Phase 3 PR 69 -- OPC UA Client ISubscribable + IHostConnectivityProbe. Completes the OpcUaClientDriver capability surface — now matches the Galaxy + Modbus + S7 driver coverage. ISubscribable: SubscribeAsync creates a new upstream Subscription via the non-obsolete Subscription(ITelemetryContext, SubscriptionOptions) ctor + AddItem/CreateItemsAsync flow, wires each MonitoredItem's Notification event into OnDataChange. Tag strings round-trip through MonitoredItem.Handle so the notification handler can identify which tag changed without a second lookup. Publishing interval floored at 50ms (servers negotiate up anyway; sub-50ms wastes round-trip). SubscriptionOptions uses KeepAliveCount=10, LifetimeCount=1000, TimestampsToReturn=Both so SourceTimestamp passthrough for the cascading-quality rule works through subscription paths too. UnsubscribeAsync calls Subscription.DeleteAsync(silent:true) and tolerates unknown handles (returns cleanly) because the caller's race with server-side cleanup after a session drop shouldn't crash either side. Session shutdown explicitly deletes every remote subscription before closing — avoids BadSubscriptionIdInvalid noise in the upstream server's log on Close. IHostConnectivityProbe: HostName surfaced as the EndpointUrl (not host:port like the Modbus/S7 drivers) so the Admin /hosts dashboard can render the full opc.tcp:// URL as a clickable target back at the remote server. HostState tracked via session.KeepAlive event — OPC UA's built-in keep-alive is authoritative for session liveness (the SDK pings on KeepAliveInterval, sets KeepAliveStopped after N missed pings), strictly better than a driver-side polling probe: no extra wire round-trip, no duplicate semantic with the native protocol. Handler transitions Running on healthy keep-alives and Stopped on any Bad service-result. Initial Running raised at end of InitializeAsync once the session is up; Shutdown transitions back to Unknown + unwires the handler. Unit tests (OpcUaClientSubscribeAndProbeTests, 3 facts): SubscribeAsync_without_initialize_throws_InvalidOperationException, UnsubscribeAsync_with_unknown_handle_is_noop (session-drop-race safety), GetHostStatuses_returns_endpoint_url_row_pre_init (asserts EndpointUrl as the host identity -- the full opc.tcp://plc.example:4840 URL). Live-session subscribe/unsubscribe round-trip + keep-alive state transition coverage lands in a follow-up PR once we scaffold the in-process OPC UA server fixture. 13/13 OpcUaClient.Tests pass. dotnet build clean. All six capability interfaces (IDriver / ITagDiscovery / IReadable / IWritable / ISubscribable / IHostConnectivityProbe) implemented — OPC UA Client driver surface complete.

ZB.MOM.WW.OtOpcUa.slnx

@@ -9,6 +9,8 @@
         <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.Driver.S7/ZB.MOM.WW.OtOpcUa.Driver.S7.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"/>
         <Project Path="src/ZB.MOM.WW.OtOpcUa.Client.UI/ZB.MOM.WW.OtOpcUa.Client.UI.csproj"/>
@@ -26,6 +28,8 @@
         <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.Driver.Modbus.IntegrationTests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.csproj"/>
+        <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.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"/>
         <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/V1_ARCHIVE_STATUS.md

@@ -1,56 +1,47 @@
-# V1 Archive Status (Phase 2 Stream D, 2026-04-18)
+# V1 Archive Status — CLOSED (Phase 2 Streams D + E complete)
 
-This document inventories every v1 surface that's been **functionally superseded** by v2 but
-**physically retained** in the build until the deletion PR (Phase 2 PR 3). Rationale: cascading
-references mean a single deletion is high blast-radius; archive-marking lets the v2 stack ship
-on its own merits while the v1 surface stays as parity reference.
+> **Status as of 2026-04-18: the v1 archive has been fully removed from the tree.**
+> This document is retained as historical record of the Phase 2 Stream D / E closure.
 
-## Archived projects
+## Final state
 
-| Path | Status | Replaced by | Build behavior |
-|---|---|---|---|
-| `src/ZB.MOM.WW.OtOpcUa.Host/` | Archive (executable in build) | `OtOpcUa.Server` + `Driver.Galaxy.Host` + `Driver.Galaxy.Proxy` | Builds; not deployed by v2 install scripts |
-| `src/ZB.MOM.WW.OtOpcUa.Historian.Aveva/` | Archive (plugin in build) | TODO: port into `Driver.Galaxy.Host/Backend/Historian/` (Task B.1.h follow-up) | Builds; loaded only by archived Host |
-| `tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive/` | Archive | `Driver.Galaxy.E2E` + per-component test projects | `<IsTestProject>false</IsTestProject>` — `dotnet test slnx` skips |
-| `tests/ZB.MOM.WW.OtOpcUa.IntegrationTests/` | Archive | `Driver.Galaxy.E2E` | `<IsTestProject>false</IsTestProject>` — `dotnet test slnx` skips |
+All five v1 archive directories have been deleted:
 
-## How to run the archived suites explicitly
+| Path | Deleted | Replaced by |
+|---|---|---|
+| `src/ZB.MOM.WW.OtOpcUa.Host/` | ✅ | `OtOpcUa.Server` + `Driver.Galaxy.Host` + `Driver.Galaxy.Proxy` |
+| `src/ZB.MOM.WW.OtOpcUa.Historian.Aveva/` | ✅ | `Driver.Galaxy.Host/Backend/Historian/` (ported in Phase 3 PRs 51-55) |
+| `tests/ZB.MOM.WW.OtOpcUa.Historian.Aveva.Tests/` | ✅ | `Driver.Galaxy.Host.Tests/Historian/` |
+| `tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive/` | ✅ | Per-component `*.Tests` projects + `Driver.Galaxy.E2E` |
+| `tests/ZB.MOM.WW.OtOpcUa.IntegrationTests/` | ✅ | `Driver.Galaxy.E2E` + `Driver.Modbus.IntegrationTests` |
 
-```powershell
-# v1 unit tests (494):
-dotnet test tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive
+## Closure timeline
 
-# v1 integration tests (6):
-dotnet test tests/ZB.MOM.WW.OtOpcUa.IntegrationTests
-```
+- **PR 2 (2026-04-18, phase-2-stream-d)** — archive-marked the four v1 projects with
+  `<IsTestProject>false</IsTestProject>` so solution builds and `dotnet test slnx` bypassed
+  them. Capture: `docs/v2/implementation/exit-gate-phase-2-final.md`.
+- **Phase 3 PR 18 (2026-04-18)** — deleted the archived project source trees. Leftover
+  `bin/` and `obj/` residue remained on disk from pre-deletion builds.
+- **Phase 2 PR 61 (2026-04-18, this closure PR)** — scrubbed the empty residue directories
+  and confirmed `dotnet build ZB.MOM.WW.OtOpcUa.slnx` clean with 0 errors.
 
-Both still pass on this dev box — they're the parity reference for Phase 2 PR 3's deletion
-decision.
+## Parity validation (Stream E)
 
-## Deletion plan (Phase 2 PR 3)
+The original 494 v1 tests + 6 v1 integration tests are **not** preserved in the v2 branch.
+Their parity-bar role is now filled by:
 
-Pre-conditions:
-- [ ] `Driver.Galaxy.E2E` test count covers the v1 IntegrationTests' 6 integration scenarios
-      at minimum (currently 7 tests; expand as needed)
-- [ ] `Driver.Galaxy.Host/Backend/Historian/` ports the Wonderware Historian plugin
-      so `MxAccessGalaxyBackend.HistoryReadAsync` returns real data (Task B.1.h)
-- [ ] Operator review on a separate PR — destructive change
-
-Steps:
-1. `git rm -r src/ZB.MOM.WW.OtOpcUa.Host/`
-2. `git rm -r src/ZB.MOM.WW.OtOpcUa.Historian.Aveva/`
-   (or move it under Driver.Galaxy.Host first if the lift is part of the same PR)
-3. `git rm -r tests/ZB.MOM.WW.OtOpcUa.Tests.v1Archive/`
-4. `git rm -r tests/ZB.MOM.WW.OtOpcUa.IntegrationTests/`
-5. Edit `ZB.MOM.WW.OtOpcUa.slnx` — remove the four project lines
-6. `dotnet build ZB.MOM.WW.OtOpcUa.slnx` → confirm clean
-7. `dotnet test ZB.MOM.WW.OtOpcUa.slnx` → confirm 470+ pass / 1 baseline (or whatever the
-   current count is plus any new E2E coverage)
-8. Commit: "Phase 2 Stream D — delete v1 archive (Host + Historian.Aveva + v1Tests + IntegrationTests)"
-9. PR 3 against `v2`, link this doc + exit-gate-phase-2-final.md
-10. One reviewer signoff
+- `Driver.Galaxy.E2E` — cross-FX subprocess parity (spawns the net48 x86 Galaxy.Host.exe
+  + connects via real named pipe, exercises every `IDriver` capability through the
+  supervisor). Stability-findings regression tests (4 × 2026-04-13 findings) live here.
+- Per-component `*.Tests` projects — cover the code that moved out of the monolith into
+  discrete v2 projects. Running `dotnet test ZB.MOM.WW.OtOpcUa.slnx` executes all of them
+  as one solution-level gate.
+- `Driver.Modbus.IntegrationTests` — adds Modbus TCP driver coverage that didn't exist in
+  v1 (DL205, S7-1500, Mitsubishi MELSEC via pymodbus sim profiles — PRs 30, 56-60).
+- Live-stack smoke tests (`Driver.Galaxy.E2E/LiveStack/`) — optional, gated on presence
+  of the `OtOpcUaGalaxyHost` service + Galaxy repository on the dev box (PRs 33, 36, 37).
 
 ## Rollback
 
-If Phase 2 PR 3 surfaces downstream consumer regressions, `git revert` the deletion commit
-restores the four projects intact. The v2 stack continues to ship from the v2 branch.
+`git revert` of the deletion commits restores the projects intact. The v2 stack continues
+to ship from the `v2` branch regardless.

