                                       on ? (byte)0xFF : (byte)0x00, 0x00 };
+                await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+                return;
+            }
+            case ModbusRegion.HoldingRegisters:
+            {
+                var bytes = EncodeRegister(value, tag);
+                if (bytes.Length == 2)
+                {
+                    var pdu = new byte[] { 0x06, (byte)(tag.Address >> 8), (byte)(tag.Address & 0xFF),
+                                           bytes[0], bytes[1] };
+                    await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+                }
+                else
+                {
+                    // FC 16 (Write Multiple Registers) for 32-bit types
+                    var qty = (ushort)(bytes.Length / 2);
+                    var pdu = new byte[6 + 1 + bytes.Length];
+                    pdu[0] = 0x10;
+                    pdu[1] = (byte)(tag.Address >> 8); pdu[2] = (byte)(tag.Address & 0xFF);
+                    pdu[3] = (byte)(qty >> 8); pdu[4] = (byte)(qty & 0xFF);
+                    pdu[5] = (byte)bytes.Length;
+                    Buffer.BlockCopy(bytes, 0, pdu, 6, bytes.Length);
+                    await transport.SendAsync(_options.UnitId, pdu, ct).ConfigureAwait(false);
+                }
+                return;
+            }
+            default:
+                throw new InvalidOperationException($"Writes not supported for region {tag.Region}");
+        }
+    }
+
+    // ---- ISubscribable (polling overlay) ----
+
+    public Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+    {
+        var id = Interlocked.Increment(ref _nextSubscriptionId);
+        var cts = new CancellationTokenSource();
+        var interval = publishingInterval < TimeSpan.FromMilliseconds(100)
+            ? TimeSpan.FromMilliseconds(100) // floor — Modbus can't sustain < 100ms polling reliably
+            : publishingInterval;
+        var handle = new ModbusSubscriptionHandle(id);
+        var state = new SubscriptionState(handle, [.. fullReferences], interval, cts);
+        _subscriptions[id] = state;
+        _ = Task.Run(() => PollLoopAsync(state, cts.Token), cts.Token);
+        return Task.FromResult<ISubscriptionHandle>(handle);
+    }
+
+    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    {
+        if (handle is ModbusSubscriptionHandle h && _subscriptions.TryRemove(h.Id, out var state))
+        {
+            state.Cts.Cancel();
+            state.Cts.Dispose();
+        }
+        return Task.CompletedTask;
+    }
+
+    private async Task PollLoopAsync(SubscriptionState state, CancellationToken ct)
+    {
+        // Initial-data push: read every tag once at subscribe time so OPC UA clients see the
+        // current value per Part 4 convention, even if the value never changes thereafter.
+        try { await PollOnceAsync(state, forceRaise: true, ct).ConfigureAwait(false); }
+        catch (OperationCanceledException) { return; }
+        catch { /* first-read error — polling continues */ }
+
+        while (!ct.IsCancellationRequested)
+        {
+            try { await Task.Delay(state.Interval, ct).ConfigureAwait(false); }
+            catch (OperationCanceledException) { return; }
+
+            try { await PollOnceAsync(state, forceRaise: false, ct).ConfigureAwait(false); }
+            catch (OperationCanceledException) { return; }
+            catch { /* transient polling error — loop continues, health surface reflects it */ }
+        }
+    }
+
+    private async Task PollOnceAsync(SubscriptionState state, bool forceRaise, CancellationToken ct)
+    {
+        var snapshots = await ReadAsync(state.TagReferences, ct).ConfigureAwait(false);
+        for (var i = 0; i < state.TagReferences.Count; i++)
+        {
+            var tagRef = state.TagReferences[i];
+            var current = snapshots[i];
+            var lastSeen = state.LastValues.TryGetValue(tagRef, out var prev) ? prev : default;
+
+            // Raise on first read (forceRaise) OR when the boxed value differs from last-known.
+            if (forceRaise || !Equals(lastSeen?.Value, current.Value) || lastSeen?.StatusCode != current.StatusCode)
+            {
+                state.LastValues[tagRef] = current;
+                OnDataChange?.Invoke(this, new DataChangeEventArgs(state.Handle, tagRef, current));
+            }
+        }
+    }
+
+    private sealed record SubscriptionState(
+        ModbusSubscriptionHandle Handle,
+        IReadOnlyList<string> TagReferences,
+        TimeSpan Interval,
+        CancellationTokenSource Cts)
+    {
+        public System.Collections.Concurrent.ConcurrentDictionary<string, DataValueSnapshot> LastValues { get; }
+            = new(StringComparer.OrdinalIgnoreCase);
+    }
+
+    private sealed record ModbusSubscriptionHandle(long Id) : ISubscriptionHandle
+    {
+        public string DiagnosticId => $"modbus-sub-{Id}";
+    }
+
+    // ---- IHostConnectivityProbe ----
+
+    public IReadOnlyList<HostConnectivityStatus> GetHostStatuses()
+    {
+        lock (_probeLock)
+            return [new HostConnectivityStatus(HostName, _hostState, _hostStateChangedUtc)];
+    }
+
+    /// <summary>
+    ///     Host identifier surfaced to <c>IHostConnectivityProbe.GetHostStatuses</c> and the Admin UI.
+    ///     Formatted as <c>host:port</c> so multiple Modbus drivers in the same server disambiguate
+    ///     by endpoint without needing the driver-instance-id in the Admin dashboard.
+    /// </summary>
+    public string HostName => $"{_options.Host}:{_options.Port}";
+
+    private async Task ProbeLoopAsync(CancellationToken ct)
+    {
+        var transport = _transport; // captured reference; disposal tears the loop down via ct
+        while (!ct.IsCancellationRequested)
+        {
+            var success = false;
+            try
+            {
+                using var probeCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+                probeCts.CancelAfter(_options.Probe.Timeout);
+                var pdu = new byte[] { 0x03,
+                    (byte)(_options.Probe.ProbeAddress >> 8),
+                    (byte)(_options.Probe.ProbeAddress & 0xFF), 0x00, 0x01 };
+                _ = await transport!.SendAsync(_options.UnitId, pdu, probeCts.Token).ConfigureAwait(false);
+                success = true;
+            }
+            catch (OperationCanceledException) when (ct.IsCancellationRequested)
+            {
+                return;
+            }
+            catch
+            {
+                // transport / timeout / exception PDU — treated as Stopped below
+            }
+
+            TransitionTo(success ? HostState.Running : HostState.Stopped);
+
+            try { await Task.Delay(_options.Probe.Interval, ct).ConfigureAwait(false); }
+            catch (OperationCanceledException) { return; }
+        }
+    }
+
+    private void TransitionTo(HostState newState)
+    {
+        HostState old;
+        lock (_probeLock)
+        {
+            old = _hostState;
+            if (old == newState) return;
+            _hostState = newState;
+            _hostStateChangedUtc = DateTime.UtcNow;
+        }
+        OnHostStatusChanged?.Invoke(this, new HostStatusChangedEventArgs(HostName, old, newState));
+    }
+
+    // ---- codec ----
+
+    /// <summary>
+    ///     How many 16-bit registers a given tag occupies. Accounts for multi-register logical
+    ///     types (Int32/Float32 = 2 regs, Int64/Float64 = 4 regs) and for strings (rounded up
+    ///     from 2 chars per register).
