Phase 6 — Draft 4 implementation plans covering v2 unimplemented features + adversarial review + adjustments. After drivers were paused per user direction, audited the v2 plan for features documented-but-unshipped and identified four coherent tracks that had no implementation plan at all. Each plan follows the docs/v2/implementation/phase-*.md template (DRAFT status, branch name, Stream A-E task breakdown, Compliance Checks, Risks, Completion Checklist). docs/v2/implementation/phase-6-1-resilience-and-observability.md (243 lines) covers Polly resilience pipelines wired to every capability interface, Tier A/B/C runtime enforcement (memory watchdog generalized beyond Galaxy, scheduled recycle per decision #67, wedge detection), health endpoints on :4841, structured Serilog with correlation IDs, LiteDB local-cache fallback per decision #36. phase-6-2-authorization-runtime.md (145 lines) wires ACL enforcement on every OPC UA Read/Write/Subscribe/Call path + LDAP-group-to-admin-role grants per decisions #105 and #129 -- runtime permission-trie evaluator over the 6-level Cluster/Namespace/UnsArea/UnsLine/Equipment/Tag hierarchy, per-session cache invalidated on generation-apply + LDAP-cache expiry. phase-6-3-redundancy-runtime.md (165 lines) lands the non-transparent warm/hot redundancy runtime per decisions #79-85: dynamic ServiceLevel node, ServerUriArray peer broadcast, mid-apply dip via sp_PublishGeneration hook, operator-driven role transition (no auto-election -- plan remains explicit about what's out of scope). phase-6-4-admin-ui-completion.md (178 lines) closes Phase 1 Stream E completion-checklist items that never landed: UNS drag-reorder + impact preview, Equipment CSV import, 5-identifier search, draft-diff viewer enhancements, OPC 40010 _base Identification field exposure per decisions #138-139. Each plan then got a Codex adversarial-review pass (codex mcp tool, read-only sandbox, synchronous). Reviews explicitly targeted decision-log conflicts, API-shape assumptions, unbounded blast radius, under-specified state transitions, and testing holes. Appended 'Adversarial Review — 2026-04-19' section to each plan with numbered findings (severity / finding / why-it-matters / adjustment accepted). Review surfaced real substantive issues that the initial drafts glossed over: Phase 6.1 auto-retry conflicting with decisions #44-45 no-auto-write-retry rule; Phase 6.1 per-driver-instance pipeline breaking decision #35's per-device isolation; Phase 6.1 recycle/watchdog at Tier A/B breaching decisions #73-74 Tier-C-only constraint; Phase 6.2 conflating control-plane LdapGroupRoleMapping with data-plane ACL grants; Phase 6.2 missing Browse enforcement entirely; Phase 6.2 subscription re-authorization policy unresolved between create-time-only and per-publish; Phase 6.3 ServiceLevel=0 colliding with OPC UA Part 5 Maintenance semantics; Phase 6.3 ServerUriArray excluding self (spec-bug); Phase 6.3 apply-window counter race on cancellation; Phase 6.3 client cutover for Kepware/Aveva OI Gateway is unverified hearsay; Phase 6.4 stale UNS impact preview overwriting concurrent draft edits; Phase 6.4 identifier contract drifting from admin-ui.md canonical set (ZTag/MachineCode/SAPID/EquipmentId/EquipmentUuid, not ZTag/SAPID/UniqueId/Alias1/Alias2); Phase 6.4 CSV import atomicity internally contradictory (single txn vs chunked inserts); Phase 6.4 OPC 40010 field list not matching decision #139. Every finding has an adjustment in the plan doc -- plans are meant to be executable from the next session with the critique already baked in rather than a clean draft that would run into the same issues at implementation time. Codex thread IDs cited in each plan's review section for reproducibility. Pure documentation PR -- no code changes. Plans are DRAFT status; each becomes its own implementation phase with its own entry-gate + exit-gate when business prioritizes.

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"/>

_p54.json

@@ -0,0 +1 @@
+{"title":"Phase 3 PR 54 -- Siemens S7 Modbus TCP quirks research doc","body":"## Summary\n\nAdds `docs/v2/s7.md` (485 lines) covering Siemens SIMATIC S7 family Modbus TCP behavior. Mirrors the `docs/v2/dl205.md` template for future per-quirk implementation PRs.\n\n## Key findings for the implementation track\n\n- **No fixed memory map** — every S7 Modbus server is user-wired via `MB_SERVER`/`MODBUSCP`/`MODBUSPN` library blocks. Driver must accept per-site config, not assume a vendor layout.\n- **MB_SERVER requires non-optimized DBs** (STATUS `0x8383` if optimized). Most common field bug.\n- **Word order default = ABCD** (opposite of DL260). Driver's S7 profile default must be `ByteOrder.BigEndian`, not `WordSwap`.\n- **One port per MB_SERVER instance** — multi-client requires parallel FBs on 503/504/… Most clients assume port 502 multiplexes (wrong on S7).\n- **CP 343-1 Lean is server-only**, requires the `2XV9450-1MB00` license.\n- **FC20/21/22/23/43 all return Illegal Function** on every S7 variant — driver must not attempt FC23 bulk-read optimization for S7.\n- **STOP-mode behavior non-deterministic** across firmware bands — treat both read/write STOP-mode responses as unavailable.\n\nTwo items flagged as unconfirmed rumour (V2.0+ float byte-order claim, STOP-mode caching location).\n\nNo code, no tests — implementation lands in PRs 56+.\n\n## Test plan\n- [x] Doc renders as markdown\n- [x] 31 citations present\n- [x] Section structure matches dl205.md template","head":"phase-3-pr54-s7-research-doc","base":"v2"}

_p55.json

@@ -0,0 +1 @@
+{"title":"Phase 3 PR 55 -- Mitsubishi MELSEC Modbus TCP quirks research doc","body":"## Summary\n\nAdds `docs/v2/mitsubishi.md` (451 lines) covering MELSEC Q/L/iQ-R/iQ-F/FX3U Modbus TCP behavior. Mirrors `docs/v2/dl205.md` template for per-quirk implementation PRs.\n\n## Key findings for the implementation track\n\n- **Module naming trap** — `QJ71MB91` is SERIAL RTU, not TCP. TCP module is `QJ71MT91`. Surface clearly in driver docs.\n- **No canonical mapping** — per-site 'Modbus Device Assignment Parameter' block (up to 16 entries). Treat mapping as runtime config.\n- **X/Y hex vs octal depends on family** — Q/L/iQ-R use HEX (X20 = decimal 32); FX/iQ-F use OCTAL (X20 = decimal 16). Helper must take a family selector.\n- **Word order CDAB default** across all MELSEC families (opposite of Siemens S7). Driver Mitsubishi profile default: `ByteOrder.WordSwap`.\n- **D-registers binary by default** (opposite of DL205's BCD default). Caller opts in to `Bcd16`/`Bcd32` when ladder uses BCD.\n- **FX5U needs firmware ≥ 1.060** for Modbus TCP server — older is client-only.\n- **FX3U-ENET vs FX3U-ENET-P502 vs FX3U-ENET-ADP** — only the middle one binds port 502; the last has no Modbus at all. Common operator mis-purchase.\n- **QJ71MT91 does NOT support FC22 / FC23** — iQ-R / iQ-F do. Bulk-read optimization must gate on capability.\n- **STOP-mode writes configurable** on Q/L/iQ-R/iQ-F (default accept), always rejected on FX3U-ENET.\n\nThree unconfirmed rumours flagged separately.\n\nNo code, no tests — implementation lands in PRs 58+.\n\n## Test plan\n- [x] Doc renders as markdown\n- [x] 17 citations present\n- [x] Per-model test naming matrix included (`Mitsubishi_QJ71MT91_*`, `Mitsubishi_FX5U_*`, `Mitsubishi_FX3U_ENET_*`, shared `Mitsubishi_Common_*`)","head":"phase-3-pr55-mitsubishi-research-doc","base":"v2"}

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.