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

@@ -0,0 +1,666 @@
+using Opc.Ua;
+using Opc.Ua.Client;
+using Opc.Ua.Configuration;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
+
+/// <summary>
+///     OPC UA Client (gateway) driver. Opens a <see cref="Session"/> against a remote OPC UA
+///     server and re-exposes its address space through the local OtOpcUa server. PR 66 ships
+///     the scaffold: <see cref="IDriver"/> only (connect / close / health). Browse, read,
+///     write, subscribe, and probe land in PRs 67-69.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Builds its own <see cref="ApplicationConfiguration"/> rather than reusing
+///         <c>Client.Shared</c> — Client.Shared is oriented at the interactive CLI; this
+///         driver is an always-on service component with different session-lifetime needs
+///         (keep-alive monitor, session transfer on reconnect, multi-year uptime).
+///     </para>
+///     <para>
+///         <b>Session lifetime</b>: a single <see cref="Session"/> per driver instance.
+///         Subscriptions multiplex onto that session; SDK reconnect handler takes the session
+///         down and brings it back up on remote-server restart — the driver must re-send
+///         subscriptions + TransferSubscriptions on reconnect to avoid dangling
+///         monitored-item handles. That mechanic lands in PR 69.
+///     </para>
+/// </remarks>
+public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string driverInstanceId)
+    : IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IDisposable, IAsyncDisposable
+{
+    // ---- ISubscribable + IHostConnectivityProbe state ----
+
+    private readonly System.Collections.Concurrent.ConcurrentDictionary<long, RemoteSubscription> _subscriptions = new();
+    private long _nextSubscriptionId;
+    private readonly object _probeLock = new();
+    private HostState _hostState = HostState.Unknown;
+    private DateTime _hostStateChangedUtc = DateTime.UtcNow;
+    private KeepAliveEventHandler? _keepAliveHandler;
+
+    public event EventHandler<DataChangeEventArgs>? OnDataChange;
+    public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;
+
+    // OPC UA StatusCode constants the driver surfaces for local-side faults. Upstream-server
+    // StatusCodes are passed through verbatim per driver-specs.md §8 "cascading quality" —
+    // downstream clients need to distinguish 'remote source down' from 'local driver failure'.
+    private const uint StatusBadNodeIdInvalid = 0x80330000u;
+    private const uint StatusBadInternalError = 0x80020000u;
+    private const uint StatusBadCommunicationError = 0x80050000u;
+
+    private readonly OpcUaClientDriverOptions _options = options;
+    private readonly SemaphoreSlim _gate = new(1, 1);
+
+    /// <summary>Active OPC UA session. Null until <see cref="InitializeAsync"/> returns cleanly.</summary>
+    internal ISession? Session { get; private set; }
+
+    /// <summary>Per-connection gate. PRs 67+ serialize read/write/browse on this.</summary>
+    internal SemaphoreSlim Gate => _gate;
+
+    private DriverHealth _health = new(DriverState.Unknown, null, null);
+    private bool _disposed;
+
+    public string DriverInstanceId => driverInstanceId;
+    public string DriverType => "OpcUaClient";
+
+    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        _health = new DriverHealth(DriverState.Initializing, null, null);
+        try
+        {
+            var appConfig = await BuildApplicationConfigurationAsync(cancellationToken).ConfigureAwait(false);
+
+            // Endpoint selection: let the stack pick the best matching endpoint for the
+            // requested security policy/mode so the driver doesn't have to hand-validate.
+            // UseSecurity=false when SecurityMode=None shortcuts around cert validation
+            // entirely and is the typical dev-bench configuration.
+            var useSecurity = _options.SecurityMode != OpcUaSecurityMode.None;
+            // The non-obsolete SelectEndpointAsync overloads all require an ITelemetryContext
+            // parameter. Passing null is valid — the SDK falls through to its built-in default
+            // trace sink. Plumbing a telemetry context through every driver surface is out of
+            // scope; the driver emits its own logs via the health surface anyway.
+            var selected = await CoreClientUtils.SelectEndpointAsync(
+                appConfig, _options.EndpointUrl, useSecurity,
+                telemetry: null!,
+                ct: cancellationToken).ConfigureAwait(false);
+            var endpointConfig = EndpointConfiguration.Create(appConfig);
+            endpointConfig.OperationTimeout = (int)_options.Timeout.TotalMilliseconds;
+            var endpoint = new ConfiguredEndpoint(null, selected, endpointConfig);
+
+            var identity = _options.AuthType switch
+            {
+                OpcUaAuthType.Anonymous => new UserIdentity(new AnonymousIdentityToken()),
+                // The UserIdentity(string, string) overload was removed in favour of
+                // (string, byte[]) to make the password encoding explicit. UTF-8 is the
+                // overwhelmingly common choice for Basic256Sha256-secured sessions.
+                OpcUaAuthType.Username => new UserIdentity(
+                    _options.Username ?? string.Empty,
+                    System.Text.Encoding.UTF8.GetBytes(_options.Password ?? string.Empty)),
+                OpcUaAuthType.Certificate => throw new NotSupportedException(
+                    "Certificate authentication lands in a follow-up PR; for now use Anonymous or Username"),
+                _ => new UserIdentity(new AnonymousIdentityToken()),
+            };
+
+            // All Session.Create* static methods are marked [Obsolete] in SDK 1.5.378; the
+            // non-obsolete path is DefaultSessionFactory.Instance.CreateAsync (which is the
+            // 8-arg signature matching our driver config — ApplicationConfiguration +
+            // ConfiguredEndpoint, no transport-waiting-connection or reverse-connect-manager
+            // required for the standard opc.tcp direct-connect case).
+            // DefaultSessionFactory's parameterless ctor is also obsolete in 1.5.378; the
+            // current constructor requires an ITelemetryContext. Passing null is tolerated —
+            // the factory falls back to its internal default sink, same as the telemetry:null
+            // on SelectEndpointAsync above.
+            var session = await new DefaultSessionFactory(telemetry: null!).CreateAsync(
+                appConfig,
+                endpoint,
+                false,                                              // updateBeforeConnect
+                _options.SessionName,
+                (uint)_options.SessionTimeout.TotalMilliseconds,
+                identity,
+                null,                                               // preferredLocales
+                cancellationToken).ConfigureAwait(false);
+
+            session.KeepAliveInterval = (int)_options.KeepAliveInterval.TotalMilliseconds;
+
+            // Wire the session's keep-alive channel into HostState. OPC UA keep-alives are
+            // authoritative for session liveness: the SDK pings on KeepAliveInterval and sets
+            // KeepAliveStopped when N intervals elapse without a response. That's strictly
+            // better than a driver-side polling probe — no extra round-trip, no duplicate
+            // semantic.
+            _keepAliveHandler = (_, e) =>
+            {
+                var healthy = !ServiceResult.IsBad(e.Status);
+                TransitionTo(healthy ? HostState.Running : HostState.Stopped);
+            };
+            session.KeepAlive += _keepAliveHandler;
+
+            Session = session;
+            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
+            TransitionTo(HostState.Running);
+        }
+        catch (Exception ex)
+        {
+            try { if (Session is Session s) await s.CloseAsync().ConfigureAwait(false); } catch { }
+            Session = null;
+            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
+            throw;
+        }
+    }
+
+    /// <summary>
+    ///     Build a minimal in-memory <see cref="ApplicationConfiguration"/>. Certificates live
+    ///     under the OS user profile — on Windows that's <c>%LocalAppData%\OtOpcUa\pki</c>
+    ///     — so multiple driver instances in the same OtOpcUa server process share one
+    ///     certificate store without extra config.
+    /// </summary>
+    private async Task<ApplicationConfiguration> BuildApplicationConfigurationAsync(CancellationToken ct)
+    {
+        // The default ctor is obsolete in favour of the ITelemetryContext overload; suppress
+        // locally rather than plumbing a telemetry context all the way through the driver
+        // surface — the driver emits no per-request telemetry of its own and the SDK's
+        // internal fallback is fine for a gateway use case.
+#pragma warning disable CS0618
+        var app = new ApplicationInstance
+        {
+            ApplicationName = _options.SessionName,
+            ApplicationType = ApplicationType.Client,
+        };
+#pragma warning restore CS0618
+
+        var pkiRoot = Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+            "OtOpcUa", "pki");
+
+        var config = new ApplicationConfiguration
+        {
+            ApplicationName = _options.SessionName,
+            ApplicationType = ApplicationType.Client,
+            ApplicationUri = _options.ApplicationUri,
+            SecurityConfiguration = new SecurityConfiguration
+            {
+                ApplicationCertificate = new CertificateIdentifier
+                {
+                    StoreType = CertificateStoreType.Directory,
+                    StorePath = Path.Combine(pkiRoot, "own"),
+                    SubjectName = $"CN={_options.SessionName}",
+                },
+                TrustedPeerCertificates = new CertificateTrustList
+                {
+                    StoreType = CertificateStoreType.Directory,
+                    StorePath = Path.Combine(pkiRoot, "trusted"),
+                },
+                TrustedIssuerCertificates = new CertificateTrustList
+                {
+                    StoreType = CertificateStoreType.Directory,
+                    StorePath = Path.Combine(pkiRoot, "issuers"),
+                },
+                RejectedCertificateStore = new CertificateTrustList
+                {
+                    StoreType = CertificateStoreType.Directory,
+                    StorePath = Path.Combine(pkiRoot, "rejected"),
+                },
+                AutoAcceptUntrustedCertificates = _options.AutoAcceptCertificates,
+            },
+            TransportQuotas = new TransportQuotas { OperationTimeout = (int)_options.Timeout.TotalMilliseconds },
+            ClientConfiguration = new ClientConfiguration
+            {
+                DefaultSessionTimeout = (int)_options.SessionTimeout.TotalMilliseconds,
+            },
+            DisableHiResClock = true,
+        };
+
+        await config.ValidateAsync(ApplicationType.Client, ct).ConfigureAwait(false);
+
+        // Attach a cert-validator handler that honours the AutoAccept flag. Without this,
+        // AutoAcceptUntrustedCertificates on the config alone isn't always enough in newer
+        // SDK versions — the validator raises an event the app has to handle.
+        if (_options.AutoAcceptCertificates)
+        {
+            config.CertificateValidator.CertificateValidation += (s, e) =>
+            {
+                if (e.Error.StatusCode == StatusCodes.BadCertificateUntrusted)
+                    e.Accept = true;
+            };
+        }
+
+        // Ensure an application certificate exists. The SDK auto-generates one if missing.
+        app.ApplicationConfiguration = config;
+        await app.CheckApplicationInstanceCertificatesAsync(silent: true, lifeTimeInMonths: null, ct)
+            .ConfigureAwait(false);
+
+        return config;
+    }
+
+    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
+        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task ShutdownAsync(CancellationToken cancellationToken)
+    {
+        // Tear down remote subscriptions first — otherwise Session.Close will try and may fail
+        // with BadSubscriptionIdInvalid noise in the upstream log. _subscriptions is cleared
+        // whether or not the wire-side delete succeeds since the local handles are useless
+        // after close anyway.
+        foreach (var rs in _subscriptions.Values)
+        {
+            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
+            catch { /* best-effort */ }
+        }
+        _subscriptions.Clear();
+
+        if (_keepAliveHandler is not null && Session is not null)
+        {
+            try { Session.KeepAlive -= _keepAliveHandler; } catch { }
+        }
+        _keepAliveHandler = null;
+
+        try { if (Session is Session s) await s.CloseAsync(cancellationToken).ConfigureAwait(false); }
+        catch { /* best-effort */ }
+        try { Session?.Dispose(); } catch { }
+        Session = null;
+
+        TransitionTo(HostState.Unknown);
+        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
+    }
+
+    public DriverHealth GetHealth() => _health;
+    public long GetMemoryFootprint() => 0;
+    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    // ---- IReadable ----
+
+    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        var session = RequireSession();
+        var results = new DataValueSnapshot[fullReferences.Count];
+        var now = DateTime.UtcNow;
+
+        // Parse NodeIds up-front. Tags whose reference doesn't parse get BadNodeIdInvalid
+        // and are omitted from the wire request — saves a round-trip against the upstream
+        // server for a fault the driver can detect locally.
+        var toSend = new ReadValueIdCollection();
+        var indexMap = new List<int>(fullReferences.Count); // maps wire-index -> results-index
+        for (var i = 0; i < fullReferences.Count; i++)
+        {
+            if (!TryParseNodeId(session, fullReferences[i], out var nodeId))
+            {
+                results[i] = new DataValueSnapshot(null, StatusBadNodeIdInvalid, null, now);
+                continue;
+            }
+            toSend.Add(new ReadValueId { NodeId = nodeId, AttributeId = Attributes.Value });
+            indexMap.Add(i);
+        }
+
+        if (toSend.Count == 0) return results;
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            try
+            {
+                var resp = await session.ReadAsync(
+                    requestHeader: null,
+                    maxAge: 0,
+                    timestampsToReturn: TimestampsToReturn.Both,
+                    nodesToRead: toSend,
+                    ct: cancellationToken).ConfigureAwait(false);
+
+                var values = resp.Results;
+                for (var w = 0; w < values.Count; w++)
+                {
+                    var r = indexMap[w];
+                    var dv = values[w];
+                    // Preserve the upstream StatusCode verbatim — including Bad codes per
+                    // §8's cascading-quality rule. Also preserve SourceTimestamp so downstream
+                    // clients can detect stale upstream data.
+                    results[r] = new DataValueSnapshot(
+                        Value: dv.Value,
+                        StatusCode: dv.StatusCode.Code,
+                        SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
+                        ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? now : dv.ServerTimestamp);
+                }
+                _health = new DriverHealth(DriverState.Healthy, now, null);
+            }
+            catch (Exception ex)
+            {
+                // Transport / timeout / session-dropped — fan out the same fault across every
+                // tag in this batch. Per-tag StatusCode stays BadCommunicationError (not
+                // BadInternalError) so operators distinguish "upstream unreachable" from
+                // "driver bug".
+                for (var w = 0; w < indexMap.Count; w++)
+                {
+                    var r = indexMap[w];
+                    results[r] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
+                }
+                _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+            }
+        }
+        finally { _gate.Release(); }
+        return results;
+    }
+
+    // ---- IWritable ----
+
+    public async Task<IReadOnlyList<WriteResult>> WriteAsync(
+        IReadOnlyList<Core.Abstractions.WriteRequest> writes, CancellationToken cancellationToken)
+    {
+        var session = RequireSession();
+        var results = new WriteResult[writes.Count];
+
+        var toSend = new WriteValueCollection();
+        var indexMap = new List<int>(writes.Count);
+        for (var i = 0; i < writes.Count; i++)
+        {
+            if (!TryParseNodeId(session, writes[i].FullReference, out var nodeId))
+            {
+                results[i] = new WriteResult(StatusBadNodeIdInvalid);
+                continue;
+            }
+            toSend.Add(new WriteValue
+            {
+                NodeId = nodeId,
+                AttributeId = Attributes.Value,
+                Value = new DataValue(new Variant(writes[i].Value)),
+            });
+            indexMap.Add(i);
+        }
+
+        if (toSend.Count == 0) return results;
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            try
+            {
+                var resp = await session.WriteAsync(
+                    requestHeader: null,
+                    nodesToWrite: toSend,
+                    ct: cancellationToken).ConfigureAwait(false);
+
+                var codes = resp.Results;
+                for (var w = 0; w < codes.Count; w++)
+                {
+                    var r = indexMap[w];
+                    // Pass upstream WriteResult StatusCode through verbatim. Success codes
+                    // include Good (0) and any warning-level Good* variants; anything with
+                    // the severity bits set is a Bad.
+                    results[r] = new WriteResult(codes[w].Code);
+                }
+            }
+            catch (Exception)
+            {
+                for (var w = 0; w < indexMap.Count; w++)
+                    results[indexMap[w]] = new WriteResult(StatusBadCommunicationError);
+            }
+        }
+        finally { _gate.Release(); }
+        return results;
+    }
+
+    /// <summary>
+    ///     Parse a tag's full-reference string as a NodeId. Accepts the standard OPC UA
+    ///     serialized forms (<c>ns=2;s=…</c>, <c>i=2253</c>, <c>ns=4;g=…</c>, <c>ns=3;b=…</c>).
+    ///     Empty + malformed strings return false; the driver surfaces that as
+    ///     <see cref="StatusBadNodeIdInvalid"/> without a wire round-trip.
+    /// </summary>
+    internal static bool TryParseNodeId(ISession session, string fullReference, out NodeId nodeId)
+    {
+        nodeId = NodeId.Null;
+        if (string.IsNullOrWhiteSpace(fullReference)) return false;
+        try
+        {
+            nodeId = NodeId.Parse(session.MessageContext, fullReference);
+            return !NodeId.IsNull(nodeId);
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    private ISession RequireSession() =>
+        Session ?? throw new InvalidOperationException("OpcUaClientDriver not initialized");
+
+    // ---- ITagDiscovery ----
+
+    public async Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+        var session = RequireSession();
+
+        var root = !string.IsNullOrEmpty(_options.BrowseRoot)
+            ? NodeId.Parse(session.MessageContext, _options.BrowseRoot)
+            : ObjectIds.ObjectsFolder;
+
+        var rootFolder = builder.Folder("Remote", "Remote");
+        var visited = new HashSet<NodeId>();
+        var discovered = 0;
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            await BrowseRecursiveAsync(session, root, rootFolder, visited,
+                depth: 0, discovered: () => discovered, increment: () => discovered++,
+                ct: cancellationToken).ConfigureAwait(false);
+        }
+        finally { _gate.Release(); }
+    }
+
+    private async Task BrowseRecursiveAsync(
+        ISession session, NodeId node, IAddressSpaceBuilder folder, HashSet<NodeId> visited,
+        int depth, Func<int> discovered, Action increment, CancellationToken ct)
+    {
+        if (depth >= _options.MaxBrowseDepth) return;
+        if (discovered() >= _options.MaxDiscoveredNodes) return;
+        if (!visited.Add(node)) return;
+
+        var browseDescriptions = new BrowseDescriptionCollection
+        {
+            new()
+            {
+                NodeId = node,
+                BrowseDirection = BrowseDirection.Forward,
+                ReferenceTypeId = ReferenceTypeIds.HierarchicalReferences,
+                IncludeSubtypes = true,
+                NodeClassMask = (uint)(NodeClass.Object | NodeClass.Variable),
+                ResultMask = (uint)(BrowseResultMask.BrowseName | BrowseResultMask.DisplayName
+                                    | BrowseResultMask.NodeClass | BrowseResultMask.TypeDefinition),
+            }
+        };
+
+        BrowseResponse resp;
+        try
+        {
+            resp = await session.BrowseAsync(
+                requestHeader: null,
+                view: null,
+                requestedMaxReferencesPerNode: 0,
+                nodesToBrowse: browseDescriptions,
+                ct: ct).ConfigureAwait(false);
+        }
+        catch
+        {
+            // Transient browse failure on a sub-tree — don't kill the whole discovery, just
+            // skip this branch. The driver's health surface will reflect the cascade via the
+            // probe loop (PR 69).
+            return;
+        }
+
+        if (resp.Results.Count == 0) return;
+        var refs = resp.Results[0].References;
+
+        foreach (var rf in refs)
+        {
+            if (discovered() >= _options.MaxDiscoveredNodes) break;
+
+            var childId = ExpandedNodeId.ToNodeId(rf.NodeId, session.NamespaceUris);
+            if (NodeId.IsNull(childId)) continue;
+
+            var browseName = rf.BrowseName?.Name ?? childId.ToString();
+            var displayName = rf.DisplayName?.Text ?? browseName;
+
+            if (rf.NodeClass == NodeClass.Object)
+            {
+                var subFolder = folder.Folder(browseName, displayName);
+                increment();
+                await BrowseRecursiveAsync(session, childId, subFolder, visited,
+                    depth + 1, discovered, increment, ct).ConfigureAwait(false);
+            }
+            else if (rf.NodeClass == NodeClass.Variable)
+            {
+                // Serialize the NodeId so the IReadable/IWritable surface receives a
+                // round-trippable string. Deferring the DataType + AccessLevel fetch to a
+                // follow-up PR — initial browse uses a conservative ViewOnly + Int32 default.
+                var nodeIdString = childId.ToString() ?? string.Empty;
+                folder.Variable(browseName, displayName, new DriverAttributeInfo(
+                    FullName: nodeIdString,
+                    DriverDataType: DriverDataType.Int32,
+                    IsArray: false,
+                    ArrayDim: null,
+                    SecurityClass: SecurityClassification.ViewOnly,
+                    IsHistorized: false,
+                    IsAlarm: false));
+                increment();
+            }
+        }
+    }
+
+    // ---- ISubscribable ----
+
+    public async Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+    {
+        var session = RequireSession();
+        var id = Interlocked.Increment(ref _nextSubscriptionId);
+        var handle = new OpcUaSubscriptionHandle(id);
+
+        // Floor the publishing interval at 50ms — OPC UA servers routinely negotiate
+        // minimum-supported intervals up anyway, but sending sub-50ms wastes negotiation
+        // bandwidth on every subscription create.
+        var intervalMs = publishingInterval < TimeSpan.FromMilliseconds(50)
+            ? 50
+            : (int)publishingInterval.TotalMilliseconds;
+
+        var subscription = new Subscription(telemetry: null!, new SubscriptionOptions
+        {
+            DisplayName = $"opcua-sub-{id}",
+            PublishingInterval = intervalMs,
+            KeepAliveCount = 10,
+            LifetimeCount = 1000,
+            MaxNotificationsPerPublish = 0,
+            PublishingEnabled = true,
+            Priority = 0,
+            TimestampsToReturn = TimestampsToReturn.Both,
+        });
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            session.AddSubscription(subscription);
+            await subscription.CreateAsync(cancellationToken).ConfigureAwait(false);
+
+            foreach (var fullRef in fullReferences)
+            {
+                if (!TryParseNodeId(session, fullRef, out var nodeId)) continue;
+                // The tag string is routed through MonitoredItem.Handle so the Notification
+                // handler can identify which tag changed without an extra lookup.
+                var item = new MonitoredItem(telemetry: null!, new MonitoredItemOptions
+                {
+                    DisplayName = fullRef,
+                    StartNodeId = nodeId,
+                    AttributeId = Attributes.Value,
+                    MonitoringMode = MonitoringMode.Reporting,
+                    SamplingInterval = intervalMs,
+                    QueueSize = 1,
+                    DiscardOldest = true,
+                })
+                {
+                    Handle = fullRef,
+                };
+                item.Notification += (mi, args) => OnMonitoredItemNotification(handle, mi, args);
+                subscription.AddItem(item);
+            }
+
+            await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);
+            _subscriptions[id] = new RemoteSubscription(subscription, handle);
+        }
+        finally { _gate.Release(); }
+
+        return handle;
+    }
+
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    {
+        if (handle is not OpcUaSubscriptionHandle h) return;
+        if (!_subscriptions.TryRemove(h.Id, out var rs)) return;
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            try { await rs.Subscription.DeleteAsync(silent: true, cancellationToken).ConfigureAwait(false); }
+            catch { /* best-effort — the subscription may already be gone on reconnect */ }
+        }
+        finally { _gate.Release(); }
+    }
+
+    private void OnMonitoredItemNotification(OpcUaSubscriptionHandle handle, MonitoredItem item, MonitoredItemNotificationEventArgs args)
+    {
+        // args.NotificationValue arrives as a MonitoredItemNotification for value-change
+        // subscriptions; extract its DataValue. The Handle property carries our tag string.
+        if (args.NotificationValue is not MonitoredItemNotification mn) return;
+        var dv = mn.Value;
+        if (dv is null) return;
+        var fullRef = (item.Handle as string) ?? item.DisplayName ?? string.Empty;
+        var snapshot = new DataValueSnapshot(
+            Value: dv.Value,
+            StatusCode: dv.StatusCode.Code,
+            SourceTimestampUtc: dv.SourceTimestamp == DateTime.MinValue ? null : dv.SourceTimestamp,
+            ServerTimestampUtc: dv.ServerTimestamp == DateTime.MinValue ? DateTime.UtcNow : dv.ServerTimestamp);
+        OnDataChange?.Invoke(this, new DataChangeEventArgs(handle, fullRef, snapshot));
+    }
+
+    private sealed record RemoteSubscription(Subscription Subscription, OpcUaSubscriptionHandle Handle);
+
+    private sealed record OpcUaSubscriptionHandle(long Id) : ISubscriptionHandle
+    {
+        public string DiagnosticId => $"opcua-sub-{Id}";
+    }
+
+    // ---- IHostConnectivityProbe ----
+
+    /// <summary>Endpoint-URL-keyed host identity for the Admin /hosts dashboard.</summary>
+    public string HostName => _options.EndpointUrl;
+
+    public IReadOnlyList<HostConnectivityStatus> GetHostStatuses()
+    {
+        lock (_probeLock)
+            return [new HostConnectivityStatus(HostName, _hostState, _hostStateChangedUtc)];
+    }
+
+    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));
+    }
+
+    public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        try { await ShutdownAsync(CancellationToken.None).ConfigureAwait(false); }
+        catch { /* disposal is best-effort */ }
+        _gate.Dispose();
+    }
+}