+    /// </summary>
+    internal static ushort RegisterCount(ModbusTagDefinition tag) => tag.DataType switch
+    {
+        ModbusDataType.Int16 or ModbusDataType.UInt16 or ModbusDataType.BitInRegister => 1,
+        ModbusDataType.Int32 or ModbusDataType.UInt32 or ModbusDataType.Float32 => 2,
+        ModbusDataType.Int64 or ModbusDataType.UInt64 or ModbusDataType.Float64 => 4,
+        ModbusDataType.String => (ushort)((tag.StringLength + 1) / 2), // 2 chars per register
+        _ => throw new InvalidOperationException($"Non-register data type {tag.DataType}"),
+    };
+
+    /// <summary>
+    ///     Word-swap the input into the big-endian layout the decoders expect. For 2-register
+    ///     types this reverses the two words; for 4-register types it reverses the four words
+    ///     (PLC stored [hi-mid, low-mid, hi-high, low-high] → memory [hi-high, low-high, hi-mid, low-mid]).
+    /// </summary>
+    private static byte[] NormalizeWordOrder(ReadOnlySpan<byte> data, ModbusByteOrder order)
+    {
+        if (order == ModbusByteOrder.BigEndian) return data.ToArray();
+        var result = new byte[data.Length];
+        for (var word = 0; word < data.Length / 2; word++)
+        {
+            var srcWord = data.Length / 2 - 1 - word;
+            result[word * 2] = data[srcWord * 2];
+            result[word * 2 + 1] = data[srcWord * 2 + 1];
+        }
+        return result;
+    }
+
+    internal static object DecodeRegister(ReadOnlySpan<byte> data, ModbusTagDefinition tag)
+    {
+        switch (tag.DataType)
+        {
+            case ModbusDataType.Int16: return BinaryPrimitives.ReadInt16BigEndian(data);
+            case ModbusDataType.UInt16: return BinaryPrimitives.ReadUInt16BigEndian(data);
+            case ModbusDataType.BitInRegister:
+            {
+                var raw = BinaryPrimitives.ReadUInt16BigEndian(data);
+                return (raw & (1 << tag.BitIndex)) != 0;
+            }
+            case ModbusDataType.Int32:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                return BinaryPrimitives.ReadInt32BigEndian(b);
+            }
+            case ModbusDataType.UInt32:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                return BinaryPrimitives.ReadUInt32BigEndian(b);
+            }
+            case ModbusDataType.Float32:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                return BinaryPrimitives.ReadSingleBigEndian(b);
+            }
+            case ModbusDataType.Int64:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                return BinaryPrimitives.ReadInt64BigEndian(b);
+            }
+            case ModbusDataType.UInt64:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                return BinaryPrimitives.ReadUInt64BigEndian(b);
+            }
+            case ModbusDataType.Float64:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                return BinaryPrimitives.ReadDoubleBigEndian(b);
+            }
+            case ModbusDataType.String:
+            {
+                // ASCII, 2 chars per register, packed high byte = first char.
+                // Respect the caller's StringLength (truncate nul-padded regions).
+                var chars = new char[tag.StringLength];
+                for (var i = 0; i < tag.StringLength; i++)
+                {
+                    var b = data[i];
+                    if (b == 0) { return new string(chars, 0, i); }
+                    chars[i] = (char)b;
+                }
+                return new string(chars);
+            }
+            default:
+                throw new InvalidOperationException($"Non-register data type {tag.DataType}");
+        }
+    }
+
+    internal static byte[] EncodeRegister(object? value, ModbusTagDefinition tag)
+    {
+        switch (tag.DataType)
+        {
+            case ModbusDataType.Int16:
+            {
+                var v = Convert.ToInt16(value);
+                var b = new byte[2]; BinaryPrimitives.WriteInt16BigEndian(b, v); return b;
+            }
+            case ModbusDataType.UInt16:
+            {
+                var v = Convert.ToUInt16(value);
+                var b = new byte[2]; BinaryPrimitives.WriteUInt16BigEndian(b, v); return b;
+            }
+            case ModbusDataType.Int32:
+            {
+                var v = Convert.ToInt32(value);
+                var b = new byte[4]; BinaryPrimitives.WriteInt32BigEndian(b, v);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
+            case ModbusDataType.UInt32:
+            {
+                var v = Convert.ToUInt32(value);
+                var b = new byte[4]; BinaryPrimitives.WriteUInt32BigEndian(b, v);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
+            case ModbusDataType.Float32:
+            {
+                var v = Convert.ToSingle(value);
+                var b = new byte[4]; BinaryPrimitives.WriteSingleBigEndian(b, v);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
+            case ModbusDataType.Int64:
+            {
+                var v = Convert.ToInt64(value);
+                var b = new byte[8]; BinaryPrimitives.WriteInt64BigEndian(b, v);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
+            case ModbusDataType.UInt64:
+            {
+                var v = Convert.ToUInt64(value);
+                var b = new byte[8]; BinaryPrimitives.WriteUInt64BigEndian(b, v);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
+            case ModbusDataType.Float64:
+            {
+                var v = Convert.ToDouble(value);
+                var b = new byte[8]; BinaryPrimitives.WriteDoubleBigEndian(b, v);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
+            case ModbusDataType.String:
+            {
+                var s = Convert.ToString(value) ?? string.Empty;
+                var regs = (tag.StringLength + 1) / 2;
+                var b = new byte[regs * 2];
+                for (var i = 0; i < tag.StringLength && i < s.Length; i++) b[i] = (byte)s[i];
+                // remaining bytes stay 0 — nul-padded per PLC convention
+                return b;
+            }
+            case ModbusDataType.BitInRegister:
+                throw new InvalidOperationException(
+                    "BitInRegister writes require a read-modify-write; not supported in PR 24 (separate follow-up).");
+            default:
+                throw new InvalidOperationException($"Non-register data type {tag.DataType}");
+        }
+    }
+
+    private static DriverDataType MapDataType(ModbusDataType t) => t switch
+    {
+        ModbusDataType.Bool or ModbusDataType.BitInRegister => DriverDataType.Boolean,
+        ModbusDataType.Int16 or ModbusDataType.Int32 => DriverDataType.Int32,
+        ModbusDataType.UInt16 or ModbusDataType.UInt32 => DriverDataType.Int32,
+        ModbusDataType.Int64 or ModbusDataType.UInt64 => DriverDataType.Int32, // widening to Int32 loses precision; PR 25 adds Int64 to DriverDataType
+        ModbusDataType.Float32 => DriverDataType.Float32,
+        ModbusDataType.Float64 => DriverDataType.Float64,
+        ModbusDataType.String => DriverDataType.String,
+        _ => DriverDataType.Int32,
+    };
+
+    private IModbusTransport RequireTransport() =>
+        _transport ?? throw new InvalidOperationException("ModbusDriver not initialized");
+
+    private const uint StatusBadInternalError = 0x80020000u;
+    private const uint StatusBadNodeIdUnknown = 0x80340000u;
+    private const uint StatusBadNotWritable = 0x803B0000u;
+
+    public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();
+    public async ValueTask DisposeAsync()
+    {
+        if (_transport is not null) await _transport.DisposeAsync().ConfigureAwait(false);
+        _transport = null;
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverOptions.cs

@@ -0,0 +1,97 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Modbus TCP driver configuration. Bound from the driver's <c>DriverConfig</c> JSON at
+///     <c>DriverHost.RegisterAsync</c>. Every register the driver exposes appears in
+///     <see cref="Tags"/>; names become the OPC UA browse name + full reference.