docs/v2/implementation/phase-6-1-resilience-and-observability.md

@@ -0,0 +1,137 @@
+# Phase 6.1 — Resilience & Observability Runtime
+
+> **Status**: DRAFT — implementation plan for a cross-cutting phase that was never formalised. The v2 `plan.md` specifies Polly, Tier A/B/C protections, structured logging, and local-cache fallback by decision; none are wired end-to-end.
+>
+> **Branch**: `v2/phase-6-1-resilience-observability`
+> **Estimated duration**: 3 weeks
+> **Predecessor**: Phase 5 (drivers) — partial; S7 + OPC UA Client shipped, AB/TwinCAT/FOCAS paused
+> **Successor**: Phase 6.2 (Authorization runtime)
+
+## Phase Objective
+
+Land the cross-cutting runtime protections + operability features that `plan.md` + `driver-stability.md` specify by decision but that no driver-phase actually wires. End-state: every driver goes through the same Polly resilience layer, health endpoints render the live driver fleet, structured logs carry per-request correlation IDs, and the config substrate survives a central DB outage via a LiteDB local cache.
+
+Closes these gaps flagged in the 2026-04-19 audit:
+
+1. Polly v8 resilience pipelines wired to every `IDriver` capability (no-op per-driver today; Galaxy has a hand-rolled `CircuitBreaker` only).
+2. Tier A/B/C enforcement at runtime — `driver-stability.md` §2–4 and decisions #63–73 define memory watchdog, bounded queues, scheduled recycle, wedge detection; `MemoryWatchdog` exists only inside `Driver.Galaxy.Host`.
+3. Health endpoints (`/healthz`, `/readyz`) on `OtOpcUa.Server`.
+4. Structured Serilog with per-request correlation IDs (driver instance, OPC UA session, IPC call).
+5. LiteDB local cache + Polly retry + fallback on central-DB outage (decision #36).
+
+## Scope — What Changes
+
+| Concern | Change |
+|---------|--------|
+| `Core` → new `Core.Resilience` sub-namespace | Shared Polly pipeline builder (`DriverResiliencePipelines`), per-capability policy (Read / Write / Subscribe / HistoryRead / Discover / Probe / Alarm). One pipeline per driver instance; driver-options decide tuning. |
+| Every `IDriver*` consumer in the server | Wrap capability calls in the shared pipeline. Policy composition order: timeout → retry (with jitter, bounded by capability-specific `MaxRetries`) → circuit breaker (per driver instance, opens on N consecutive failures) → bulkhead (ceiling on in-flight requests per driver). |
+| `Core` → new `Core.Stability` sub-namespace | Generalise `MemoryWatchdog` (`Driver.Galaxy.Host`) into `DriverMemoryWatchdog` consuming `IDriver.GetMemoryFootprint()`. Add `ScheduledRecycleScheduler` (decision #67) for weekly/time-of-day recycle. Add `WedgeDetector` that flips a driver to Faulted when no successful Read in N × PublishingInterval. |
+| `DriverTypeRegistry` | Each driver type registers its `DriverTier` {A, B, C}. Tier C drivers must also advertise their out-of-process topology; the registry enforces invariants (Tier C has a `Proxy` + `Host` pair). |
+| `OtOpcUa.Server` → new Minimal API endpoints | `/healthz` (liveness — process alive + config DB reachable or LiteDB cache warm), `/readyz` (readiness — every driver instance reports `DriverState.Healthy`). JSON bodies cite individual driver health per instance. |
+| Serilog configuration | Centralize enrichers in `OtOpcUa.Server/Observability/LogContextEnricher.cs`. Every driver call runs inside a `LogContext.PushProperty` scope with {DriverInstanceId, DriverType, CapabilityName, CorrelationId (UA RequestHandle or internal GUID)}. Sink config stays rolling-file per CLAUDE.md; JSON-formatted output added alongside plain-text so SIEM ingestion works. |
+| `Configuration` project | Add `LiteDbConfigCache` adapter. Wrap EF Core queries in a Polly pipeline: timeout (2 s) → retry (3×, jittered) → fallback-to-cache. Cache refresh on successful DB query + after `sp_PublishGeneration`. Cache lives at `%ProgramData%/OtOpcUa/config-cache/<cluster-id>.db` per node. |
+| `DriverHostStatus` entity | Extend to carry `LastCircuitBreakerOpenUtc`, `ConsecutiveFailures`, `CurrentBulkheadDepth`, `LastRecycleUtc`. Admin `/hosts` page reads these. |
+
+## Scope — What Does NOT Change
+
+| Item | Reason |
+|------|--------|
+| Driver wire protocols | Resilience is a server-side wrapper; individual drivers don't see Polly. Their existing retry logic (ModbusTcpTransport reconnect, SessionReconnectHandler) stays in place as inner layers. |
+| Config DB schema | LiteDB cache is a read-only mirror; no new central tables except `DriverHostStatus` column additions. |
+| OPC UA wire behavior visible to clients | Health endpoints live on a separate HTTP port (4841 by convention); the OPC UA server on 4840 is unaffected. |
+| The four 2026-04-13 Galaxy stability findings | Already closed in Phase 2. Phase 6.1 *generalises* the pattern, doesn't re-fix Galaxy. |
+| Driver-layer SafeHandle usage | Existing Galaxy `SafeMxAccessHandle` + Modbus `TcpClient` disposal stay — they're driver-internal, not part of the cross-cutting layer. |
+
+## Entry Gate Checklist
+
+- [ ] Phases 0–5 exit gates cleared (or explicitly deferred with task reference)
+- [ ] `driver-stability.md` §2–4 re-read; decisions #63–73 + #34–36 re-skimmed
+- [ ] Polly v8 NuGet available (`Microsoft.Extensions.Resilience` + `Polly.Core`) — verify package restore before task breakdown
+- [ ] LiteDB 5.x NuGet confirmed MIT + actively maintained
+- [ ] Existing drivers catalogued: Galaxy.Proxy, Modbus, S7, OpcUaClient — confirm test counts baseline so the resilience layer doesn't regress any
+- [ ] Serilog configuration inventory: locate every `Log.ForContext` call site that will need `LogContext` rewrap
+- [ ] Admin `/hosts` page's current `DriverHostStatus` consumption reviewed so the schema extensions don't break it
+
+## Task Breakdown
+
+### Stream A — Resilience layer (1 week)
+
+1. **A.1** Add `Polly.Core` + `Microsoft.Extensions.Resilience` to `Core`. Build `DriverResiliencePipelineBuilder` that composes Timeout → Retry (exponential backoff + jitter, capability-specific max retries) → CircuitBreaker (consecutive-failure threshold; half-open probe) → Bulkhead (max in-flight per driver instance). Unit tests cover each policy in isolation + composed pipeline.