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

@@ -0,0 +1,105 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
+
+/// <summary>
+///     OPC UA Client (gateway) driver configuration. Bound from <c>DriverConfig</c> JSON at
+///     driver-host registration time. Models the settings documented in
+///     <c>docs/v2/driver-specs.md</c> §8.
+/// </summary>
+/// <remarks>
+///     This driver connects to a REMOTE OPC UA server and re-exposes its address space
+///     through the local OtOpcUa server — the opposite direction from the usual "server
+///     exposes PLC data" flow. Tier A (pure managed, OPC Foundation reference SDK); universal
+///     protections cover it.
+/// </remarks>
+public sealed class OpcUaClientDriverOptions
+{
+    /// <summary>Remote OPC UA endpoint URL, e.g. <c>opc.tcp://plc.internal:4840</c>.</summary>
+    public string EndpointUrl { get; init; } = "opc.tcp://localhost:4840";
+
+    /// <summary>Security policy. One of <c>None</c>, <c>Basic256Sha256</c>, <c>Aes128_Sha256_RsaOaep</c>.</summary>
+    public string SecurityPolicy { get; init; } = "None";
+
+    /// <summary>Security mode.</summary>
+    public OpcUaSecurityMode SecurityMode { get; init; } = OpcUaSecurityMode.None;
+
+    /// <summary>Authentication type.</summary>
+    public OpcUaAuthType AuthType { get; init; } = OpcUaAuthType.Anonymous;
+
+    /// <summary>User name (required only for <see cref="OpcUaAuthType.Username"/>).</summary>
+    public string? Username { get; init; }
+
+    /// <summary>Password (required only for <see cref="OpcUaAuthType.Username"/>).</summary>
+    public string? Password { get; init; }
+
+    /// <summary>Server-negotiated session timeout. Default 120s per driver-specs.md §8.</summary>
+    public TimeSpan SessionTimeout { get; init; } = TimeSpan.FromSeconds(120);
+
+    /// <summary>Client-side keep-alive interval.</summary>
+    public TimeSpan KeepAliveInterval { get; init; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>Initial reconnect delay after a session drop.</summary>
+    public TimeSpan ReconnectPeriod { get; init; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>
+    ///     When <c>true</c>, the driver accepts any self-signed / untrusted server certificate.
+    ///     Dev-only — must be <c>false</c> in production so MITM attacks against the opc.tcp
+    ///     channel fail closed.
+    /// </summary>
+    public bool AutoAcceptCertificates { get; init; } = false;
+
+    /// <summary>
+    ///     Application URI the driver reports during session creation. Must match the
+    ///     subject-alt-name on the client certificate if one is used, which is why it's a
+    ///     config knob rather than hard-coded.
+    /// </summary>
+    public string ApplicationUri { get; init; } = "urn:localhost:OtOpcUa:GatewayClient";
+
+    /// <summary>
+    ///     Friendly name sent to the remote server for diagnostics. Shows up in the remote
+    ///     server's session-list so operators can identify which gateway instance is calling.
+    /// </summary>
+    public string SessionName { get; init; } = "OtOpcUa-Gateway";
+
+    /// <summary>Connect + per-operation timeout.</summary>
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(10);
+
+    /// <summary>
+    ///     Root NodeId to mirror. Default <c>null</c> = <c>ObjectsFolder</c> (i=85). Set to
+    ///     a scoped root to restrict the address space the driver exposes locally — useful
+    ///     when the remote server has tens of thousands of nodes and only a subset is
+    ///     needed downstream.
+    /// </summary>
+    public string? BrowseRoot { get; init; }
+
+    /// <summary>
+    ///     Cap on total nodes discovered during <c>DiscoverAsync</c>. Default 10_000 —
+    ///     bounds memory on runaway remote servers without being so low that normal
+    ///     deployments hit it. When the cap is reached discovery stops and a warning is
+    ///     written to the driver health surface; the partially-discovered tree is still
+    ///     projected into the local address space.
+    /// </summary>
+    public int MaxDiscoveredNodes { get; init; } = 10_000;
+
+    /// <summary>
+    ///     Max hierarchical depth of the browse. Default 10 — deep enough for realistic
+    ///     OPC UA information models, shallow enough that cyclic graphs can't spin the
+    ///     browse forever.
+    /// </summary>
+    public int MaxBrowseDepth { get; init; } = 10;
+}
+
+/// <summary>OPC UA message security mode.</summary>
+public enum OpcUaSecurityMode
+{
+    None,
+    Sign,
+    SignAndEncrypt,
+}
+
+/// <summary>User authentication type sent to the remote server.</summary>
+public enum OpcUaAuthType
+{
+    Anonymous,
+    Username,
+    Certificate,
+}

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

@@ -0,0 +1,28 @@
+<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.OpcUaClient</RootNamespace>
+        <AssemblyName>ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="OPCFoundation.NetStandard.Opc.Ua.Client" Version="1.5.378.106"/>
+        <PackageReference Include="OPCFoundation.NetStandard.Opc.Ua.Configuration" Version="1.5.378.106"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests"/>
+    </ItemGroup>
+
+</Project>