+/// </summary>
+public sealed class ModbusDriverOptions
+{
+    public string Host { get; init; } = "127.0.0.1";
+    public int Port { get; init; } = 502;
+    public byte UnitId { get; init; } = 1;
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+
+    /// <summary>Pre-declared tag map. Modbus has no discovery protocol — the driver returns exactly these.</summary>
+    public IReadOnlyList<ModbusTagDefinition> Tags { get; init; } = [];
+
+    /// <summary>
+    ///     Background connectivity-probe settings. When <see cref="ModbusProbeOptions.Enabled"/>
+    ///     is true the driver runs a tick loop that issues a cheap FC03 at register 0 every
+    ///     <see cref="ModbusProbeOptions.Interval"/> and raises <c>OnHostStatusChanged</c> on
+    ///     Running ↔ Stopped transitions. The Admin UI / OPC UA clients see the state through
+    ///     <see cref="IHostConnectivityProbe"/>.
+    /// </summary>
+    public ModbusProbeOptions Probe { get; init; } = new();
+}
+
+public sealed class ModbusProbeOptions
+{
+    public bool Enabled { get; init; } = true;
+    public TimeSpan Interval { get; init; } = TimeSpan.FromSeconds(5);
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+    /// <summary>Register to read for the probe. Zero is usually safe; override for PLCs that lock register 0.</summary>
+    public ushort ProbeAddress { get; init; } = 0;
+}
+
+/// <summary>
+///     One Modbus-backed OPC UA variable. Address is zero-based (Modbus spec numbering, not
+///     the documentation's 1-based coil/register conventions). Multi-register types
+///     (Int32/UInt32/Float32 = 2 regs; Int64/UInt64/Float64 = 4 regs) respect the
+///     <see cref="ByteOrder"/> field — real-world PLCs disagree on word ordering.
+/// </summary>
+/// <param name="Name">
+///     Tag name, used for both the OPC UA browse name and the driver's full reference. Must be
+///     unique within the driver.
+/// </param>
+/// <param name="Region">Coils / DiscreteInputs / InputRegisters / HoldingRegisters.</param>
+/// <param name="Address">Zero-based address within the region.</param>
+/// <param name="DataType">
+///     Logical data type. See <see cref="ModbusDataType"/> for the register count each encodes.
+/// </param>
+/// <param name="Writable">When true and Region supports writes (Coils / HoldingRegisters), IWritable routes writes here.</param>
+/// <param name="ByteOrder">Word ordering for multi-register types. Ignored for Bool / Int16 / UInt16 / BitInRegister / String.</param>
+/// <param name="BitIndex">For <c>DataType = BitInRegister</c>: which bit of the holding register (0-15, LSB-first).</param>
+/// <param name="StringLength">For <c>DataType = String</c>: number of ASCII characters (2 per register, rounded up).</param>
+public sealed record ModbusTagDefinition(
+    string Name,
+    ModbusRegion Region,
+    ushort Address,
+    ModbusDataType DataType,
+    bool Writable = true,
+    ModbusByteOrder ByteOrder = ModbusByteOrder.BigEndian,
+    byte BitIndex = 0,
+    ushort StringLength = 0);
+
+public enum ModbusRegion { Coils, DiscreteInputs, InputRegisters, HoldingRegisters }
+
+public enum ModbusDataType
+{
+    Bool,
+    Int16,
+    UInt16,
+    Int32,
+    UInt32,
+    Int64,
+    UInt64,
+    Float32,
+    Float64,
+    /// <summary>Single bit within a holding register. <see cref="ModbusTagDefinition.BitIndex"/> selects 0-15 LSB-first.</summary>
+    BitInRegister,
+    /// <summary>ASCII string packed 2 chars per register, <see cref="ModbusTagDefinition.StringLength"/> characters long.</summary>
+    String,
+}
+
+/// <summary>
+///     Word ordering for multi-register types. Modbus TCP standard is <see cref="BigEndian"/>
+///     (ABCD for 32-bit: high word at the lower address). Many PLCs — Siemens S7, several
+///     Allen-Bradley series, some Modicon families — use <see cref="WordSwap"/> (CDAB), which
+///     keeps bytes big-endian within each register but reverses the word pair(s).
+/// </summary>
+public enum ModbusByteOrder
+{
+    BigEndian,
+    WordSwap,
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusTcpTransport.cs

@@ -0,0 +1,113 @@
+using System.Net.Sockets;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Concrete Modbus TCP transport. Wraps a single <see cref="TcpClient"/> and serializes
+///     requests so at most one transaction is in-flight at a time — Modbus servers typically
+///     support concurrent transactions, but the single-flight model keeps the wire trace
+///     easy to diagnose and avoids interleaved-response correlation bugs.