+2. **A.2** `DriverResilienceOptions` record bound from `DriverInstance.ResilienceConfig` JSON column (new nullable). Defaults encoded per-tier: Tier A (OPC UA Client, S7) — 3 retries, 2 s timeout, 5-failure breaker; Tier B (Modbus) — same except 4 s timeout; Tier C (Galaxy) — 1 retry (inner supervisor handles restart), 10 s timeout, circuit-breaker trips but doesn't kill the driver (the Proxy supervisor already handles that).
+3. **A.3** `DriverCapabilityInvoker<T>` wraps every `IDriver*` method call. Existing server-side dispatch (whatever currently calls `driver.ReadAsync`) routes through the invoker. Policy injection via DI.
+4. **A.4** Remove the hand-rolled `CircuitBreaker` + `Backoff` from `Driver.Galaxy.Proxy/Supervisor/` — replaced by the shared layer. Keep `HeartbeatMonitor` (different concern: IPC liveness, not data-path resilience).
+5. **A.5** Unit tests: per-policy, per-composition. Integration test: Modbus driver under a FlakeyTransport that fails 5×, succeeds on 6th; invoker surfaces the eventual success. Bench: no-op overhead < 1% under nominal load.
+
+### Stream B — Tier A/B/C stability runtime (1 week, can parallel with Stream A after A.1)
+
+1. **B.1** `Core.Abstractions` → `DriverTier` enum {A, B, C}. Extend `DriverTypeRegistry` to require `DriverTier` at registration. Existing driver types get their tier stamped (Galaxy = C, Modbus = B, S7 = B, OpcUaClient = A).
+2. **B.2** Generalise `DriverMemoryWatchdog` (lift from `Driver.Galaxy.Host/MemoryWatchdog.cs`). Tier-specific thresholds: A = 256 MB RSS soft / 512 MB hard, B = 512 MB soft / 1 GB hard, C = 1 GB soft / 2 GB hard (decision #70 hybrid multiplier + floor). Soft threshold → log + metric; hard threshold → mark driver Faulted + trigger recycle.
+3. **B.3** `ScheduledRecycleScheduler` (decision #67): each driver instance can opt-in to a weekly recycle at a configured cron. Recycle = `ShutdownAsync` → `InitializeAsync`. Tier C drivers get the Proxy-side recycle; Tier A/B recycle in-process.
+4. **B.4** `WedgeDetector`: polling thread per driver instance; if `LastSuccessfulRead` older than `WedgeThreshold` (default 5 × PublishingInterval, minimum 60 s) AND driver state is `Healthy`, flag as wedged → force `ReinitializeAsync`. Prevents silent dead-subscriptions.
+5. **B.5** Tests: watchdog unit tests drive synthetic allocation; scheduler uses a virtual clock; wedge detector tests use a fake IClock + driver stub.
+
+### Stream C — Health endpoints + structured logging (4 days)
+
+1. **C.1** `OtOpcUa.Server/Observability/HealthEndpoints.cs` — Minimal API on a second Kestrel binding (default `http://+:4841`). `/healthz` reports process uptime + config-DB reachability (or cache-warm). `/readyz` enumerates `DriverInstance` rows + reports each driver's `DriverHealth.State`; returns 503 if ANY driver is Faulted. JSON body per `docs/v2/acl-design.md` §"Operator Dashboards" shape.
+2. **C.2** `LogContextEnricher` installed at Serilog config time. Every driver-capability call site wraps its body in `using (LogContext.PushProperty("DriverInstanceId", id)) using (LogContext.PushProperty("CorrelationId", correlationId))`. Correlation IDs: reuse OPC UA `RequestHeader.RequestHandle` when in-flight; otherwise generate `Guid.NewGuid().ToString("N")[..12]`.
+3. **C.3** Add JSON-formatted Serilog sink alongside the existing rolling-file plain-text sink so SIEMs (Splunk, Datadog) can ingest without a regex parser. Sink switchable via `Serilog:WriteJson` appsetting.
+4. **C.4** Integration test: boot server, issue Modbus read, assert log line contains `DriverInstanceId` + `CorrelationId` structured fields.
+
+### Stream D — Config DB LiteDB fallback (1 week)
+
+1. **D.1** `LiteDbConfigCache` adapter. Wraps `ConfigurationDbContext` queries that are safe to serve stale (cluster membership, generation metadata, driver instance definitions, LDAP role mapping). Write-path queries (draft save, publish) bypass the cache and fail hard on DB outage.
+2. **D.2** Cache refresh strategy: refresh on every successful read (write-through-cache), full refresh after `sp_PublishGeneration` confirmation. Cache entries carry `CachedAtUtc`; served entries older than 24 h trigger a synthetic `Warning` log line so operators see stale data in effect.
+3. **D.3** Polly pipeline in `Configuration` project: EF Core query → retry 3× → fallback to cache. On fallback, driver state stays `Healthy` but a `UsingStaleConfig` flag on the cluster's health report flips true.
+4. **D.4** Tests: in-memory SQL Server failure injected via `TestContainers`-ish double; cache returns last-known values; Admin UI banners reflect `UsingStaleConfig`.
+
+### Stream E — Admin `/hosts` page refresh (3 days)
+
+1. **E.1** Extend `DriverHostStatus` schema with Stream A resilience columns. Generate EF migration.
+2. **E.2** `Admin/FleetStatusHub` SignalR hub pushes `LastCircuitBreakerOpenUtc` + `CurrentBulkheadDepth` + `LastRecycleUtc` on change.
+3. **E.3** `/hosts` Blazor page renders new columns; red badge if `ConsecutiveFailures > breakerThreshold / 2`.
+
+## Compliance Checks (run at exit gate)
+
+- [ ] **Polly coverage**: every `IDriver*` method call in the server dispatch layer routes through `DriverCapabilityInvoker`. Enforce via a Roslyn analyzer added to `Core.Abstractions` build (error on direct `IDriver.ReadAsync` calls outside the invoker).
+- [ ] **Tier registry**: every driver type registered in `DriverTypeRegistry` has a non-null `Tier`. Unit test walks the registry + asserts no gaps.
+- [ ] **Health contract**: `/healthz` + `/readyz` respond within 500 ms even with one driver Faulted.
+- [ ] **Structured log**: CI grep on `tests/` output asserts at least one log line contains `"DriverInstanceId"` + `"CorrelationId"` JSON fields.
+- [ ] **Cache fallback**: Integration test kills the SQL container mid-operation; driver health stays `Healthy`, `UsingStaleConfig` flips true.
+- [ ] No regression in existing test suites — `dotnet test ZB.MOM.WW.OtOpcUa.slnx` count equal-or-greater than pre-Phase-6.1 baseline.