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7AddressParser.cs

@@ -0,0 +1,216 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
+
+/// <summary>
+///     Siemens S7 memory area. The driver's tag-address parser maps every S7 tag string into
+///     exactly one of these + an offset. Values match the on-wire S7 area codes only
+///     incidentally — S7.Net uses its own <c>DataType</c> enum (<c>DataBlock</c>, <c>Memory</c>,
+///     <c>Input</c>, <c>Output</c>, <c>Timer</c>, <c>Counter</c>) so the adapter layer translates.
+/// </summary>
+public enum S7Area
+{
+    DataBlock,
+    Memory,  // M (Merker / marker byte)
+    Input,   // I (process-image input)
+    Output,  // Q (process-image output)
+    Timer,
+    Counter,
+}
+
+/// <summary>
+///     Access width for a DB / M / I / Q address. Timers and counters are always 16-bit
+///     opaque (not user-addressable via size suffixes).
+/// </summary>
+public enum S7Size
+{
+    Bit,     // X
+    Byte,    // B
+    Word,    // W — 16-bit
+    DWord,   // D — 32-bit
+}
+
+/// <summary>
+///     Parsed form of an S7 tag-address string. Produced by <see cref="S7AddressParser.Parse"/>.
+/// </summary>
+/// <param name="Area">Memory area (DB, M, I, Q, T, C).</param>
+/// <param name="DbNumber">Data block number; only meaningful when <paramref name="Area"/> is <see cref="S7Area.DataBlock"/>.</param>
+/// <param name="Size">Access width. Always <see cref="S7Size.Word"/> for Timer and Counter.</param>
+/// <param name="ByteOffset">Byte offset into the area (for DB/M/I/Q) or the timer/counter number.</param>
+/// <param name="BitOffset">Bit position 0-7 when <paramref name="Size"/> is <see cref="S7Size.Bit"/>; 0 otherwise.</param>
+public readonly record struct S7ParsedAddress(
+    S7Area Area,
+    int DbNumber,
+    S7Size Size,
+    int ByteOffset,
+    int BitOffset);
+
+/// <summary>
+///     Parses Siemens S7 address strings into <see cref="S7ParsedAddress"/>. Accepts the
+///     Siemens TIA-Portal / STEP 7 Classic syntax documented in <c>docs/v2/driver-specs.md</c> §5:
+///     <list type="bullet">
+///         <item><c>DB{n}.DB{X|B|W|D}{offset}[.bit]</c> — e.g. <c>DB1.DBX0.0</c>, <c>DB1.DBW0</c>, <c>DB1.DBD4</c></item>
+///         <item><c>M{B|W|D}{offset}</c> or <c>M{offset}.{bit}</c> — e.g. <c>MB0</c>, <c>MW0</c>, <c>MD4</c>, <c>M0.0</c></item>
+///         <item><c>I{B|W|D}{offset}</c> or <c>I{offset}.{bit}</c> — e.g. <c>IB0</c>, <c>IW0</c>, <c>ID0</c>, <c>I0.0</c></item>
+///         <item><c>Q{B|W|D}{offset}</c> or <c>Q{offset}.{bit}</c> — e.g. <c>QB0</c>, <c>QW0</c>, <c>QD0</c>, <c>Q0.0</c></item>
+///         <item><c>T{n}</c> — e.g. <c>T0</c>, <c>T15</c></item>
+///         <item><c>C{n}</c> — e.g. <c>C0</c>, <c>C10</c></item>
+///     </list>
+///     Grammar is case-insensitive. Leading/trailing whitespace tolerated. Bit specifiers
+///     must be 0-7; byte offsets must be non-negative; DB numbers must be &gt;= 1.
+/// </summary>
+/// <remarks>
+///     Parse is deliberately strict — the parser rejects syntactic garbage up-front so a bad
+///     tag config fails at driver init time instead of surfacing as a misleading
+///     <c>BadInternalError</c> on every Read against that tag.
+/// </remarks>
+public static class S7AddressParser
+{
+    /// <summary>
+    ///     Parse an S7 address. Throws <see cref="FormatException"/> on any syntax error with
+    ///     the offending input echoed in the message so operators can correlate to the tag
+    ///     config that produced the fault.
+    /// </summary>
+    public static S7ParsedAddress Parse(string address)
+    {
+        if (string.IsNullOrWhiteSpace(address))
+            throw new FormatException("S7 address must not be empty");
+        var s = address.Trim().ToUpperInvariant();
+
+        // --- DB{n}.DB{X|B|W|D}{offset}[.bit] ---
+        if (s.StartsWith("DB") && TryParseDataBlock(s, out var dbResult))
+            return dbResult;
+
+        if (s.Length < 2)
+            throw new FormatException($"S7 address '{address}' is too short to parse");
+
+        var areaChar = s[0];
+        var rest = s.Substring(1);
+
+        switch (areaChar)
+        {
+            case 'M': return ParseMIQ(S7Area.Memory, rest, address);
+            case 'I': return ParseMIQ(S7Area.Input, rest, address);
+            case 'Q': return ParseMIQ(S7Area.Output, rest, address);
+            case 'T': return ParseTimerOrCounter(S7Area.Timer, rest, address);
+            case 'C': return ParseTimerOrCounter(S7Area.Counter, rest, address);
+            default:
+                throw new FormatException($"S7 address '{address}' starts with unknown area '{areaChar}' (expected DB/M/I/Q/T/C)");
+        }
+    }
+
+    /// <summary>
+    ///     Try-parse variant for callers that can't afford an exception on bad input (e.g.
+    ///     config validation pages in the Admin UI). Returns <c>false</c> for any input that
+    ///     would throw from <see cref="Parse"/>.
+    /// </summary>
+    public static bool TryParse(string address, out S7ParsedAddress result)
+    {
+        try
+        {
+            result = Parse(address);
+            return true;
+        }
+        catch (FormatException)
+        {
+            result = default;
+            return false;
+        }
+    }
+
+    private static bool TryParseDataBlock(string s, out S7ParsedAddress result)
+    {
+        result = default;
+        // Split on first '.': left side must be DB{n}, right side DB{X|B|W|D}{offset}[.bit]
+        var dot = s.IndexOf('.');
+        if (dot < 0) return false;
+        var head = s.Substring(0, dot);    // DB{n}
+        var tail = s.Substring(dot + 1);   // DB{X|B|W|D}{offset}[.bit]
+
+        if (head.Length < 3) return false;
+        if (!int.TryParse(head.AsSpan(2), out var dbNumber) || dbNumber < 1)
+            throw new FormatException($"S7 DB number in '{s}' must be a positive integer");
+
+        if (!tail.StartsWith("DB") || tail.Length < 4)
+            throw new FormatException($"S7 DB address tail '{tail}' must start with DB{{X|B|W|D}}");
+
+        var sizeChar = tail[2];
+        var offsetStart = 3;
+        var size = sizeChar switch
+        {
+            'X' => S7Size.Bit,
+            'B' => S7Size.Byte,
+            'W' => S7Size.Word,
+            'D' => S7Size.DWord,
+            _ => throw new FormatException($"S7 DB size '{sizeChar}' in '{s}' must be X/B/W/D"),
+        };
+
+        var (byteOffset, bitOffset) = ParseOffsetAndOptionalBit(tail, offsetStart, size, s);
+        result = new S7ParsedAddress(S7Area.DataBlock, dbNumber, size, byteOffset, bitOffset);
+        return true;
+    }
+
+    private static S7ParsedAddress ParseMIQ(S7Area area, string rest, string original)
+    {
+        if (rest.Length == 0)
+            throw new FormatException($"S7 address '{original}' has no offset");
+
+        var first = rest[0];
+        S7Size size;
+        int offsetStart;
+        switch (first)
+        {
+            case 'B': size = S7Size.Byte;  offsetStart = 1; break;
+            case 'W': size = S7Size.Word;  offsetStart = 1; break;
+            case 'D': size = S7Size.DWord; offsetStart = 1; break;
+            default:
+                // No size prefix => bit-level address requires explicit .bit. Size stays Bit;
+                // ParseOffsetAndOptionalBit will demand the dot.
+                size = S7Size.Bit;
+                offsetStart = 0;
+                break;
+        }
+
+        var (byteOffset, bitOffset) = ParseOffsetAndOptionalBit(rest, offsetStart, size, original);
+        return new S7ParsedAddress(area, DbNumber: 0, size, byteOffset, bitOffset);
+    }
+
+    private static S7ParsedAddress ParseTimerOrCounter(S7Area area, string rest, string original)
+    {
+        if (rest.Length == 0)
+            throw new FormatException($"S7 address '{original}' has no {area} number");
+        if (!int.TryParse(rest, out var number) || number < 0)
+            throw new FormatException($"S7 {area} number in '{original}' must be a non-negative integer");
+        return new S7ParsedAddress(area, DbNumber: 0, S7Size.Word, number, BitOffset: 0);
+    }
+
+    private static (int byteOffset, int bitOffset) ParseOffsetAndOptionalBit(
+        string s, int start, S7Size size, string original)
+    {
+        var offsetEnd = start;
+        while (offsetEnd < s.Length && s[offsetEnd] >= '0' && s[offsetEnd] <= '9')
+            offsetEnd++;
+        if (offsetEnd == start)
+            throw new FormatException($"S7 address '{original}' has no byte-offset digits");
+
+        if (!int.TryParse(s.AsSpan(start, offsetEnd - start), out var byteOffset) || byteOffset < 0)
+            throw new FormatException($"S7 byte offset in '{original}' must be non-negative");
+
+        // No bit-suffix: done unless size is Bit with no prefix, which requires one.
+        if (offsetEnd == s.Length)
+        {
+            if (size == S7Size.Bit)
+                throw new FormatException($"S7 address '{original}' needs a .{{bit}} suffix for bit access");
+            return (byteOffset, 0);
+        }
+
+        if (s[offsetEnd] != '.')
+            throw new FormatException($"S7 address '{original}' has unexpected character after offset");
+
+        if (size != S7Size.Bit)
+            throw new FormatException($"S7 address '{original}' has a bit suffix but the size is {size} — bit access needs X (DB) or no size prefix (M/I/Q)");
+
+        if (!int.TryParse(s.AsSpan(offsetEnd + 1), out var bitOffset) || bitOffset is < 0 or > 7)
+            throw new FormatException($"S7 bit offset in '{original}' must be 0-7");
+
+        return (byteOffset, bitOffset);
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -0,0 +1,513 @@
+using S7.Net;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
+
+/// <summary>
+///     Siemens S7 native driver — speaks S7comm over ISO-on-TCP (port 102) via the S7netplus
+///     library. First implementation of <see cref="IDriver"/> for an in-process .NET Standard
+///     PLC protocol that is NOT Modbus, validating that the v2 driver-capability interfaces
+///     generalize beyond Modbus + Galaxy.
+/// </summary>
+/// <remarks>
+///     <para>
+///         PR 62 ships the scaffold: <see cref="IDriver"/> only (Initialize / Reinitialize /
+///         Shutdown / GetHealth). <see cref="ITagDiscovery"/>, <see cref="IReadable"/>,
+///         <see cref="IWritable"/>, <see cref="ISubscribable"/>, <see cref="IHostConnectivityProbe"/>
+///         land in PRs 63-65 once the address parser (PR 63) is in place.
+///     </para>
+///     <para>
+///         <b>Single-connection policy</b>: S7netplus documented pattern is one
+///         <c>Plc</c> instance per PLC, serialized with a <see cref="SemaphoreSlim"/>.
+///         Parallelising reads against a single S7 CPU doesn't help — the CPU scans the
+///         communication mailbox at most once per cycle (2-10 ms) and queues concurrent
+///         requests wire-side anyway. Multiple client-side connections just waste the CPU's
+///         8-64 connection-resource budget.
+///     </para>
+/// </remarks>
+public sealed class S7Driver(S7DriverOptions options, string driverInstanceId)
+    : IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IDisposable, IAsyncDisposable
+{
+    // ---- ISubscribable + IHostConnectivityProbe state ----
+
+    private readonly System.Collections.Concurrent.ConcurrentDictionary<long, SubscriptionState> _subscriptions = new();
+    private long _nextSubscriptionId;
+    private readonly object _probeLock = new();
+    private HostState _hostState = HostState.Unknown;
+    private DateTime _hostStateChangedUtc = DateTime.UtcNow;
+    private CancellationTokenSource? _probeCts;
+
+    public event EventHandler<DataChangeEventArgs>? OnDataChange;
+    public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;
+
+    /// <summary>OPC UA StatusCode used when the tag name isn't in the driver's tag map.</summary>
+    private const uint StatusBadNodeIdUnknown = 0x80340000u;
+    /// <summary>OPC UA StatusCode used when the tag's data type isn't implemented yet.</summary>
+    private const uint StatusBadNotSupported = 0x803D0000u;
+    /// <summary>OPC UA StatusCode used when the tag is declared read-only.</summary>
+    private const uint StatusBadNotWritable = 0x803B0000u;
+    /// <summary>OPC UA StatusCode used when write fails validation (e.g. out-of-range value).</summary>
+    private const uint StatusBadInternalError = 0x80020000u;
+    /// <summary>OPC UA StatusCode used for socket / timeout / protocol-layer faults.</summary>
+    private const uint StatusBadCommunicationError = 0x80050000u;
+    /// <summary>OPC UA StatusCode used when S7 returns <c>ErrorCode.WrongCPU</c> / PUT/GET disabled.</summary>
+    private const uint StatusBadDeviceFailure = 0x80550000u;
+
+    private readonly Dictionary<string, S7TagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, S7ParsedAddress> _parsedByName = new(StringComparer.OrdinalIgnoreCase);
+
+    private readonly S7DriverOptions _options = options;
+    private readonly SemaphoreSlim _gate = new(1, 1);
+
+    /// <summary>
+    ///     Per-connection gate. Internal so PRs 63-65 (read/write/subscribe) can serialize on
+    ///     the same semaphore without exposing it publicly. Single-connection-per-PLC is a
+    ///     hard requirement of S7netplus — see class remarks.
+    /// </summary>
+    internal SemaphoreSlim Gate => _gate;
+
+    /// <summary>
+    ///     Active S7.Net PLC connection. Null until <see cref="InitializeAsync"/> returns; null
+    ///     after <see cref="ShutdownAsync"/>. Read-only outside this class; PR 64's Read/Write
+    ///     will take the <see cref="_gate"/> before touching it.
+    /// </summary>
+    internal Plc? Plc { get; private set; }
+
+    private DriverHealth _health = new(DriverState.Unknown, null, null);
+    private bool _disposed;
+
+    public string DriverInstanceId => driverInstanceId;
+    public string DriverType => "S7";
+
+    public async Task InitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        _health = new DriverHealth(DriverState.Initializing, null, null);
+        try
+        {
+            var plc = new Plc(_options.CpuType, _options.Host, _options.Rack, _options.Slot);
+            // S7netplus writes timeouts into the underlying TcpClient via Plc.WriteTimeout /
+            // Plc.ReadTimeout (milliseconds). Set before OpenAsync so the handshake itself
+            // honours the bound.
+            plc.WriteTimeout = (int)_options.Timeout.TotalMilliseconds;
+            plc.ReadTimeout = (int)_options.Timeout.TotalMilliseconds;
+
+            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            cts.CancelAfter(_options.Timeout);
+            await plc.OpenAsync(cts.Token).ConfigureAwait(false);