+/// </summary>
+public sealed class ModbusTcpTransport : IModbusTransport
+{
+    private readonly string _host;
+    private readonly int _port;
+    private readonly TimeSpan _timeout;
+    private readonly SemaphoreSlim _gate = new(1, 1);
+    private TcpClient? _client;
+    private NetworkStream? _stream;
+    private ushort _nextTx;
+    private bool _disposed;
+
+    public ModbusTcpTransport(string host, int port, TimeSpan timeout)
+    {
+        _host = host;
+        _port = port;
+        _timeout = timeout;
+    }
+
+    public async Task ConnectAsync(CancellationToken ct)
+    {
+        _client = new TcpClient();
+        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        cts.CancelAfter(_timeout);
+        await _client.ConnectAsync(_host, _port, cts.Token).ConfigureAwait(false);
+        _stream = _client.GetStream();
+    }
+
+    public async Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+    {
+        if (_disposed) throw new ObjectDisposedException(nameof(ModbusTcpTransport));
+        if (_stream is null) throw new InvalidOperationException("Transport not connected");
+
+        await _gate.WaitAsync(ct).ConfigureAwait(false);
+        try
+        {
+            var txId = ++_nextTx;
+
+            // MBAP: [TxId(2)][Proto=0(2)][Length(2)][UnitId(1)] + PDU
+            var adu = new byte[7 + pdu.Length];
+            adu[0] = (byte)(txId >> 8);
+            adu[1] = (byte)(txId & 0xFF);
+            // protocol id already zero
+            var len = (ushort)(1 + pdu.Length); // unit id + pdu
+            adu[4] = (byte)(len >> 8);
+            adu[5] = (byte)(len & 0xFF);
+            adu[6] = unitId;
+            Buffer.BlockCopy(pdu, 0, adu, 7, pdu.Length);
+
+            using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+            cts.CancelAfter(_timeout);
+            await _stream.WriteAsync(adu.AsMemory(), cts.Token).ConfigureAwait(false);
+            await _stream.FlushAsync(cts.Token).ConfigureAwait(false);
+
+            var header = new byte[7];
+            await ReadExactlyAsync(_stream, header, cts.Token).ConfigureAwait(false);
+            var respTxId = (ushort)((header[0] << 8) | header[1]);
+            if (respTxId != txId)
+                throw new InvalidDataException($"Modbus TxId mismatch: expected {txId} got {respTxId}");
+            var respLen = (ushort)((header[4] << 8) | header[5]);
+            if (respLen < 1) throw new InvalidDataException($"Modbus response length too small: {respLen}");
+            var respPdu = new byte[respLen - 1];
+            await ReadExactlyAsync(_stream, respPdu, cts.Token).ConfigureAwait(false);
+
+            // Exception PDU: function code has high bit set.
+            if ((respPdu[0] & 0x80) != 0)
+            {
+                var fc = (byte)(respPdu[0] & 0x7F);
+                var ex = respPdu[1];
+                throw new ModbusException(fc, ex, $"Modbus exception fc={fc} code={ex}");
+            }
+
+            return respPdu;
+        }
+        finally
+        {
+            _gate.Release();
+        }
+    }
+
+    private static async Task ReadExactlyAsync(Stream s, byte[] buf, CancellationToken ct)
+    {
+        var read = 0;
+        while (read < buf.Length)
+        {
+            var n = await s.ReadAsync(buf.AsMemory(read), ct).ConfigureAwait(false);
+            if (n == 0) throw new EndOfStreamException("Modbus socket closed mid-response");
+            read += n;
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        try
+        {
+            if (_stream is not null) await _stream.DisposeAsync().ConfigureAwait(false);
+        }
+        catch { /* best-effort */ }
+        _client?.Dispose();
+        _gate.Dispose();
+    }
+}

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

@@ -0,0 +1,27 @@
+<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.Modbus</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-37gx-xxp4-5rgx"/>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-w3x6-4m5h-cxqf"/>
+    </ItemGroup>
+
+</Project>

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusDataTypeTests.cs

@@ -0,0 +1,175 @@
+using System.Buffers.Binary;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class ModbusDataTypeTests
+{
+    /// <summary>
+    ///     Register-count lookup is per-tag now (strings need StringLength; Int64/Float64 need 4).
+    /// </summary>
+    [Theory]
+    [InlineData(ModbusDataType.BitInRegister, 1)]
+    [InlineData(ModbusDataType.Int16, 1)]
+    [InlineData(ModbusDataType.UInt16, 1)]
+    [InlineData(ModbusDataType.Int32, 2)]
+    [InlineData(ModbusDataType.UInt32, 2)]
+    [InlineData(ModbusDataType.Float32, 2)]
+    [InlineData(ModbusDataType.Int64, 4)]
+    [InlineData(ModbusDataType.UInt64, 4)]
+    [InlineData(ModbusDataType.Float64, 4)]
+    public void RegisterCount_returns_correct_register_count_per_type(ModbusDataType t, int expected)
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, t);
+        ModbusDriver.RegisterCount(tag).ShouldBe((ushort)expected);
+    }
+
+    [Theory]
+    [InlineData(0, 1)]   // 0 chars → still 1 byte / 1 register (pathological but well-defined: length 0 is 0 bytes)
+    [InlineData(1, 1)]
+    [InlineData(2, 1)]
+    [InlineData(3, 2)]
+    [InlineData(10, 5)]
+    public void RegisterCount_for_String_rounds_up_to_register_pair(ushort chars, int expectedRegs)
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String, StringLength: chars);
+        // 0-char is encoded as 0 regs; the test case expects 1 for lengths 1-2, 2 for 3-4, etc.
+        if (chars == 0) ModbusDriver.RegisterCount(tag).ShouldBe((ushort)0);
+        else ModbusDriver.RegisterCount(tag).ShouldBe((ushort)expectedRegs);
+    }
+
+    // --- Int32 / UInt32 / Float32 with byte-order variants ---
+
+    [Fact]
+    public void Int32_BigEndian_decodes_ABCD_layout()
+    {
+        // Value 0x12345678 → bytes [0x12, 0x34, 0x56, 0x78] as PLC wrote them.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int32,
+            ByteOrder: ModbusByteOrder.BigEndian);
+        var bytes = new byte[] { 0x12, 0x34, 0x56, 0x78 };
+        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe(0x12345678);
+    }
+
+    [Fact]
+    public void Int32_WordSwap_decodes_CDAB_layout()
+    {
+        // Siemens/AB PLC stored 0x12345678 as register[0] = 0x5678, register[1] = 0x1234.