+
+## Risks and Mitigations
+
+| Risk | Likelihood | Impact | Mitigation |
+|------|:----------:|:------:|------------|
+| Polly pipeline adds per-request latency on hot path | Medium | Medium | Benchmark Stream A.5 before merging; 1 % overhead budget; inline hot path short-circuits when retry count = 0 |
+| LiteDB cache diverges from central DB | Medium | High | Stale-data banner in Admin UI; `UsingStaleConfig` flag surfaced on `/readyz`; cache refresh on every successful DB round-trip; 24-hour synthetic warning |
+| Tier watchdog false-positive-kills a legitimate batch load | Low | High | Soft/hard threshold split; soft only logs; hard triggers recycle; thresholds configurable per-instance |
+| Wedge detector races with slow-but-healthy drivers | Medium | High | Minimum 60 s threshold; detector only activates if driver claims `Healthy`; add circuit-breaker feedback so rapid oscillation trips instead of thrashing |
+| Roslyn analyzer breaks external driver authors | Low | Medium | Release analyzer as warning-level initially; upgrade to error in Phase 6.1+1 after one release cycle |
+
+## Completion Checklist
+
+- [ ] Stream A: Polly shared pipeline + per-tier defaults + driver-capability invoker + tests
+- [ ] Stream B: Tier registry + generalised watchdog + scheduled recycle + wedge detector
+- [ ] Stream C: `/healthz` + `/readyz` + structured logging + JSON Serilog sink
+- [ ] Stream D: LiteDB cache + Polly fallback in Configuration
+- [ ] Stream E: Admin `/hosts` page refresh
+- [ ] Cross-cutting: `phase-6-1-compliance.ps1` exits 0; full solution `dotnet test` passes; exit-gate doc recorded
+
+## Adversarial Review — 2026-04-19 (Codex, thread `019da489-e317-7aa1-ab1f-6335e0be2447`)
+
+Plan substantially rewritten before implementation to address these findings. Each entry: severity · verdict · adjustment.
+
+1. **Crit · ACCEPT** — Auto-retry collides with decisions #44/#45 (no auto-write-retry; opt-in via `WriteIdempotent` + CAS). Pipeline now **capability-specific**: Read/HistoryRead/Discover/Probe/Alarm-subscribe all get retries; **Write does not** unless the tag metadata carries `WriteIdempotent=true`. New `WriteIdempotentAttribute` surfaces on `ModbusTagDefinition` / `S7TagDefinition` / etc.
+2. **Crit · ACCEPT** — "One pipeline per driver instance" breaks decision #35's per-device isolation. **Change**: pipeline key is `(DriverInstanceId, HostName)` not just `DriverInstanceId`. One dead PLC behind a multi-device Modbus driver no longer opens the breaker for healthy siblings.
+3. **Crit · ACCEPT** — Memory watchdog + scheduled recycle at Tier A/B breaches decisions #73/#74 (process-kill protections are Tier-C-only). **Change**: Stream B splits into two — `MemoryTracking` (all tiers, soft/hard thresholds log + surface to Admin `/hosts`; never kills) and `MemoryRecycle` (Tier C only, requires out-of-process topology). Tier A/B overrun paths escalate to Tier C via a future PR, not auto-kill.
+4. **High · ACCEPT** — Removing Galaxy's hand-rolled `CircuitBreaker` drops decision #68 host-supervision crash-loop protection. **Change**: keep `Driver.Galaxy.Proxy/Supervisor/CircuitBreaker.cs` + `Backoff.cs` — they guard the IPC *process* re-spawn, not the per-call data path. Data-path Polly is an orthogonal layer.
+5. **High · ACCEPT** — Roslyn analyzer targeting `IDriver` misses the hot paths (`IReadable.ReadAsync`, `IWritable.WriteAsync`, `ISubscribable.SubscribeAsync` etc.). **Change**: analyzer rule now matches every method on the capability interfaces; compliance doc enumerates the full call-site list.
+6. **High · ACCEPT** — `/healthz` + `/readyz` under-specified for degraded-running. **Change**: add a state-matrix sub-section explicitly covering `Unknown` (pre-init: `/readyz` 503), `Initializing` (503), `Healthy` (200), `Degraded` (200 with JSON body flagging the degraded driver; `/readyz` is OR across drivers), `Faulted` (503), plus cached-config-serving (`/healthz` returns 200 + `UsingStaleConfig: true` in JSON body).
+7. **High · ACCEPT** — `WedgeDetector` based on "no successful Read" false-fires on write-only subscriptions + idle systems. **Change**: wedge criteria now `(hasPendingWork AND noProgressIn > threshold)` where `hasPendingWork` comes from the Polly bulkhead depth + active MonitoredItem count. Idle driver stays Healthy.
+8. **High · ACCEPT** — LiteDB cache serving mixed-generation reads breaks publish atomicity. **Change**: cache is snapshot-per-generation. Each published generation writes a sealed snapshot into `<cache-root>/<cluster>/<generationId>.db`; reads serve the last-known-sealed generation and never mix. Central DB outage during a *publish* means that publish fails (write path doesn't use cache); reads continue from the prior sealed snapshot.
+9. **Med · ACCEPT** — `DriverHostStatus` schema conflates per-host connectivity with per-driver-instance resilience counters. **Change**: new `DriverInstanceResilienceStatus` table separate from `DriverHostStatus`. Admin `/hosts` joins both for display.
+10. **Med · ACCEPT** — Compliance says analyzer-error; risks say analyzer-warning. **Change**: phase 6.1 ships at **error** level (this phase is the gate); warning-mode option removed.
+11. **Med · ACCEPT** — Hardcoded per-tier MB bands ignore decision #70's `max(multiplier × baseline, baseline + floor)` formula with observed-baseline capture. **Change**: watchdog captures baseline at post-init plateau (median of first 5 min GetMemoryFootprint samples) + applies the hybrid formula. Tier constants now encode the multiplier + floor, not raw MB.
+12. **Med · ACCEPT** — Tests mostly cover happy path. **Change**: Stream A.5 adds negative tests for duplicate-write-replay-under-timeout; Stream B.5 adds false-wedge-on-idle-subscription + false-wedge-on-slow-historic-backfill; Stream D.4 adds mixed-generation cache test + corrupt-first-boot cache test.
+

docs/v2/implementation/phase-6-2-authorization-runtime.md

@@ -0,0 +1,126 @@
+# Phase 6.2 — Authorization Runtime (ACL + LDAP grants)
+
+> **Status**: DRAFT — the v2 `plan.md` decision #129 + `acl-design.md` specify a 6-level permission-trie evaluator with `NodePermissions` bitmask grants, but no runtime evaluator exists. ACL tables are schematized but unread by the data path.
+>
+> **Branch**: `v2/phase-6-2-authorization-runtime`
+> **Estimated duration**: 2.5 weeks
+> **Predecessor**: Phase 6.1 (Resilience & Observability) — reuses the Polly pipeline for ACL-cache refresh retries
+> **Successor**: Phase 6.3 (Redundancy)
+
+## Phase Objective
+
+Wire ACL enforcement on every OPC UA Read / Write / Subscribe / Call path + LDAP group → admin role grants that the v2 plan specified but never ran. End-state: a user's effective permissions resolve through a per-session permission-trie over the 6-level `Cluster / Namespace / UnsArea / UnsLine / Equipment / Tag` hierarchy, cached per session, invalidated on generation-apply + LDAP group expiry.