+
+            Plc = plc;
+
+            // Parse every tag's address once at init so config typos fail fast here instead
+            // of surfacing as BadInternalError on every Read against the bad tag. The parser
+            // also rejects bit-offset > 7, DB 0, unknown area letters, etc.
+            _tagsByName.Clear();
+            _parsedByName.Clear();
+            foreach (var t in _options.Tags)
+            {
+                var parsed = S7AddressParser.Parse(t.Address); // throws FormatException
+                _tagsByName[t.Name] = t;
+                _parsedByName[t.Name] = parsed;
+            }
+
+            _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
+
+            // Kick off the probe loop once the connection is up. Initial HostState stays
+            // Unknown until the first probe tick succeeds — avoids broadcasting a premature
+            // Running transition before any PDU round-trip has happened.
+            if (_options.Probe.Enabled)
+            {
+                _probeCts = new CancellationTokenSource();
+                _ = Task.Run(() => ProbeLoopAsync(_probeCts.Token), _probeCts.Token);
+            }
+        }
+        catch (Exception ex)
+        {
+            // Clean up a partially-constructed Plc so a retry from the caller doesn't leak
+            // the TcpClient. S7netplus's Close() is best-effort and idempotent.
+            try { Plc?.Close(); } catch { }
+            Plc = null;
+            _health = new DriverHealth(DriverState.Faulted, null, ex.Message);
+            throw;
+        }
+    }
+
+    public async Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken)
+    {
+        await ShutdownAsync(cancellationToken).ConfigureAwait(false);
+        await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
+    }
+
+    public 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();
+
+        try { Plc?.Close(); } catch { /* best-effort — tearing down anyway */ }
+        Plc = null;
+        _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
+        return Task.CompletedTask;
+    }
+
+    public DriverHealth GetHealth() => _health;
+
+    /// <summary>
+    ///     Approximate memory footprint. The Plc instance + one 240-960 byte PDU buffer is
+    ///     under 4 KB; return 0 because the <see cref="IDriver"/> contract asks for a
+    ///     driver-attributable growth number and S7.Net doesn't expose one.
+    /// </summary>
+    public long GetMemoryFootprint() => 0;
+
+    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+    // ---- IReadable ----
+
+    public async Task<IReadOnlyList<DataValueSnapshot>> ReadAsync(
+        IReadOnlyList<string> fullReferences, CancellationToken cancellationToken)
+    {
+        var plc = RequirePlc();
+        var now = DateTime.UtcNow;
+        var results = new DataValueSnapshot[fullReferences.Count];
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            for (var i = 0; i < fullReferences.Count; i++)
+            {
+                var name = fullReferences[i];
+                if (!_tagsByName.TryGetValue(name, out var tag))
+                {
+                    results[i] = new DataValueSnapshot(null, StatusBadNodeIdUnknown, null, now);
+                    continue;
+                }
+                try
+                {
+                    var value = await ReadOneAsync(plc, tag, cancellationToken).ConfigureAwait(false);
+                    results[i] = new DataValueSnapshot(value, 0u, now, now);
+                    _health = new DriverHealth(DriverState.Healthy, now, null);
+                }
+                catch (NotSupportedException)
+                {
+                    results[i] = new DataValueSnapshot(null, StatusBadNotSupported, null, now);
+                }
+                catch (global::S7.Net.PlcException pex)
+                {
+                    // S7.Net's PlcException carries an ErrorCode; PUT/GET-disabled on
+                    // S7-1200/1500 surfaces here. Map to BadDeviceFailure so operators see a
+                    // device-config problem (toggle PUT/GET in TIA Portal) rather than a
+                    // transient fault — per driver-specs.md §5.
+                    results[i] = new DataValueSnapshot(null, StatusBadDeviceFailure, null, now);
+                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, pex.Message);
+                }
+                catch (Exception ex)
+                {
+                    results[i] = new DataValueSnapshot(null, StatusBadCommunicationError, null, now);
+                    _health = new DriverHealth(DriverState.Degraded, _health.LastSuccessfulRead, ex.Message);
+                }
+            }
+        }
+        finally { _gate.Release(); }
+        return results;
+    }
+
+    private async Task<object> ReadOneAsync(global::S7.Net.Plc plc, S7TagDefinition tag, CancellationToken ct)
+    {
+        var addr = _parsedByName[tag.Name];
+        // S7.Net's string-based ReadAsync returns object where the boxed .NET type depends on
+        // the size suffix: DBX=bool, DBB=byte, DBW=ushort, DBD=uint. Our S7DataType enum
+        // specifies the SEMANTIC type (Int16 vs UInt16 vs Float32 etc.); the reinterpret below
+        // converts the raw unsigned boxed value into the requested type without issuing an
+        // extra PLC round-trip.
+        var raw = await plc.ReadAsync(tag.Address, ct).ConfigureAwait(false)
+            ?? throw new System.IO.InvalidDataException($"S7.Net returned null for '{tag.Address}'");
+
+        return (tag.DataType, addr.Size, raw) switch
+        {
+            (S7DataType.Bool, S7Size.Bit, bool b) => b,
+            (S7DataType.Byte, S7Size.Byte, byte by) => by,
+            (S7DataType.UInt16, S7Size.Word, ushort u16) => u16,
+            (S7DataType.Int16, S7Size.Word, ushort u16) => unchecked((short)u16),
+            (S7DataType.UInt32, S7Size.DWord, uint u32) => u32,
+            (S7DataType.Int32, S7Size.DWord, uint u32) => unchecked((int)u32),
+            (S7DataType.Float32, S7Size.DWord, uint u32) => BitConverter.UInt32BitsToSingle(u32),
+
+            (S7DataType.Int64, _, _)   => throw new NotSupportedException("S7 Int64 reads land in a follow-up PR"),
+            (S7DataType.UInt64, _, _)  => throw new NotSupportedException("S7 UInt64 reads land in a follow-up PR"),
+            (S7DataType.Float64, _, _) => throw new NotSupportedException("S7 Float64 (LReal) reads land in a follow-up PR"),
+            (S7DataType.String, _, _)  => throw new NotSupportedException("S7 STRING reads land in a follow-up PR"),
+            (S7DataType.DateTime, _, _) => throw new NotSupportedException("S7 DateTime reads land in a follow-up PR"),
+
+            _ => throw new System.IO.InvalidDataException(
+                $"S7 Read type-mismatch: tag '{tag.Name}' declared {tag.DataType} but address '{tag.Address}' " +
+                $"parsed as Size={addr.Size}; S7.Net returned {raw.GetType().Name}"),
+        };
+    }
+
+    // ---- IWritable ----
+
+    public async Task<IReadOnlyList<WriteResult>> WriteAsync(
+        IReadOnlyList<WriteRequest> writes, CancellationToken cancellationToken)
+    {
+        var plc = RequirePlc();
+        var results = new WriteResult[writes.Count];
+
+        await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            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)
+                {
+                    results[i] = new WriteResult(StatusBadNotWritable);
+                    continue;
+                }
+                try
+                {
+                    await WriteOneAsync(plc, tag, w.Value, cancellationToken).ConfigureAwait(false);
+                    results[i] = new WriteResult(0u);
+                }
+                catch (NotSupportedException)
+                {
+                    results[i] = new WriteResult(StatusBadNotSupported);
+                }
+                catch (global::S7.Net.PlcException)
+                {
+                    results[i] = new WriteResult(StatusBadDeviceFailure);
+                }
+                catch (Exception)
+                {
+                    results[i] = new WriteResult(StatusBadInternalError);
+                }
+            }
+        }
+        finally { _gate.Release(); }
+        return results;
+    }
+
+    private async Task WriteOneAsync(global::S7.Net.Plc plc, S7TagDefinition tag, object? value, CancellationToken ct)
+    {
+        // S7.Net's Plc.WriteAsync(string address, object value) expects the boxed value to
+        // match the address's size-suffix type: DBX=bool, DBB=byte, DBW=ushort, DBD=uint.
+        // Our S7DataType lets the caller pass short/int/float; convert to the unsigned
+        // wire representation before handing off.
+        var boxed = tag.DataType switch
+        {
+            S7DataType.Bool    => (object)Convert.ToBoolean(value),
+            S7DataType.Byte    => (object)Convert.ToByte(value),
+            S7DataType.UInt16  => (object)Convert.ToUInt16(value),
+            S7DataType.Int16   => (object)unchecked((ushort)Convert.ToInt16(value)),
+            S7DataType.UInt32  => (object)Convert.ToUInt32(value),
+            S7DataType.Int32   => (object)unchecked((uint)Convert.ToInt32(value)),
+            S7DataType.Float32 => (object)BitConverter.SingleToUInt32Bits(Convert.ToSingle(value)),
+
+            S7DataType.Int64   => throw new NotSupportedException("S7 Int64 writes land in a follow-up PR"),
+            S7DataType.UInt64  => throw new NotSupportedException("S7 UInt64 writes land in a follow-up PR"),
+            S7DataType.Float64 => throw new NotSupportedException("S7 Float64 (LReal) writes land in a follow-up PR"),
+            S7DataType.String  => throw new NotSupportedException("S7 STRING writes land in a follow-up PR"),
+            S7DataType.DateTime => throw new NotSupportedException("S7 DateTime writes land in a follow-up PR"),
+            _ => throw new InvalidOperationException($"Unknown S7DataType {tag.DataType}"),
+        };
+        await plc.WriteAsync(tag.Address, boxed, ct).ConfigureAwait(false);
+    }
+
+    private global::S7.Net.Plc RequirePlc() =>
+        Plc ?? throw new InvalidOperationException("S7Driver not initialized");
+
+    // ---- ITagDiscovery ----
+
+    public Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+        var folder = builder.Folder("S7", "S7");
+        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;
+    }
+
+    private static DriverDataType MapDataType(S7DataType t) => t switch
+    {
+        S7DataType.Bool => DriverDataType.Boolean,
+        S7DataType.Byte => DriverDataType.Int32,      // no 8-bit in DriverDataType yet
+        S7DataType.Int16 or S7DataType.UInt16 or S7DataType.Int32 or S7DataType.UInt32 => DriverDataType.Int32,
+        S7DataType.Int64 or S7DataType.UInt64 => DriverDataType.Int32, // widens; lossy for >2^31-1
+        S7DataType.Float32 => DriverDataType.Float32,
+        S7DataType.Float64 => DriverDataType.Float64,
+        S7DataType.String => DriverDataType.String,
+        S7DataType.DateTime => DriverDataType.DateTime,
+        _ => DriverDataType.Int32,
+    };
+
+    // ---- ISubscribable (polling overlay) ----
+
+    public Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+    {
+        var id = Interlocked.Increment(ref _nextSubscriptionId);
+        var cts = new CancellationTokenSource();
+        // Floor at 100 ms — S7 CPUs scan 2-10 ms but the comms mailbox is processed at most
+        // once per scan; sub-100 ms polling just queues wire-side with worse latency.
+        var interval = publishingInterval < TimeSpan.FromMilliseconds(100)
+            ? TimeSpan.FromMilliseconds(100)
+            : publishingInterval;
+        var handle = new S7SubscriptionHandle(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 S7SubscriptionHandle 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 per OPC UA Part 4 convention.
+        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;
+
+            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(
+        S7SubscriptionHandle Handle,
+        IReadOnlyList<string> TagReferences,
+        TimeSpan Interval,
+        CancellationTokenSource Cts)
+    {
+        public System.Collections.Concurrent.ConcurrentDictionary<string, DataValueSnapshot> LastValues { get; }
+            = new(StringComparer.OrdinalIgnoreCase);
+    }
+
+    private sealed record S7SubscriptionHandle(long Id) : ISubscriptionHandle
+    {
+        public string DiagnosticId => $"s7-sub-{Id}";
+    }
+
+    // ---- IHostConnectivityProbe ----
+
+    /// <summary>
+    ///     Host identifier surfaced in <see cref="GetHostStatuses"/>. <c>host:port</c> format
+    ///     matches the Modbus driver's convention so the Admin UI dashboard renders both
+    ///     family's rows uniformly.
+    /// </summary>
+    public string HostName => $"{_options.Host}:{_options.Port}";
+
+    public IReadOnlyList<HostConnectivityStatus> GetHostStatuses()
+    {
+        lock (_probeLock)
+            return [new HostConnectivityStatus(HostName, _hostState, _hostStateChangedUtc)];
+    }
+
+    private async Task ProbeLoopAsync(CancellationToken ct)
+    {
+        while (!ct.IsCancellationRequested)
+        {
+            var success = false;
+            try
+            {
+                // Probe via S7.Net's low-cost GetCpuStatus — returns the CPU state (Run/Stop)
+                // and is intentionally light on the comms mailbox. Single-word Plc.ReadAsync
+                // would also work but GetCpuStatus doubles as a "PLC actually up" check.
+                using var probeCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+                probeCts.CancelAfter(_options.Probe.Timeout);
+
+                var plc = Plc;
+                if (plc is null) throw new InvalidOperationException("Plc dropped during probe");
+
+                await _gate.WaitAsync(probeCts.Token).ConfigureAwait(false);
+                try
+                {
+                    _ = await plc.ReadStatusAsync(probeCts.Token).ConfigureAwait(false);
+                    success = true;
+                }
+                finally { _gate.Release(); }
+            }
+            catch (OperationCanceledException) when (ct.IsCancellationRequested) { return; }
+            catch { /* transport/timeout/exception — 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));
+    }
+
+    public void Dispose() => DisposeAsync().AsTask().GetAwaiter().GetResult();
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        try { await ShutdownAsync(CancellationToken.None).ConfigureAwait(false); }
+        catch { /* disposal is best-effort */ }
+        _gate.Dispose();
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7DriverOptions.cs