+        // Wire bytes are [0x56, 0x78, 0x12, 0x34]; with ByteOrder=WordSwap we get 0x12345678 back.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int32,
+            ByteOrder: ModbusByteOrder.WordSwap);
+        var bytes = new byte[] { 0x56, 0x78, 0x12, 0x34 };
+        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe(0x12345678);
+    }
+
+    [Fact]
+    public void Float32_WordSwap_encode_decode_roundtrips()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Float32,
+            ByteOrder: ModbusByteOrder.WordSwap);
+        var wire = ModbusDriver.EncodeRegister(25.5f, tag);
+        wire.Length.ShouldBe(4);
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe(25.5f);
+    }
+
+    // --- Int64 / UInt64 / Float64 ---
+
+    [Fact]
+    public void Int64_BigEndian_roundtrips()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int64);
+        var wire = ModbusDriver.EncodeRegister(0x0123456789ABCDEFL, tag);
+        wire.Length.ShouldBe(8);
+        BinaryPrimitives.ReadInt64BigEndian(wire).ShouldBe(0x0123456789ABCDEFL);
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe(0x0123456789ABCDEFL);
+    }
+
+    [Fact]
+    public void UInt64_WordSwap_reverses_four_words()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.UInt64,
+            ByteOrder: ModbusByteOrder.WordSwap);
+        var value = 0xAABBCCDDEEFF0011UL;
+
+        var wireBE = new byte[8];
+        BinaryPrimitives.WriteUInt64BigEndian(wireBE, value);
+
+        // Word-swap layout: [word3, word2, word1, word0] where each word keeps its bytes big-endian.
+        var wireWS = new byte[] { wireBE[6], wireBE[7], wireBE[4], wireBE[5], wireBE[2], wireBE[3], wireBE[0], wireBE[1] };
+        ModbusDriver.DecodeRegister(wireWS, tag).ShouldBe(value);
+
+        var roundtrip = ModbusDriver.EncodeRegister(value, tag);
+        roundtrip.ShouldBe(wireWS);
+    }
+
+    [Fact]
+    public void Float64_roundtrips_under_word_swap()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Float64,
+            ByteOrder: ModbusByteOrder.WordSwap);
+        var wire = ModbusDriver.EncodeRegister(3.14159265358979d, tag);
+        wire.Length.ShouldBe(8);
+        ((double)ModbusDriver.DecodeRegister(wire, tag)!).ShouldBe(3.14159265358979d, tolerance: 1e-12);
+    }
+
+    // --- BitInRegister ---
+
+    [Theory]
+    [InlineData(0b0000_0000_0000_0001, 0, true)]
+    [InlineData(0b0000_0000_0000_0001, 1, false)]
+    [InlineData(0b1000_0000_0000_0000, 15, true)]
+    [InlineData(0b0100_0000_0100_0000, 6, true)]
+    [InlineData(0b0100_0000_0100_0000, 14, true)]
+    [InlineData(0b0100_0000_0100_0000, 7, false)]
+    public void BitInRegister_extracts_bit_at_index(ushort raw, byte bitIndex, bool expected)
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.BitInRegister,
+            BitIndex: bitIndex);
+        var bytes = new byte[] { (byte)(raw >> 8), (byte)(raw & 0xFF) };
+        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void BitInRegister_write_is_not_supported_in_PR24()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.BitInRegister,
+            BitIndex: 5);
+        Should.Throw<InvalidOperationException>(() => ModbusDriver.EncodeRegister(true, tag))
+            .Message.ShouldContain("read-modify-write");
+    }
+
+    // --- String ---
+
+    [Fact]
+    public void String_decodes_ASCII_packed_two_chars_per_register()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 6);
+        // "HELLO!" = 0x48 0x45 0x4C 0x4C 0x4F 0x21 across 3 registers.
+        var bytes = "HELLO!"u8.ToArray();
+        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe("HELLO!");
+    }
+
+    [Fact]
+    public void String_decode_truncates_at_first_nul()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 10);
+        var bytes = new byte[] { 0x48, 0x69, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
+        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe("Hi");
+    }
+
+    [Fact]
+    public void String_encode_nul_pads_remaining_bytes()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 8);
+        var wire = ModbusDriver.EncodeRegister("Hi", tag);
+        wire.Length.ShouldBe(8);
+        wire[0].ShouldBe((byte)'H');
+        wire[1].ShouldBe((byte)'i');
+        for (var i = 2; i < 8; i++) wire[i].ShouldBe((byte)0);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusDriverTests.cs

@@ -0,0 +1,244 @@
+using System.Buffers.Binary;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class ModbusDriverTests
+{
+    /// <summary>
+    ///     In-memory Modbus TCP server impl that speaks the function codes the driver uses.
+    ///     Maintains a register/coil bank so Read/Write round-trips work.