+
+Closes these gaps:
+
+1. **Data-path ACL enforcement** — `NodeAcl` table + `NodePermissions` flags shipped; `NodeAclService.cs` present as a CRUD surface; no code consults ACLs at `Read`/`Write` time. OPC UA server answers everything to everyone.
+2. **`LdapGroupRoleMapping` for cluster-scoped admin grants** — decision #105 shipped as the *design*; admin roles are hardcoded (`FleetAdmin` / `ConfigEditor` / `ReadOnly`) with no cluster-scoping and no LDAP-to-grant table. Decision #105 explicitly lifts this from v2.1 into v2.0.
+3. **Explicit Deny pathway** — deferred to v2.1 (decision #129 note). Phase 6.2 ships *grants only*; `Deny` stays out.
+4. **Admin UI ACL grant editor** — `AclsTab.razor` exists but edits the now-unused `NodeAcl` table; needs to wire to the runtime evaluator + the new `LdapGroupRoleMapping` table.
+
+## Scope — What Changes
+
+| Concern | Change |
+|---------|--------|
+| `Configuration` project | New entity `LdapGroupRoleMapping { Id, LdapGroup, Role, ClusterId? (nullable = system-wide), IsSystemWide, GeneratedAtUtc }`. Migration. Admin CRUD. |
+| `Core` → new `Core.Authorization` sub-namespace | `IPermissionEvaluator` interface; concrete `PermissionTrieEvaluator` implementation loads ACLs + LDAP mappings from Configuration, builds a trie keyed on the 6-level scope hierarchy, evaluates a `(UserClaim[], NodeId, NodePermissions)` → `bool` decision in O(depth × group-count). |
+| `Core.Authorization` cache | `PermissionTrieCache` — one trie per `(ClusterId, GenerationId)`. Rebuilt on `sp_PublishGeneration` confirmation; served from memory thereafter. Per-session evaluator keeps a reference to the current trie + user's LDAP groups. |
+| OPC UA server dispatch | `OtOpcUa.Server/OpcUa/DriverNodeManager.cs` Read/Write/HistoryRead/MonitoredItem-create paths call `PermissionEvaluator.Authorize(session.Identity, nodeId, NodePermissions.Read)` etc. before delegating to the driver. Unauthorized returns `BadUserAccessDenied` (0x80210000) — not a silent no-op per corrections-doc B1. |
+| `LdapAuthService` (existing) | On cookie-auth success, resolves the user's LDAP groups via `LdapGroupService.GetMemberships` + loads the matching `LdapGroupRoleMapping` rows → produces a role-claim list + cluster-scope claim list. Stored on the auth cookie. |
+| Admin UI `AclsTab.razor` | Repoint edits at the new `NodeAclService` API that writes through to the same table the evaluator reads. Add a "test this permission" probe that runs a dummy evaluator against a chosen `(user, nodeId, action)` so ops can sanity-check grants before publishing a draft. |
+| Admin UI new tab `RoleGrantsTab.razor` | CRUD over `LdapGroupRoleMapping`. Per-cluster + system-wide grants. FleetAdmin only. |
+| Audit log | Every Grant/Revoke/Publish on `LdapGroupRoleMapping` or `NodeAcl` writes an `AuditLog` row with old/new state + user. |
+
+## Scope — What Does NOT Change
+
+| Item | Reason |
+|------|--------|
+| OPC UA authn | Already done (PR 19 LDAP user identity + Basic256Sha256 profile). Phase 6.2 is authorization only. |
+| Explicit `Deny` grants | Decision #129 note explicitly defers to v2.1. Default-deny + additive grants only. |
+| Driver-side `SecurityClassification` metadata | Drivers keep reporting `Operate` / `ViewOnly` / etc. — the evaluator uses them as *part* of the decision but doesn't replace them. |
+| Galaxy namespace (SystemPlatform kind) | UNS levels don't apply; evaluator treats Galaxy nodes as `Cluster → Namespace → Tag` (skip UnsArea/UnsLine/Equipment). |
+
+## Entry Gate Checklist
+
+- [ ] Phase 6.1 merged (reuse `Core.Resilience` Polly pipeline for the ACL cache-refresh retries)
+- [ ] `acl-design.md` re-read in full
+- [ ] Decision log #105, #129, corrections-doc B1 re-skimmed
+- [ ] Existing `NodeAcl` + `NodePermissions` flag enum audited; confirm bitmask flags match `acl-design.md` table
+- [ ] Existing `LdapAuthService` group-resolution code path traced end-to-end — confirm it already queries group memberships (we only need the caller to consume the result)
+- [ ] Test DB scenarios catalogued: two clusters, three LDAP groups per cluster, mixed grant shapes; captured as seed-data fixtures
+
+## Task Breakdown
+
+### Stream A — `LdapGroupRoleMapping` table + migration (3 days)
+
+1. **A.1** Entity + EF Core migration. Columns per §Scope table. Unique constraint on `(LdapGroup, ClusterId)` with null-tolerant comparer for the system-wide case. Index on `LdapGroup` for the hot-path lookup on auth.
+2. **A.2** `ILdapGroupRoleMappingService` CRUD. Wrap in the Phase 6.1 Polly pipeline (timeout → retry → fallback-to-cache).
+3. **A.3** Seed-data migration: preserve the current hardcoded `FleetAdmin` / `ConfigEditor` / `ReadOnly` mappings by seeding rows for the existing LDAP groups the dev box uses (`cn=fleet-admin,…`, `cn=config-editor,…`, `cn=read-only,…`). Op no-op migration for existing deployments.
+
+### Stream B — Permission-trie evaluator (1 week)
+
+1. **B.1** `IPermissionEvaluator.Authorize(IEnumerable<Claim> identity, NodeId nodeId, NodePermissions needed)` — returns `bool`. Phase 6.2 returns only `true` / `false`; v2.1 can widen to `Allow`/`Deny`/`Indeterminate` if Deny lands.
+2. **B.2** `PermissionTrieBuilder` builds the trie from `NodeAcl` + `LdapGroupRoleMapping` joined to the current generation's `UnsArea` + `UnsLine` + `Equipment` + `Tag` tables. One trie per `(ClusterId, GenerationId)` so rollback doesn't smear permissions across generations.
+3. **B.3** Trie node structure: `{ Level: enum, ScopeId: Guid, AllowedPermissions: NodePermissions, ChildrenByLevel: Dictionary<Guid, TrieNode> }`. Evaluation walks from Cluster → Namespace → UnsArea → UnsLine → Equipment → Tag, ORing allowed permissions at each level. Additive semantics: a grant at Cluster level cascades to every descendant tag.
+4. **B.4** `PermissionTrieCache` service scoped as singleton; exposes `GetTrieAsync(ClusterId, ct)` that returns the current-generation trie. Invalidated on `sp_PublishGeneration` via an in-process event bus; also on TTL expiry (24 h safety net).
+5. **B.5** Per-session cached evaluator: OPC UA Session authentication produces `UserAuthorizationState { ClusterId, LdapGroups[], Trie }`; cached on the session until session close or generation-apply.