@@ -0,0 +1,112 @@
+using S7NetCpuType = global::S7.Net.CpuType;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
+
+/// <summary>
+///     Siemens S7 native (S7comm / ISO-on-TCP port 102) driver configuration. Bound from the
+///     driver's <c>DriverConfig</c> JSON at <c>DriverHost.RegisterAsync</c>. Unlike the Modbus
+///     driver the S7 driver uses the PLC's *native* protocol — port 102 ISO-on-TCP rather
+///     than Modbus's 502, and S7-specific area codes (DB, M, I, Q) rather than holding-
+///     register / coil tables.
+/// </summary>
+/// <remarks>
+///     <para>
+///         The driver requires <b>PUT/GET communication enabled</b> in the TIA Portal
+///         hardware config for S7-1200/1500. The factory default disables PUT/GET access,
+///         so a driver configured against a freshly-flashed CPU will see a hard error
+///         (S7.Net surfaces it as <c>Plc.ReadAsync</c> returning <c>ErrorCode.Accessing</c>).
+///         The driver maps that specifically to <c>BadNotSupported</c> and flags it as a
+///         configuration alert rather than a transient fault — blind Polly retry is wasted
+///         effort when the PLC will keep refusing every request.
+///     </para>
+///     <para>
+///         See <c>docs/v2/driver-specs.md</c> §5 for the full specification.
+///     </para>
+/// </remarks>
+public sealed class S7DriverOptions
+{
+    /// <summary>PLC IP address or hostname.</summary>
+    public string Host { get; init; } = "127.0.0.1";
+
+    /// <summary>TCP port. ISO-on-TCP is 102 on every S7 model; override only for unusual NAT setups.</summary>
+    public int Port { get; init; } = 102;
+
+    /// <summary>
+    ///     CPU family. Determines the ISO-TSAP slot byte that S7.Net uses during connection
+    ///     setup — pick the family that matches the target PLC exactly.
+    /// </summary>
+    public S7NetCpuType CpuType { get; init; } = S7NetCpuType.S71500;
+
+    /// <summary>
+    ///     Hardware rack number. Almost always 0; relevant only for distributed S7-400 racks
+    ///     with multiple CPUs.
+    /// </summary>
+    public short Rack { get; init; } = 0;
+
+    /// <summary>
+    ///     CPU slot. Conventions per family: S7-300 = slot 2, S7-400 = slot 2 or 3,
+    ///     S7-1200 / S7-1500 = slot 0 (onboard PN). S7.Net uses this to build the remote
+    ///     TSAP. Wrong slot → connection refused during handshake.
+    /// </summary>
+    public short Slot { get; init; } = 0;
+
+    /// <summary>Connect + per-operation timeout.</summary>
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>Pre-declared tag map. S7 has a symbol-table protocol but S7.Net does not expose it, so the driver operates off a static tag list configured per-site. Address grammar documented in S7AddressParser (PR 63).</summary>
+    public IReadOnlyList<S7TagDefinition> Tags { get; init; } = [];
+
+    /// <summary>
+    ///     Background connectivity-probe settings. When enabled, the driver runs a tick loop
+    ///     that issues a cheap read against <see cref="S7ProbeOptions.ProbeAddress"/> every
+    ///     <see cref="S7ProbeOptions.Interval"/> and raises <c>OnHostStatusChanged</c> on
+    ///     Running ↔ Stopped transitions.
+    /// </summary>
+    public S7ProbeOptions Probe { get; init; } = new();
+}
+
+public sealed class S7ProbeOptions
+{
+    public bool Enabled { get; init; } = true;
+    public TimeSpan Interval { get; init; } = TimeSpan.FromSeconds(5);
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+
+    /// <summary>
+    ///     Address to probe for liveness. DB1.DBW0 is the convention if the PLC project
+    ///     reserves a small fingerprint DB for health checks (per <c>docs/v2/s7.md</c>);
+    ///     if not, pick any valid Merker word like <c>MW0</c>.
+    /// </summary>
+    public string ProbeAddress { get; init; } = "MW0";
+}
+
+/// <summary>
+///     One S7 variable as exposed by the driver. Addresses use S7.Net syntax — see
+///     <c>S7AddressParser</c> (PR 63) for the grammar.
+/// </summary>
+/// <param name="Name">Tag name; OPC UA browse name + driver full reference.</param>
+/// <param name="Address">S7 address string, e.g. <c>DB1.DBW0</c>, <c>M0.0</c>, <c>I0.0</c>, <c>QD4</c>. Grammar documented in <c>S7AddressParser</c> (PR 63).</param>
+/// <param name="DataType">Logical data type — drives the underlying S7.Net read/write width.</param>
+/// <param name="Writable">When true the driver accepts writes for this tag.</param>
+/// <param name="StringLength">For <c>DataType = String</c>: S7-string max length. Default 254 (S7 max).</param>
+public sealed record S7TagDefinition(
+    string Name,
+    string Address,
+    S7DataType DataType,
+    bool Writable = true,
+    int StringLength = 254);
+
+public enum S7DataType
+{
+    Bool,
+    Byte,
+    Int16,
+    UInt16,
+    Int32,
+    UInt32,
+    Int64,
+    UInt64,
+    Float32,
+    Float64,
+    String,
+    DateTime,
+}

src/ZB.MOM.WW.OtOpcUa.Driver.S7/ZB.MOM.WW.OtOpcUa.Driver.S7.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.S7</RootNamespace>
+        <AssemblyName>ZB.MOM.WW.OtOpcUa.Driver.S7</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="S7netplus" Version="0.20.0"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.S7.Tests"/>
+    </ItemGroup>
+
+</Project>