+    /// </summary>
+    private sealed class FakeTransport : IModbusTransport
+    {
+        public readonly ushort[] HoldingRegisters = new ushort[256];
+        public readonly ushort[] InputRegisters = new ushort[256];
+        public readonly bool[] Coils = new bool[256];
+        public readonly bool[] DiscreteInputs = new bool[256];
+        public bool ForceConnectFail { get; set; }
+
+        public Task ConnectAsync(CancellationToken ct)
+            => ForceConnectFail ? Task.FromException(new InvalidOperationException("connect refused")) : Task.CompletedTask;
+
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+        {
+            var fc = pdu[0];
+            return fc switch
+            {
+                0x01 => Task.FromResult(ReadBits(pdu, Coils)),
+                0x02 => Task.FromResult(ReadBits(pdu, DiscreteInputs)),
+                0x03 => Task.FromResult(ReadRegs(pdu, HoldingRegisters)),
+                0x04 => Task.FromResult(ReadRegs(pdu, InputRegisters)),
+                0x05 => Task.FromResult(WriteCoil(pdu)),
+                0x06 => Task.FromResult(WriteSingleReg(pdu)),
+                0x10 => Task.FromResult(WriteMultipleRegs(pdu)),
+                _ => Task.FromException<byte[]>(new ModbusException(fc, 0x01, $"fc={fc} not supported by fake")),
+            };
+        }
+
+        private byte[] ReadBits(byte[] pdu, bool[] bank)
+        {
+            var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+            var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+            var byteCount = (byte)((qty + 7) / 8);
+            var resp = new byte[2 + byteCount];
+            resp[0] = pdu[0];
+            resp[1] = byteCount;
+            for (var i = 0; i < qty; i++)
+                if (bank[addr + i]) resp[2 + (i / 8)] |= (byte)(1 << (i % 8));
+            return resp;
+        }
+
+        private byte[] ReadRegs(byte[] pdu, ushort[] bank)
+        {
+            var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+            var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+            var byteCount = (byte)(qty * 2);
+            var resp = new byte[2 + byteCount];
+            resp[0] = pdu[0];
+            resp[1] = byteCount;
+            for (var i = 0; i < qty; i++)
+            {
+                resp[2 + i * 2] = (byte)(bank[addr + i] >> 8);
+                resp[3 + i * 2] = (byte)(bank[addr + i] & 0xFF);
+            }
+            return resp;
+        }
+
+        private byte[] WriteCoil(byte[] pdu)
+        {
+            var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+            Coils[addr] = pdu[3] == 0xFF;
+            return pdu; // Modbus echoes the request on write success
+        }
+
+        private byte[] WriteSingleReg(byte[] pdu)
+        {
+            var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+            HoldingRegisters[addr] = (ushort)((pdu[3] << 8) | pdu[4]);
+            return pdu;
+        }
+
+        private byte[] WriteMultipleRegs(byte[] pdu)
+        {
+            var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+            var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+            for (var i = 0; i < qty; i++)
+                HoldingRegisters[addr + i] = (ushort)((pdu[6 + i * 2] << 8) | pdu[7 + i * 2]);
+            return new byte[] { 0x10, pdu[1], pdu[2], pdu[3], pdu[4] };
+        }
+
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    private static (ModbusDriver driver, FakeTransport fake) NewDriver(params ModbusTagDefinition[] tags)
+    {
+        var fake = new FakeTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = tags };
+        var drv = new ModbusDriver(opts, "modbus-1", _ => fake);
+        return (drv, fake);
+    }
+
+    [Fact]
+    public async Task Initialize_connects_and_populates_tag_map()
+    {
+        var (drv, _) = NewDriver(
+            new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16),
+            new ModbusTagDefinition("Run", ModbusRegion.Coils, 0, ModbusDataType.Bool));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        drv.GetHealth().State.ShouldBe(DriverState.Healthy);
+    }
+
+    [Fact]
+    public async Task Read_Int16_holding_register_returns_BigEndian_value()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 10, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        fake.HoldingRegisters[10] = 12345;
+
+        var r = await drv.ReadAsync(["Level"], CancellationToken.None);
+        r[0].Value.ShouldBe((short)12345);
+        r[0].StatusCode.ShouldBe(0u);
+    }
+
+    [Fact]
+    public async Task Read_Float32_spans_two_registers_BigEndian()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Temp", ModbusRegion.HoldingRegisters, 4, ModbusDataType.Float32));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // IEEE 754 single for 25.5f is 0x41CC0000 — [41 CC][00 00] big-endian across two regs.
+        var bytes = new byte[4];
+        BinaryPrimitives.WriteSingleBigEndian(bytes, 25.5f);
+        fake.HoldingRegisters[4] = (ushort)((bytes[0] << 8) | bytes[1]);
+        fake.HoldingRegisters[5] = (ushort)((bytes[2] << 8) | bytes[3]);
+
+        var r = await drv.ReadAsync(["Temp"], CancellationToken.None);
+        r[0].Value.ShouldBe(25.5f);
+    }
+
+    [Fact]
+    public async Task Read_Coil_returns_boolean()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Run", ModbusRegion.Coils, 3, ModbusDataType.Bool));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        fake.Coils[3] = true;
+
+        var r = await drv.ReadAsync(["Run"], CancellationToken.None);
+        r[0].Value.ShouldBe(true);
+    }
+
+    [Fact]
+    public async Task Unknown_tag_returns_BadNodeIdUnknown_not_an_exception()
+    {
+        var (drv, _) = NewDriver();
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var r = await drv.ReadAsync(["DoesNotExist"], CancellationToken.None);
+        r[0].StatusCode.ShouldBe(0x80340000u);
+    }
+
+    [Fact]
+    public async Task Write_UInt16_holding_register_roundtrips()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Setpoint", ModbusRegion.HoldingRegisters, 20, ModbusDataType.UInt16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync([new WriteRequest("Setpoint", (ushort)42000)], CancellationToken.None);
+        results[0].StatusCode.ShouldBe(0u);
+        fake.HoldingRegisters[20].ShouldBe((ushort)42000);
+    }
+
+    [Fact]
+    public async Task Write_Float32_uses_FC16_WriteMultipleRegisters()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Temp", ModbusRegion.HoldingRegisters, 4, ModbusDataType.Float32));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.WriteAsync([new WriteRequest("Temp", 25.5f)], CancellationToken.None);
+
+        // Decode back through the fake bank to check the two-register shape.
+        var raw = new byte[4];
+        raw[0] = (byte)(fake.HoldingRegisters[4] >> 8);
+        raw[1] = (byte)(fake.HoldingRegisters[4] & 0xFF);
+        raw[2] = (byte)(fake.HoldingRegisters[5] >> 8);
+        raw[3] = (byte)(fake.HoldingRegisters[5] & 0xFF);
+        BinaryPrimitives.ReadSingleBigEndian(raw).ShouldBe(25.5f);
+    }
+
+    [Fact]
+    public async Task Write_to_InputRegister_returns_BadNotWritable()
+    {
+        var (drv, _) = NewDriver(new ModbusTagDefinition("Ro", ModbusRegion.InputRegisters, 0, ModbusDataType.UInt16, Writable: false));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var r = await drv.WriteAsync([new WriteRequest("Ro", (ushort)7)], CancellationToken.None);
+        r[0].StatusCode.ShouldBe(0x803B0000u);
+    }
+
+    [Fact]
+    public async Task Discover_streams_one_folder_per_driver_with_a_variable_per_tag()
+    {
+        var (drv, _) = NewDriver(
+            new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16),
+            new ModbusTagDefinition("Temp", ModbusRegion.HoldingRegisters, 4, ModbusDataType.Float32),
+            new ModbusTagDefinition("Run", ModbusRegion.Coils, 0, ModbusDataType.Bool));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var builder = new RecordingBuilder();
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.Count.ShouldBe(1);
+        builder.Folders[0].BrowseName.ShouldBe("Modbus");
+        builder.Variables.Count.ShouldBe(3);
+        builder.Variables.ShouldContain(v => v.BrowseName == "Level" && v.Info.DriverDataType == DriverDataType.Int32);
+        builder.Variables.ShouldContain(v => v.BrowseName == "Temp" && v.Info.DriverDataType == DriverDataType.Float32);
+        builder.Variables.ShouldContain(v => v.BrowseName == "Run" && v.Info.DriverDataType == DriverDataType.Boolean);
+    }
+
+    // --- helpers ---
+
+    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.Modbus.Tests/ModbusProbeTests.cs

@@ -0,0 +1,208 @@
+using System.Collections.Concurrent;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class ModbusProbeTests
+{
+    /// <summary>
+    ///     Transport fake the probe tests flip between "responding" and "unreachable" to
+    ///     exercise the state machine. Calls to SendAsync with FC=0x03 count as probe traffic
+    ///     (the driver's probe loop issues exactly that shape).