+6. **B.6** Unit tests: trie-walk theory covering (a) Cluster-level grant cascades to tags, (b) Equipment-level grant doesn't leak to sibling Equipment, (c) multi-group union, (d) no-grant → deny, (e) Galaxy nodes skip UnsArea/UnsLine levels.
+
+### Stream C — OPC UA server dispatch wiring (4 days)
+
+1. **C.1** `DriverNodeManager.Read` — consult evaluator before delegating to `IReadable`. Unauthorized nodes get `BadUserAccessDenied` per-attribute, not on the whole batch.
+2. **C.2** `DriverNodeManager.Write` — same. Evaluator needs `NodePermissions.WriteOperate` / `WriteTune` / `WriteConfigure` depending on driver-reported `SecurityClassification` of the attribute.
+3. **C.3** `DriverNodeManager.HistoryRead` — ACL checks `NodePermissions.Read` (history uses the same Read flag per `acl-design.md`).
+4. **C.4** `DriverNodeManager.CreateMonitoredItem` — denies unauthorized nodes at subscription create time, not after the first publish. Cleaner than silently omitting notifications.
+5. **C.5** Alarm actions (acknowledge / confirm / shelve) — checks `AlarmAck` / `AlarmConfirm` / `AlarmShelve` flags.
+6. **C.6** Integration tests: boot server with a seed trie, auth as three distinct users with different group memberships, assert read of one tag allowed + read of another denied + write denied where Read allowed.
+
+### Stream D — Admin UI refresh (4 days)
+
+1. **D.1** `RoleGrantsTab.razor` — FleetAdmin-gated CRUD on `LdapGroupRoleMapping`. Per-cluster dropdown + system-wide checkbox. Validation: LDAP group must exist in the dev LDAP (GLAuth) before saving — best-effort probe with graceful degradation.
+2. **D.2** `AclsTab.razor` rewrites its edit path to write through the new `NodeAclService`. Adds a "Probe this permission" row: choose `(LDAP group, node, action)` → shows Allow / Deny + the reason (which grant matched).
+3. **D.3** Draft-generation diff viewer now includes an ACL section: "X grants added, Y grants removed, Z grants changed."
+4. **D.4** SignalR notification: `PermissionTrieCache` invalidation on `sp_PublishGeneration` pushes to Admin UI so operators see "this clusters permissions were just updated" within 2 s.
+
+## Compliance Checks (run at exit gate)
+
+- [ ] **Data-path enforcement**: OPC UA Read against a NodeId the current user has no grant for returns `BadUserAccessDenied` with a ServiceResult, not Good with stale data. Verified by an integration test with a Basic256Sha256-secured session + a read-only LDAP identity.
+- [ ] **Trie invariants**: `PermissionTrieBuilder` is idempotent (building twice with identical inputs produces equal tries — override `Equals` to assert).
+- [ ] **Additive grants**: Cluster-level grant on User A means User A can read every tag in that cluster *without* needing any lower-level grant.
+- [ ] **Isolation between clusters**: a grant on Cluster 1 has zero effect on Cluster 2 for the same user.
+- [ ] **Galaxy path coverage**: ACL checks work on `Galaxy` folder nodes + tag nodes where the UNS levels are absent (the trie treats them as shallow `Cluster → Namespace → Tag`).
+- [ ] No regression in driver test counts.
+
+## Risks and Mitigations
+
+| Risk | Likelihood | Impact | Mitigation |
+|------|:----------:|:------:|------------|
+| ACL evaluator latency on per-read hot path | Medium | High | Trie lookup is O(depth) = O(6); session-cached UserAuthorizationState avoids per-Read trie rebuild; benchmark in Stream B.6 |
+| Trie cache stale after a rollback | Medium | High | `sp_PublishGeneration` + `sp_RollbackGeneration` both emit the invalidation event; trie keyed on `(ClusterId, GenerationId)` so rollback fetches the prior trie cleanly |
+| `BadUserAccessDenied` returns expose sensitive browse-name metadata | Low | Medium | Server returns only the status code + NodeId; no message leak per OPC UA Part 4 §7.34 guidance |
+| LdapGroupRoleMapping migration breaks existing deployments | Low | High | Seed-migration preserves the hardcoded groups' effective grants verbatim; smoke test exercises the post-migration fleet admin login |
+| Deny semantics accidentally ship (would break `acl-design.md` defer) | Low | Medium | `IPermissionEvaluator.Authorize` returns `bool` (not tri-state) through Phase 6.2; widening to `Allow`/`Deny`/`Indeterminate` is a v2.1 ticket |
+
+## Completion Checklist
+
+- [ ] Stream A: `LdapGroupRoleMapping` entity + migration + CRUD + seed
+- [ ] Stream B: evaluator + trie builder + cache + per-session state + unit tests
+- [ ] Stream C: OPC UA dispatch wiring on Read/Write/HistoryRead/Subscribe/Alarm paths
+- [ ] Stream D: Admin UI `RoleGrantsTab` + `AclsTab` refresh + SignalR invalidation
+- [ ] `phase-6-2-compliance.ps1` exits 0; exit-gate doc recorded
+
+## Adversarial Review — 2026-04-19 (Codex, thread `019da48d-0d2b-7171-aed2-fc05f1f39ca3`)
+
+1. **Crit · ACCEPT** — Trie must not conflate `LdapGroupRoleMapping` (control-plane admin claims per decision #105) with data-plane ACLs (decision #129). **Change**: `LdapGroupRoleMapping` is consumed only by the Admin UI role router. Data-plane trie reads `NodeAcl` rows joined against the session's **resolved LDAP groups**, never admin roles. Stream B.2 updated.
+2. **Crit · ACCEPT** — Cached `UserAuthorizationState` survives LDAP group changes because memberships only refresh at cookie-auth. Change: add `MembershipFreshnessInterval` (default 15 min); past that, next hot-path authz call forces group re-resolution (fail-closed if LDAP unreachable). Session-close-wins on config-rollback.
+3. **High · ACCEPT** — Node-local invalidation doesn't extend across redundant pair. **Change**: trie keyed on `(ClusterId, GenerationId)`; hot-path authz looks up `CurrentGenerationId` from the shared config DB (Polly-wrapped + sub-second cache). A Backup that read stale generation gets a mismatched trie → forces re-load. Implementation note added to Stream B.4.
+4. **High · ACCEPT** — Browse enforcement missing. **Change**: new Stream C.7 (`Browse + TranslateBrowsePathsToNodeIds` enforcement). Ancestor visibility implied when any descendant has a grant; denied ancestors filter from browse results per `acl-design.md` §Browse.
+5. **High · ACCEPT** — `HistoryRead` should use `NodePermissions.HistoryRead` bit, not `Read`. **Change**: Stream C.3 revised; separate unit test asserts `Read+no-HistoryRead` denies HistoryRead while allowing current-value reads.
+6. **High · ACCEPT** — Galaxy shallow-path (Cluster→Namespace→Tag) loses folder hierarchy authorization. **Change**: SystemPlatform namespaces use a `FolderSegment` scope-level between Namespace and Tag, populated from `Tag.FolderPath`; UNS-kind namespaces keep the 6-level hierarchy. Trie supports both via `ScopeKind` on each node.