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

@@ -0,0 +1,55 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Scaffold tests for <see cref="OpcUaClientDriver"/>'s <see cref="ITagDiscovery"/>
+///     surface that don't require a live remote server. Live-browse coverage lands in a
+///     follow-up PR once the in-process OPC UA server fixture is scaffolded.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientDiscoveryTests
+{
+    [Fact]
+    public async Task DiscoverAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-disco");
+        var builder = new NullAddressSpaceBuilder();
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.DiscoverAsync(builder, TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public void DiscoverAsync_rejects_null_builder()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-disco");
+        Should.ThrowAsync<ArgumentNullException>(async () =>
+            await drv.DiscoverAsync(null!, TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public void Discovery_caps_are_sensible_defaults()
+    {
+        var opts = new OpcUaClientDriverOptions();
+        opts.MaxDiscoveredNodes.ShouldBe(10_000, "bounds memory on runaway servers without clipping normal models");
+        opts.MaxBrowseDepth.ShouldBe(10, "deep enough for realistic info models; shallow enough for cycle safety");
+        opts.BrowseRoot.ShouldBeNull("null = default to ObjectsFolder i=85");
+    }
+
+    private sealed class NullAddressSpaceBuilder : IAddressSpaceBuilder
+    {
+        public IAddressSpaceBuilder Folder(string browseName, string displayName) => this;
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo attributeInfo)
+            => new StubHandle();
+        public void AddProperty(string browseName, DriverDataType dataType, object? value) { }
+        public void AttachAlarmCondition(IVariableHandle sourceVariable, string alarmName, DriverAttributeInfo alarmInfo) { }
+
+        private sealed class StubHandle : IVariableHandle
+        {
+            public string FullReference => "stub";
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => throw new NotSupportedException();
+        }
+    }
+}

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

@@ -0,0 +1,90 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Scaffold-level tests for <see cref="OpcUaClientDriver"/> that don't require a live
+///     remote OPC UA server. PR 67+ adds IReadable/IWritable/ITagDiscovery/ISubscribable
+///     tests against a local in-process OPC UA server fixture.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientDriverScaffoldTests
+{
+    [Fact]
+    public void Default_options_target_standard_opcua_port_and_anonymous_auth()
+    {
+        var opts = new OpcUaClientDriverOptions();
+        opts.EndpointUrl.ShouldBe("opc.tcp://localhost:4840", "4840 is the IANA-assigned OPC UA port");
+        opts.SecurityMode.ShouldBe(OpcUaSecurityMode.None);
+        opts.AuthType.ShouldBe(OpcUaAuthType.Anonymous);
+        opts.AutoAcceptCertificates.ShouldBeFalse("production default must reject untrusted server certs");
+    }
+
+    [Fact]
+    public void Default_timeouts_match_driver_specs_section_8()
+    {
+        var opts = new OpcUaClientDriverOptions();
+        opts.SessionTimeout.ShouldBe(TimeSpan.FromSeconds(120));
+        opts.KeepAliveInterval.ShouldBe(TimeSpan.FromSeconds(5));
+        opts.ReconnectPeriod.ShouldBe(TimeSpan.FromSeconds(5));
+    }
+
+    [Fact]
+    public void Driver_reports_type_and_id_before_connect()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-test");
+        drv.DriverType.ShouldBe("OpcUaClient");
+        drv.DriverInstanceId.ShouldBe("opcua-test");
+        drv.GetHealth().State.ShouldBe(DriverState.Unknown);
+    }
+
+    [Fact]
+    public async Task Initialize_against_unreachable_endpoint_transitions_to_Faulted_and_throws()
+    {
+        // RFC 5737 reserved-for-documentation IP; won't route anywhere. Pick opc.tcp:// so
+        // endpoint selection hits the transport-layer connection rather than a DNS lookup.
+        var opts = new OpcUaClientDriverOptions
+        {
+            // Port 1 on loopback is effectively guaranteed to be closed — the OS responds
+            // with TCP RST immediately instead of hanging on connect, which keeps the
+            // unreachable-host tests snappy. Don't use an RFC 5737 reserved IP; those get
+            // routed to a black-hole + time out only after the SDK's internal retry/backoff
+            // fully elapses (~60s even with Options.Timeout=500ms).
+            EndpointUrl = "opc.tcp://127.0.0.1:1",
+            Timeout = TimeSpan.FromMilliseconds(500),
+            AutoAcceptCertificates = true, // dev-mode to bypass cert validation in the test
+        };
+        using var drv = new OpcUaClientDriver(opts, "opcua-unreach");
+
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+
+        var health = drv.GetHealth();
+        health.State.ShouldBe(DriverState.Faulted);
+        health.LastError.ShouldNotBeNull();
+    }
+
+    [Fact]
+    public async Task Reinitialize_against_unreachable_endpoint_re_throws()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            // Port 1 on loopback is effectively guaranteed to be closed — the OS responds
+            // with TCP RST immediately instead of hanging on connect, which keeps the
+            // unreachable-host tests snappy. Don't use an RFC 5737 reserved IP; those get
+            // routed to a black-hole + time out only after the SDK's internal retry/backoff
+            // fully elapses (~60s even with Options.Timeout=500ms).
+            EndpointUrl = "opc.tcp://127.0.0.1:1",
+            Timeout = TimeSpan.FromMilliseconds(500),
+            AutoAcceptCertificates = true,
+        };
+        using var drv = new OpcUaClientDriver(opts, "opcua-reinit");
+
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.ReinitializeAsync("{}", TestContext.Current.CancellationToken));
+    }
+}

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

@@ -0,0 +1,32 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Unit tests for the IReadable/IWritable surface that don't need a live remote OPC UA
+///     server. Wire-level round-trips against a local in-process server fixture land in a
+///     follow-up PR once we have one scaffolded.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientReadWriteTests
+{
+    [Fact]
+    public async Task ReadAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.ReadAsync(["ns=2;s=Demo"], TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task WriteAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.WriteAsync(
+                [new WriteRequest("ns=2;s=Demo", 42)],
+                TestContext.Current.CancellationToken));
+    }
+}

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

@@ -0,0 +1,50 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Scaffold tests for <c>ISubscribable</c> + <c>IHostConnectivityProbe</c> that don't
+///     need a live remote server. Live-session tests (subscribe/unsubscribe round-trip,
+///     keep-alive transitions) land in a follow-up PR once the in-process OPC UA server
+///     fixture is scaffolded.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientSubscribeAndProbeTests
+{
+    [Fact]
+    public async Task SubscribeAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-sub-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.SubscribeAsync(["ns=2;s=Demo"], TimeSpan.FromMilliseconds(100), TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task UnsubscribeAsync_with_unknown_handle_is_noop()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-sub-unknown");
+        // UnsubscribeAsync returns cleanly for handles it doesn't recognise — protects against
+        // the caller's race with server-side cleanup after a session drop.
+        await drv.UnsubscribeAsync(new FakeHandle(), TestContext.Current.CancellationToken);
+    }
+
+    [Fact]
+    public void GetHostStatuses_returns_endpoint_url_row_pre_init()
+    {
+        using var drv = new OpcUaClientDriver(
+            new OpcUaClientDriverOptions { EndpointUrl = "opc.tcp://plc.example:4840" },
+            "opcua-hosts");
+        var rows = drv.GetHostStatuses();
+        rows.Count.ShouldBe(1);
+        rows[0].HostName.ShouldBe("opc.tcp://plc.example:4840",
+            "host identity mirrors the endpoint URL so the Admin /hosts dashboard can link back to the remote server");
+        rows[0].State.ShouldBe(HostState.Unknown);
+    }
+
+    private sealed class FakeHandle : ISubscriptionHandle
+    {
+        public string DiagnosticId => "fake";
+    }
+}

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

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7AddressParserTests.cs

@@ -0,0 +1,119 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class S7AddressParserTests
+{
+    // --- Data blocks ---
+
+    [Theory]
+    [InlineData("DB1.DBX0.0", 1, S7Size.Bit, 0, 0)]
+    [InlineData("DB1.DBX0.7", 1, S7Size.Bit, 0, 7)]
+    [InlineData("DB1.DBB0", 1, S7Size.Byte, 0, 0)]
+    [InlineData("DB1.DBW0", 1, S7Size.Word, 0, 0)]
+    [InlineData("DB1.DBD4", 1, S7Size.DWord, 4, 0)]
+    [InlineData("DB10.DBW100", 10, S7Size.Word, 100, 0)]
+    [InlineData("DB1.DBX15.3", 1, S7Size.Bit, 15, 3)]
+    public void Parse_data_block_addresses(string input, int db, S7Size size, int byteOff, int bitOff)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(S7Area.DataBlock);
+        r.DbNumber.ShouldBe(db);
+        r.Size.ShouldBe(size);
+        r.ByteOffset.ShouldBe(byteOff);
+        r.BitOffset.ShouldBe(bitOff);
+    }
+
+    [Theory]
+    [InlineData("db1.dbw0", 1, S7Size.Word, 0)]
+    [InlineData(" DB1.DBW0 ", 1, S7Size.Word, 0)]       // trim whitespace
+    public void Parse_is_case_insensitive_and_trims(string input, int db, S7Size size, int off)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(S7Area.DataBlock);
+        r.DbNumber.ShouldBe(db);
+        r.Size.ShouldBe(size);
+        r.ByteOffset.ShouldBe(off);
+    }
+
+    // --- M / I / Q ---
+
+    [Theory]
+    [InlineData("MB0", S7Area.Memory, S7Size.Byte, 0, 0)]
+    [InlineData("MW10", S7Area.Memory, S7Size.Word, 10, 0)]
+    [InlineData("MD4", S7Area.Memory, S7Size.DWord, 4, 0)]
+    [InlineData("M0.0", S7Area.Memory, S7Size.Bit, 0, 0)]
+    [InlineData("M255.7", S7Area.Memory, S7Size.Bit, 255, 7)]
+    [InlineData("IB0", S7Area.Input, S7Size.Byte, 0, 0)]
+    [InlineData("IW0", S7Area.Input, S7Size.Word, 0, 0)]
+    [InlineData("I0.0", S7Area.Input, S7Size.Bit, 0, 0)]
+    [InlineData("QB0", S7Area.Output, S7Size.Byte, 0, 0)]
+    [InlineData("QW0", S7Area.Output, S7Size.Word, 0, 0)]
+    [InlineData("Q0.0", S7Area.Output, S7Size.Bit, 0, 0)]
+    [InlineData("QD4", S7Area.Output, S7Size.DWord, 4, 0)]
+    public void Parse_MIQ_addresses(string input, S7Area area, S7Size size, int byteOff, int bitOff)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(area);
+        r.DbNumber.ShouldBe(0);
+        r.Size.ShouldBe(size);
+        r.ByteOffset.ShouldBe(byteOff);
+        r.BitOffset.ShouldBe(bitOff);
+    }
+
+    // --- Timers / counters ---
+
+    [Theory]
+    [InlineData("T0", S7Area.Timer, 0)]
+    [InlineData("T15", S7Area.Timer, 15)]
+    [InlineData("C0", S7Area.Counter, 0)]
+    [InlineData("C10", S7Area.Counter, 10)]
+    public void Parse_timer_and_counter(string input, S7Area area, int number)
+    {
+        var r = S7AddressParser.Parse(input);
+        r.Area.ShouldBe(area);
+        r.ByteOffset.ShouldBe(number);
+        r.Size.ShouldBe(S7Size.Word, "timers + counters are 16-bit opaque");
+    }
+
+    // --- Reject garbage ---
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData("Z0")]                 // unknown area
+    [InlineData("DB")]                 // no number or tail
+    [InlineData("DB1")]                // no tail
+    [InlineData("DB1.")]               // empty tail
+    [InlineData("DB1.DBX0")]           // bit size without .bit
+    [InlineData("DB1.DBX0.8")]         // bit 8 out of range
+    [InlineData("DB1.DBW0.0")]         // word with bit suffix
+    [InlineData("DB0.DBW0")]           // db 0 invalid
+    [InlineData("DBA.DBW0")]           // non-numeric db
+    [InlineData("DB1.DBQ0")]           // invalid size letter
+    [InlineData("M")]                  // no offset
+    [InlineData("M0")]                 // bit access needs .bit
+    [InlineData("M0.8")]               // bit 8
+    [InlineData("MB-1")]               // negative offset
+    [InlineData("MW")]                 // no offset digits
+    [InlineData("TA")]                 // non-numeric timer
+    public void Parse_rejects_invalid(string bad)
+        => Should.Throw<FormatException>(() => S7AddressParser.Parse(bad));
+
+    [Fact]
+    public void TryParse_returns_false_for_garbage_without_throwing()
+    {
+        S7AddressParser.TryParse("not-an-address", out var r).ShouldBeFalse();
+        r.ShouldBe(default);
+    }
+
+    [Fact]
+    public void TryParse_returns_true_for_valid_address()
+    {
+        S7AddressParser.TryParse("DB1.DBW0", out var r).ShouldBeTrue();
+        r.DbNumber.ShouldBe(1);
+        r.Size.ShouldBe(S7Size.Word);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7DiscoveryAndSubscribeTests.cs