+    /// </summary>
+    private sealed class FlappyTransport : IModbusTransport
+    {
+        public volatile bool Reachable = true;
+        public int ProbeCount;
+
+        public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
+
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+        {
+            if (pdu[0] == 0x03) Interlocked.Increment(ref ProbeCount);
+            if (!Reachable)
+                return Task.FromException<byte[]>(new IOException("transport unreachable"));
+
+            // Happy path — return a valid FC03 response for 1 register at addr.
+            if (pdu[0] == 0x03)
+            {
+                var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+                var resp = new byte[2 + qty * 2];
+                resp[0] = 0x03;
+                resp[1] = (byte)(qty * 2);
+                return Task.FromResult(resp);
+            }
+            return Task.FromException<byte[]>(new NotSupportedException());
+        }
+
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    private static (ModbusDriver drv, FlappyTransport fake) NewDriver(ModbusProbeOptions probe)
+    {
+        var fake = new FlappyTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Port = 502, Probe = probe };
+        return (new ModbusDriver(opts, "modbus-1", _ => fake), fake);
+    }
+
+    [Fact]
+    public async Task Initial_state_is_Unknown_before_first_probe_tick()
+    {
+        var (drv, _) = NewDriver(new ModbusProbeOptions { Enabled = false });
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var statuses = drv.GetHostStatuses();
+        statuses.Count.ShouldBe(1);
+        statuses[0].State.ShouldBe(HostState.Unknown);
+        statuses[0].HostName.ShouldBe("fake:502");
+    }
+
+    [Fact]
+    public async Task First_successful_probe_transitions_to_Running()
+    {
+        var (drv, fake) = NewDriver(new ModbusProbeOptions
+        {
+            Enabled = true,
+            Interval = TimeSpan.FromMilliseconds(150),
+            Timeout = TimeSpan.FromSeconds(1),
+        });
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // Wait for the first probe to complete.
+        var deadline = DateTime.UtcNow + TimeSpan.FromSeconds(2);
+        while (fake.ProbeCount == 0 && DateTime.UtcNow < deadline) await Task.Delay(25);
+
+        // Then wait for the event to actually arrive.
+        deadline = DateTime.UtcNow + TimeSpan.FromSeconds(1);
+        while (transitions.Count == 0 && DateTime.UtcNow < deadline) await Task.Delay(25);
+
+        transitions.Count.ShouldBeGreaterThanOrEqualTo(1);
+        transitions.TryDequeue(out var t).ShouldBeTrue();
+        t!.OldState.ShouldBe(HostState.Unknown);
+        t.NewState.ShouldBe(HostState.Running);
+        drv.GetHostStatuses()[0].State.ShouldBe(HostState.Running);
+
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Transport_failure_transitions_to_Stopped()
+    {
+        var (drv, fake) = NewDriver(new ModbusProbeOptions
+        {
+            Enabled = true,
+            Interval = TimeSpan.FromMilliseconds(150),
+            Timeout = TimeSpan.FromSeconds(1),
+        });
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await WaitForStateAsync(drv, HostState.Running, TimeSpan.FromSeconds(2));
+
+        fake.Reachable = false;
+        await WaitForStateAsync(drv, HostState.Stopped, TimeSpan.FromSeconds(2));
+
+        transitions.Select(t => t.NewState).ShouldContain(HostState.Stopped);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Recovery_transitions_Stopped_back_to_Running()
+    {
+        var (drv, fake) = NewDriver(new ModbusProbeOptions
+        {
+            Enabled = true,
+            Interval = TimeSpan.FromMilliseconds(150),
+            Timeout = TimeSpan.FromSeconds(1),
+        });
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await WaitForStateAsync(drv, HostState.Running, TimeSpan.FromSeconds(2));
+
+        fake.Reachable = false;
+        await WaitForStateAsync(drv, HostState.Stopped, TimeSpan.FromSeconds(2));
+
+        fake.Reachable = true;
+        await WaitForStateAsync(drv, HostState.Running, TimeSpan.FromSeconds(2));
+
+        // We expect at minimum: Unknown→Running, Running→Stopped, Stopped→Running.
+        transitions.Count.ShouldBeGreaterThanOrEqualTo(3);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Repeated_successful_probes_do_not_generate_duplicate_Running_events()
+    {
+        var (drv, _) = NewDriver(new ModbusProbeOptions
+        {
+            Enabled = true,
+            Interval = TimeSpan.FromMilliseconds(100),
+            Timeout = TimeSpan.FromSeconds(1),
+        });
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await WaitForStateAsync(drv, HostState.Running, TimeSpan.FromSeconds(2));
+        await Task.Delay(500); // several more probe ticks, all successful — state shouldn't thrash
+
+        transitions.Count.ShouldBe(1); // only the initial Unknown→Running
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Disabled_probe_stays_Unknown_and_fires_no_events()
+    {
+        var (drv, _) = NewDriver(new ModbusProbeOptions { Enabled = false });
+        var transitions = new ConcurrentQueue<HostStatusChangedEventArgs>();
+        drv.OnHostStatusChanged += (_, e) => transitions.Enqueue(e);
+
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await Task.Delay(300);
+
+        transitions.Count.ShouldBe(0);
+        drv.GetHostStatuses()[0].State.ShouldBe(HostState.Unknown);
+        await drv.ShutdownAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Shutdown_stops_the_probe_loop()
+    {
+        var (drv, fake) = NewDriver(new ModbusProbeOptions
+        {
+            Enabled = true,
+            Interval = TimeSpan.FromMilliseconds(100),
+            Timeout = TimeSpan.FromSeconds(1),
+        });
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        await WaitForStateAsync(drv, HostState.Running, TimeSpan.FromSeconds(2));
+
+        var before = fake.ProbeCount;
+        await drv.ShutdownAsync(CancellationToken.None);
+        await Task.Delay(400);
+
+        // A handful of in-flight ticks may complete after shutdown in a narrow race; the
+        // contract is that the loop stops scheduling new ones. Tolerate ≤1 extra.