+7. **High · ACCEPT** — Subscription re-authorization policy unresolved between create-time-only (fast, wrong on revoke) and per-publish (slow). **Change**: stamp each `MonitoredItem` with `(AuthGenerationId, MembershipVersion)`; re-evaluate on Publish only when either version changed. Revoked items drop to `BadUserAccessDenied` within one publish cycle.
+8. **Med · ACCEPT** — Mixed-authorization batch `Read` / `CreateMonitoredItems` service-result semantics underspecified. **Change**: Stream C.6 explicitly tests per-`ReadValueId` + per-`MonitoredItemCreateResult` denial in mixed batches; batch never collapses to a coarse failure.
+9. **Med · ACCEPT** — Missing surfaces: `Method.Call`, `HistoryUpdate`, event filter on subscriptions, subscription-transfer on reconnect, alarm-ack. **Change**: scope expanded — every OPC UA authorization surface enumerated in Stream C: Read, Write, HistoryRead, HistoryUpdate, CreateMonitoredItems, TransferSubscriptions, Call, Acknowledge/Confirm/Shelve, Browse, TranslateBrowsePathsToNodeIds.
+10. **Med · ACCEPT** — `bool` evaluator bakes in grant-only semantics; collides with v2.1 Deny. **Change**: internal model uses `AuthorizationDecision { Allow | NotGranted | Denied, IReadOnlyList<MatchedGrant> Provenance }`. Phase 6.2 maps `Denied` → never produced; UI + audit log use the full record so v2.1 Deny lands without API break.
+11. **Med · ACCEPT** — 6.1 cache fallback is availability-oriented; applying it to auth is correctness-dangerous. **Change**: auth-specific staleness budget `AuthCacheMaxStaleness` (default 5 min, not 24 h). Past that, hot-path evaluator fails closed on cached reads; all authorization calls return `NotGranted` until fresh data lands. Documented in risks + compliance.
+12. **Low · ACCEPT** — Existing `NodeAclService` is raw CRUD. **Change**: new `ValidatedNodeAclAuthoringService` enforces scope-uniqueness + draft/publish invariants + rejects invalid (LDAP group, scope) pairs; Admin UI writes through it only. Stream D.2 adjusted.
+

docs/v2/implementation/phase-6-3-redundancy-runtime.md

@@ -0,0 +1,130 @@
+# Phase 6.3 — Redundancy Runtime
+
+> **Status**: DRAFT — `CLAUDE.md` + `docs/Redundancy.md` describe a non-transparent warm/hot redundancy model with unique ApplicationUris, `RedundancySupport` advertisement, `ServerUriArray`, and dynamic `ServiceLevel`. Entities (`ServerCluster`, `ClusterNode`, `RedundancyRole`, `RedundancyMode`) exist; the runtime behavior (actual `ServiceLevel` number computation, mid-apply dip, `ServerUriArray` broadcast) is not wired.
+>
+> **Branch**: `v2/phase-6-3-redundancy-runtime`
+> **Estimated duration**: 2 weeks
+> **Predecessor**: Phase 6.2 (Authorization) — reuses the Phase 6.1 health endpoints for cluster-peer probing
+> **Successor**: Phase 6.4 (Admin UI completion)
+
+## Phase Objective
+
+Land the non-transparent redundancy protocol end-to-end: two `OtOpcUa.Server` instances in a `ServerCluster` each expose a live `ServiceLevel` node whose value reflects that instance's suitability to serve traffic, advertise each other via `ServerUriArray`, and transition role (Primary ↔ Backup) based on health + operator intent.
+
+Closes these gaps:
+
+1. **Dynamic `ServiceLevel`** — OPC UA Part 5 §6.3.34 specifies a Byte (0..255) that clients poll to pick the healthiest server. Our server publishes it as a static value today.
+2. **`ServerUriArray` broadcast** — Part 4 specifies that every node in a redundant pair should advertise its peers' ApplicationUris. Currently advertises only its own.
+3. **Primary / Backup role coordination** — entities carry `RedundancyRole` but the runtime doesn't read it; no peer health probing; no role-transfer on primary failure.
+4. **Mid-apply dip** — decision-level expectation that a server mid-generation-apply should report a *lower* ServiceLevel so clients cut over to the peer during the apply window. Not implemented.
+
+## Scope — What Changes
+
+| Concern | Change |
+|---------|--------|
+| `OtOpcUa.Server` → new `Server.Redundancy` sub-namespace | `RedundancyCoordinator` singleton. Resolves the current node's `ClusterNode` row at startup, loads its peers from `ServerCluster`, probes each peer's `/healthz` (Phase 6.1 endpoint) every `PeerProbeInterval` (default 2 s), maintains per-peer health state. |
+| OPC UA server root | `ServiceLevel` variable node becomes a `BaseDataVariable` whose value updates on `RedundancyCoordinator` state change. `ServerUriArray` array variable refreshes on cluster-topology change. `RedundancySupport` stays static (set from `RedundancyMode` at startup). |
+| `RedundancyCoordinator` computation | `ServiceLevel` formula: 255 = Primary + fully healthy + no apply in progress; 200 = Primary + an apply in the middle (clients should prefer peer); 100 = Backup + fully healthy; 50 = Backup + mid-apply; 0 = Faulted or peer-unreachable-and-I'm-not-authoritative. Documented in `docs/Redundancy.md` update. |
+| Role transition | Split-brain avoidance: role is *declared* in the shared config DB (`ClusterNode.RedundancyRole`), not elected at runtime. An operator flips the row (or a failover script does). Coordinator only reads; never writes. |
+| `sp_PublishGeneration` hook | Before the apply starts, the coordinator sets `ApplyInProgress = true` in-memory → `ServiceLevel` drops to mid-apply band. Clears after `sp_PublishGeneration` returns. |
+| Admin UI `/cluster/{id}` page | New `RedundancyTab.razor` — shows current node's role + ServiceLevel + peer reachability. FleetAdmin can trigger a role-swap by editing `ClusterNode.RedundancyRole` + publishing a draft. |
+| Metrics | New OpenTelemetry metrics: `ot_opcua_service_level{cluster,node}`, `ot_opcua_peer_reachable{cluster,node,peer}`, `ot_opcua_apply_in_progress{cluster,node}`. Sink via Phase 6.1 observability layer. |
+
+## Scope — What Does NOT Change
+
+| Item | Reason |
+|------|--------|
+| OPC UA authn / authz | Phases 6.2 + prior. Redundancy is orthogonal. |
+| Driver layer | Drivers aren't redundancy-aware; they run on each node independently against the same equipment. The server layer handles the ServiceLevel story. |
+| Automatic failover / election | Explicitly out of scope. Non-transparent = client picks which server to use via ServiceLevel + ServerUriArray. We do NOT ship consensus, leader election, or automatic promotion. Operator-driven failover is the v2.0 model per decision #79–85. |
+| Transparent redundancy (`RedundancySupport=Transparent`) | Not supported. If the operator asks for it the server fails startup with a clear error. |
+| Historian redundancy | Galaxy Historian's own redundancy (two historians on two CPUs) is out of scope. The Galaxy driver talks to whichever historian is reachable from its node. |
+
+## Entry Gate Checklist
+
+- [ ] Phase 6.1 merged (uses `/healthz` for peer probing)
+- [ ] `CLAUDE.md` §Redundancy + `docs/Redundancy.md` re-read
+- [ ] Decisions #79–85 re-skimmed
+- [ ] `ServerCluster`/`ClusterNode`/`RedundancyRole`/`RedundancyMode` entities + existing migration reviewed
+- [ ] OPC UA Part 4 §Redundancy + Part 5 §6.3.34 (ServiceLevel) re-skimmed
+- [ ] Dev box has two OtOpcUa.Server instances configured against the same cluster — one designated Primary, one Backup — for integration testing
+
+## Task Breakdown
+
+### Stream A — Cluster topology loader (3 days)
+
+1. **A.1** `RedundancyCoordinator` startup path: reads `ClusterNode` row for the current node (identified by `appsettings.json` `Cluster:NodeId`), reads the cluster's peer list, validates invariants (no duplicate `ApplicationUri`, at most one `Primary` per cluster if `RedundancyMode.WarmActive`, at most two nodes total in v2.0 per decision #83).