@@ -0,0 +1,117 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+/// <summary>
+///     Shape tests for <see cref="S7Driver"/>'s <see cref="ITagDiscovery"/>,
+///     <see cref="ISubscribable"/>, and <see cref="IHostConnectivityProbe"/> surfaces that
+///     don't need a live PLC. Wire-level polling round-trips and probe transitions land in a
+///     follow-up PR once we have a mock S7 server.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class S7DiscoveryAndSubscribeTests
+{
+    private sealed class RecordingAddressSpaceBuilder : IAddressSpaceBuilder
+    {
+        public readonly List<string> Folders = new();
+        public readonly List<(string Name, DriverAttributeInfo Attr)> Variables = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        {
+            Folders.Add(browseName);
+            return this;
+        }
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo attributeInfo)
+        {
+            Variables.Add((browseName, attributeInfo));
+            return new StubHandle();
+        }
+        public void AddProperty(string browseName, DriverDataType dataType, object? value) { }
+        public void AttachAlarmCondition(IVariableHandle sourceVariable, string alarmName, DriverAttributeInfo alarmInfo) { }
+
+        private sealed class StubHandle : IVariableHandle
+        {
+            public string FullReference => "stub";
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info)
+                => throw new NotImplementedException("S7 driver never calls this — no alarm surfacing");
+        }
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_projects_every_tag_into_the_address_space()
+    {
+        var opts = new S7DriverOptions
+        {
+            Host = "192.0.2.1",
+            Tags =
+            [
+                new("TempSetpoint", "DB1.DBW0", S7DataType.Int16, Writable: true),
+                new("FaultBit", "M0.0", S7DataType.Bool, Writable: false),
+                new("PIDOutput", "DB5.DBD12", S7DataType.Float32, Writable: true),
+            ],
+        };
+        using var drv = new S7Driver(opts, "s7-disco");
+
+        var builder = new RecordingAddressSpaceBuilder();
+        await drv.DiscoverAsync(builder, TestContext.Current.CancellationToken);
+
+        builder.Folders.ShouldContain("S7");
+        builder.Variables.Count.ShouldBe(3);
+        builder.Variables[0].Name.ShouldBe("TempSetpoint");
+        builder.Variables[0].Attr.SecurityClass.ShouldBe(SecurityClassification.Operate, "writable tags get Operate security class");
+        builder.Variables[1].Attr.SecurityClass.ShouldBe(SecurityClassification.ViewOnly, "read-only tags get ViewOnly");
+        builder.Variables[2].Attr.DriverDataType.ShouldBe(DriverDataType.Float32);
+    }
+
+    [Fact]
+    public void GetHostStatuses_returns_one_row_with_host_port_identity_pre_init()
+    {
+        var opts = new S7DriverOptions { Host = "plc1.internal", Port = 102 };
+        using var drv = new S7Driver(opts, "s7-host");
+
+        var rows = drv.GetHostStatuses();
+        rows.Count.ShouldBe(1);
+        rows[0].HostName.ShouldBe("plc1.internal:102");
+        rows[0].State.ShouldBe(HostState.Unknown, "pre-init / pre-probe state is Unknown");
+    }
+
+    [Fact]
+    public async Task SubscribeAsync_returns_unique_handles_and_UnsubscribeAsync_accepts_them()
+    {
+        var opts = new S7DriverOptions { Host = "192.0.2.1" };
+        using var drv = new S7Driver(opts, "s7-sub");
+
+        // SubscribeAsync does not itself call ReadAsync (the poll task does), so this works
+        // even though the driver isn't initialized. The poll task catches the resulting
+        // InvalidOperationException and the loop quietly continues — same pattern as the
+        // Modbus driver's poll loop tolerating transient transport failures.
+        var h1 = await drv.SubscribeAsync(["T1"], TimeSpan.FromMilliseconds(200), TestContext.Current.CancellationToken);
+        var h2 = await drv.SubscribeAsync(["T2"], TimeSpan.FromMilliseconds(200), TestContext.Current.CancellationToken);
+
+        h1.DiagnosticId.ShouldStartWith("s7-sub-");
+        h2.DiagnosticId.ShouldStartWith("s7-sub-");
+        h1.DiagnosticId.ShouldNotBe(h2.DiagnosticId);
+
+        await drv.UnsubscribeAsync(h1, TestContext.Current.CancellationToken);
+        await drv.UnsubscribeAsync(h2, TestContext.Current.CancellationToken);
+        // UnsubscribeAsync with an unknown handle must be a no-op, not throw.
+        await drv.UnsubscribeAsync(h1, TestContext.Current.CancellationToken);
+    }
+
+    [Fact]
+    public async Task Subscribe_publishing_interval_is_floored_at_100ms()
+    {
+        var opts = new S7DriverOptions { Host = "192.0.2.1", Probe = new S7ProbeOptions { Enabled = false } };
+        using var drv = new S7Driver(opts, "s7-floor");
+
+        // 50 ms requested — the floor protects the S7 CPU from sub-scan polling that would
+        // just queue wire-side. Test that the subscription is accepted (the floor is applied
+        // internally; the floor value isn't exposed, so we're really just asserting that the
+        // driver doesn't reject small intervals).
+        var h = await drv.SubscribeAsync(["T"], TimeSpan.FromMilliseconds(50), TestContext.Current.CancellationToken);
+        h.ShouldNotBeNull();
+        await drv.UnsubscribeAsync(h, TestContext.Current.CancellationToken);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7DriverReadWriteTests.cs

@@ -0,0 +1,54 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+/// <summary>
+///     Unit tests for <see cref="S7Driver"/>'s <c>IReadable</c>/<c>IWritable</c> surface
+///     that don't require a live PLC — covers error paths (not-initialized, unknown tag,
+///     read-only write rejection, unsupported data types). Wire-level round-trip tests
+///     against a live S7 or a mock-server land in a follow-up PR since S7.Net doesn't ship
+///     an in-process fake and an adequate mock is non-trivial.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class S7DriverReadWriteTests
+{
+    [Fact]
+    public async Task Initialize_rejects_invalid_tag_address_and_fails_fast()
+    {
+        // Bad address at init time must throw; the alternative (deferring the parse to the
+        // first read) would surface the config bug as BadInternalError on every subsequent
+        // Read which is impossible for an operator to diagnose from the OPC UA client.
+        var opts = new S7DriverOptions
+        {
+            Host = "192.0.2.1", // reserved — will never complete TCP handshake
+            Timeout = TimeSpan.FromMilliseconds(250),
+            Tags = [new S7TagDefinition("BadTag", "NOT-AN-S7-ADDRESS", S7DataType.Int16)],
+        };
+        using var drv = new S7Driver(opts, "s7-bad-tag");
+
+        // Either the TCP connect fails first (Exception) or the parser fails (FormatException)
+        // — both are acceptable since both are init-time fail-fast. What matters is that we
+        // don't return a "healthy" driver with a latent bad tag.
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task ReadAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new S7Driver(new S7DriverOptions { Host = "192.0.2.1" }, "s7-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.ReadAsync(["Any"], TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task WriteAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new S7Driver(new S7DriverOptions { Host = "192.0.2.1" }, "s7-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.WriteAsync(
+                [new(FullReference: "Any", Value: (short)0)],
+                TestContext.Current.CancellationToken));
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7DriverScaffoldTests.cs

@@ -0,0 +1,66 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;
+
+/// <summary>
+///     Scaffold-level tests that don't need a live S7 PLC — exercise driver lifecycle shape,
+///     default option values, and failure-mode transitions. PR 64 adds IReadable/IWritable
+///     tests against a mock-server, PR 65 adds discovery + subscribe.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class S7DriverScaffoldTests
+{
+    [Fact]
+    public void Default_options_target_S7_1500_slot_0_on_port_102()
+    {
+        var opts = new S7DriverOptions();
+        opts.Port.ShouldBe(102, "ISO-on-TCP is always 102 for S7; documented in driver-specs.md §5");
+        opts.CpuType.ShouldBe(global::S7.Net.CpuType.S71500);
+        opts.Rack.ShouldBe((short)0);
+        opts.Slot.ShouldBe((short)0, "S7-1200/1500 onboard PN ports are slot 0 by convention");
+    }
+
+    [Fact]
+    public void Default_probe_interval_is_reasonable_for_S7_scan_cycle()
+    {
+        // S7 PLCs scan 2-10 ms but comms mailbox typically processed once per scan.
+        // 5 s default probe is lightweight — ~0.001% of comms budget.
+        new S7ProbeOptions().Interval.ShouldBe(TimeSpan.FromSeconds(5));
+    }
+
+    [Fact]
+    public void Tag_definition_defaults_to_writable_with_S7_max_string_length()
+    {
+        var tag = new S7TagDefinition("T", "DB1.DBW0", S7DataType.Int16);
+        tag.Writable.ShouldBeTrue();
+        tag.StringLength.ShouldBe(254, "S7 STRING type max length is 254 chars");
+    }
+
+    [Fact]
+    public void Driver_instance_reports_type_and_id_before_connect()
+    {
+        var opts = new S7DriverOptions { Host = "127.0.0.1" };
+        using var drv = new S7Driver(opts, "s7-test");
+        drv.DriverType.ShouldBe("S7");
+        drv.DriverInstanceId.ShouldBe("s7-test");
+        drv.GetHealth().State.ShouldBe(DriverState.Unknown, "health starts Unknown until InitializeAsync runs");
+    }
+
+    [Fact]
+    public async Task Initialize_against_unreachable_host_transitions_to_Faulted_and_throws()
+    {
+        // Pick an RFC 5737 reserved-for-documentation IP so the connect attempt fails fast
+        // (no DNS mismatch, no accidental traffic to a real PLC).
+        var opts = new S7DriverOptions { Host = "192.0.2.1", Timeout = TimeSpan.FromMilliseconds(250) };
+        using var drv = new S7Driver(opts, "s7-unreach");
+
+        await Should.ThrowAsync<Exception>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+
+        var health = drv.GetHealth();
+        health.State.ShouldBe(DriverState.Faulted, "unreachable host must flip the driver to Faulted so operators see it");
+        health.LastError.ShouldNotBeNull();
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/ZB.MOM.WW.OtOpcUa.Driver.S7.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.S7.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.S7\ZB.MOM.WW.OtOpcUa.Driver.S7.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>