+        (fake.ProbeCount - before).ShouldBeLessThanOrEqualTo(1);
+    }
+
+    private static async Task WaitForStateAsync(ModbusDriver drv, HostState expected, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            if (drv.GetHostStatuses()[0].State == expected) return;
+            await Task.Delay(25);
+        }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusSubscriptionTests.cs

@@ -0,0 +1,180 @@
+using System.Collections.Concurrent;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class ModbusSubscriptionTests
+{
+    /// <summary>
+    ///     Lightweight fake transport the subscription tests drive through — only the FC03
+    ///     (Read Holding Registers) path is used. Mutating <see cref="HoldingRegisters"/>
+    ///     between polls is how each test simulates a PLC value change.
+    /// </summary>
+    private sealed class FakeTransport : IModbusTransport
+    {
+        public readonly ushort[] HoldingRegisters = new ushort[256];
+        public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
+
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+        {
+            if (pdu[0] != 0x03) return Task.FromException<byte[]>(new NotSupportedException("FC not supported"));
+            var addr = (ushort)((pdu[1] << 8) | pdu[2]);
+            var qty = (ushort)((pdu[3] << 8) | pdu[4]);
+            var resp = new byte[2 + qty * 2];
+            resp[0] = 0x03;
+            resp[1] = (byte)(qty * 2);
+            for (var i = 0; i < qty; i++)
+            {
+                resp[2 + i * 2] = (byte)(HoldingRegisters[addr + i] >> 8);
+                resp[3 + i * 2] = (byte)(HoldingRegisters[addr + i] & 0xFF);
+            }
+            return Task.FromResult(resp);
+        }
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    private static (ModbusDriver drv, FakeTransport fake) NewDriver(params ModbusTagDefinition[] tags)
+    {
+        var fake = new FakeTransport();
+        var opts = new ModbusDriverOptions { Host = "fake", Tags = tags };
+        return (new ModbusDriver(opts, "modbus-1", _ => fake), fake);
+    }
+
+    [Fact]
+    public async Task Initial_poll_raises_OnDataChange_for_every_subscribed_tag()
+    {
+        var (drv, fake) = NewDriver(
+            new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16),
+            new ModbusTagDefinition("Temp", ModbusRegion.HoldingRegisters, 1, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        fake.HoldingRegisters[0] = 100;
+        fake.HoldingRegisters[1] = 200;
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Level", "Temp"], TimeSpan.FromMilliseconds(200), CancellationToken.None);
+        await WaitForCountAsync(events, 2, TimeSpan.FromSeconds(2));
+
+        events.Select(e => e.FullReference).ShouldContain("Level");
+        events.Select(e => e.FullReference).ShouldContain("Temp");
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Unchanged_values_do_not_raise_after_initial_poll()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        fake.HoldingRegisters[0] = 100;
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await Task.Delay(500); // ~5 poll cycles at 100ms, value stable the whole time
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        events.Count.ShouldBe(1); // only the initial-data push, no change events after
+    }
+
+    [Fact]
+    public async Task Value_change_between_polls_raises_OnDataChange()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        fake.HoldingRegisters[0] = 100;
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForCountAsync(events, 1, TimeSpan.FromSeconds(1));
+        fake.HoldingRegisters[0] = 200; // simulate PLC update
+        await WaitForCountAsync(events, 2, TimeSpan.FromSeconds(2));
+
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+        events.Count.ShouldBeGreaterThanOrEqualTo(2);
+        events.Last().Snapshot.Value.ShouldBe((short)200);
+    }
+
+    [Fact]
+    public async Task Unsubscribe_stops_the_polling_loop()
+    {
+        var (drv, fake) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForCountAsync(events, 1, TimeSpan.FromSeconds(1));
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        var countAfterUnsub = events.Count;
+        fake.HoldingRegisters[0] = 999; // would trigger a change if still polling
+        await Task.Delay(400);
+        events.Count.ShouldBe(countAfterUnsub);
+    }
+
+    [Fact]
+    public async Task SubscribeAsync_floors_intervals_below_100ms()
+    {
+        var (drv, _) = NewDriver(new ModbusTagDefinition("Level", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        // 10ms requested — implementation floors to 100ms. We verify indirectly: over 300ms, a
+        // 10ms interval would produce many more events than a 100ms interval would on a stable
+        // value. Since the value is unchanged, we only expect the initial-data push (1 event).
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Level"], TimeSpan.FromMilliseconds(10), CancellationToken.None);
+        await Task.Delay(300);
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        events.Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Multiple_subscriptions_fire_independently()
+    {
+        var (drv, fake) = NewDriver(
+            new ModbusTagDefinition("A", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16),
+            new ModbusTagDefinition("B", ModbusRegion.HoldingRegisters, 1, ModbusDataType.Int16));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var eventsA = new ConcurrentQueue<DataChangeEventArgs>();
+        var eventsB = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) =>
+        {
+            if (e.FullReference == "A") eventsA.Enqueue(e);
+            else if (e.FullReference == "B") eventsB.Enqueue(e);
+        };
+
+        var ha = await drv.SubscribeAsync(["A"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        var hb = await drv.SubscribeAsync(["B"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForCountAsync(eventsA, 1, TimeSpan.FromSeconds(1));
+        await WaitForCountAsync(eventsB, 1, TimeSpan.FromSeconds(1));
+
+        await drv.UnsubscribeAsync(ha, CancellationToken.None);
+        var aCount = eventsA.Count;
+        fake.HoldingRegisters[1] = 77; // only B should pick this up
+        await WaitForCountAsync(eventsB, 2, TimeSpan.FromSeconds(2));
+
+        eventsA.Count.ShouldBe(aCount);        // unchanged since unsubscribe
+        eventsB.Count.ShouldBeGreaterThanOrEqualTo(2);
+        await drv.UnsubscribeAsync(hb, CancellationToken.None);
+    }
+
+    private static async Task WaitForCountAsync<T>(ConcurrentQueue<T> q, int target, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (q.Count < target && DateTime.UtcNow < deadline)
+            await Task.Delay(25);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.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.Modbus.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.Modbus\ZB.MOM.WW.OtOpcUa.Driver.Modbus.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>