+2. **A.2** Topology subscription — coordinator re-reads on `sp_PublishGeneration` confirmation so an operator role-swap takes effect after publish (no process restart needed).
+3. **A.3** Tests: two-node cluster seed, one-node cluster seed (degenerate), duplicate-uri rejection.
+
+### Stream B — Peer health probing + ServiceLevel computation (4 days)
+
+1. **B.1** `PeerProbeLoop` runs per peer at `PeerProbeInterval` (2 s default, configurable via `appsettings.json`). Calls peer's `/healthz` via `HttpClient`; timeout 1 s. Exponential backoff on sustained failure.
+2. **B.2** `ServiceLevelCalculator.Compute(current role, self health, peer reachable, apply in progress) → byte`. Matrix documented in §Scope.
+3. **B.3** Calculator reacts to inputs via `IObserver` pattern so changes immediately push to the OPC UA `ServiceLevel` node.
+4. **B.4** Tests: matrix coverage for all role × health × apply permutations (32 cases); injected `IClock` + fake `HttpClient` so tests are deterministic.
+
+### Stream C — OPC UA node wiring (3 days)
+
+1. **C.1** `ServiceLevel` variable node created under `ServerStatus` at server startup. Type `Byte`, AccessLevel = CurrentRead only. Subscribe to `ServiceLevelCalculator` observable; push updates via `DataChangeNotification`.
+2. **C.2** `ServerUriArray` variable node under `ServerCapabilities`. Array of `String`, length = peer count. Updates on topology change.
+3. **C.3** `RedundancySupport` variable — static at startup from `RedundancyMode`. Values: `None`, `Cold`, `Warm`, `WarmActive`, `Hot`. Phase 6.3 supports everything except `Transparent` + `HotAndMirrored`.
+4. **C.4** Test against the Client.CLI: connect to primary, read `ServiceLevel` → expect 255; pause primary apply → expect 200; fail primary → client sees `Bad_ServerNotConnected` + reconnects to peer at 100.
+
+### Stream D — Apply-window integration (2 days)
+
+1. **D.1** `sp_PublishGeneration` caller wraps the apply in `using (coordinator.BeginApplyWindow())`. `BeginApplyWindow` increments an in-process counter; ServiceLevel drops on first increment. Dispose decrements.
+2. **D.2** Nested applies handled by the counter (rarely happens but Ignition and Kepware clients have both been observed firing rapid-succession draft publishes).
+3. **D.3** Test: mid-apply subscribe on primary; assert the subscribing client sees the ServiceLevel drop immediately after the apply starts, then restore after apply completes.
+
+### Stream E — Admin UI + metrics (3 days)
+
+1. **E.1** `RedundancyTab.razor` under `/cluster/{id}/redundancy`. Shows each node's role, current ServiceLevel, peer reachability, last apply timestamp. Role-swap button posts a draft edit on `ClusterNode.RedundancyRole`; publish applies.
+2. **E.2** OpenTelemetry meter export: three gauges per the §Scope metrics. Sink via Phase 6.1 observability.
+3. **E.3** SignalR push: `FleetStatusHub` broadcasts ServiceLevel changes so the Admin UI updates within ~1 s of the coordinator observing a peer flip.
+
+## Compliance Checks (run at exit gate)
+
+- [ ] **Primary-healthy** ServiceLevel = 255.
+- [ ] **Backup-healthy** ServiceLevel = 100.
+- [ ] **Mid-apply Primary** ServiceLevel = 200 — verified via Client.CLI subscription polling ServiceLevel during a forced draft publish.
+- [ ] **Peer-unreachable** handling: when a Primary can't probe its Backup's `/healthz`, Primary still serves at 255 (peer is the one with the problem). When a Backup can't probe Primary, Backup flips to 200 (per decision #81 — a lonely Backup promotes its advertised level to signal "I'll take over if you ask" without auto-promoting).
+- [ ] **Role transition via operator publish**: FleetAdmin swaps `RedundancyRole` rows in a draft, publishes; both nodes re-read topology on publish confirmation and flip ServiceLevel accordingly — no restart needed.
+- [ ] **ServerUriArray** returns exactly the peer node's ApplicationUri.
+- [ ] **Client.CLI cutover**: with a primary deliberately halted, a client that was connected to primary reconnects to the backup within the ServiceLevel-polling interval.
+- [ ] No regression in existing driver test suites; no regression in `/healthz` reachability under redundancy load.
+
+## Risks and Mitigations
+
+| Risk | Likelihood | Impact | Mitigation |
+|------|:----------:|:------:|------------|
+| Split-brain from operator race (both nodes marked Primary) | Low | High | Coordinator rejects startup if its cluster has >1 Primary row; logs + fails fast. Document as a publish-time validation in `sp_PublishGeneration`. |
+| ServiceLevel thrashing on flaky peer | Medium | Medium | 2 s probe interval + 3-sample smoothing window; only declares a peer unreachable after 3 consecutive failed probes |
+| Client ignores ServiceLevel and stays on broken primary | Medium | Medium | Documented in `docs/Redundancy.md` — non-transparent redundancy requires client cooperation; most SCADA clients (Ignition, Kepware, Aveva OI Gateway) honor it. Unit-test the advertised values; field behavior is client-responsibility |
+| Apply-window counter leaks on exception | Low | High | `BeginApplyWindow` returns `IDisposable`; `using` syntax enforces paired decrement; unit test for exception-in-apply path |
+| `HttpClient` probe leaks sockets | Low | Medium | Single shared `HttpClient` per coordinator (not per-probe); timeouts tight to avoid keeping connections open during peer downtime |
+
+## Completion Checklist
+
+- [ ] Stream A: topology loader + tests
+- [ ] Stream B: peer probe + ServiceLevel calculator + 32-case matrix tests
+- [ ] Stream C: ServiceLevel / ServerUriArray / RedundancySupport node wiring + Client.CLI smoke test
+- [ ] Stream D: apply-window integration + nested-apply counter
+- [ ] Stream E: Admin `RedundancyTab` + OpenTelemetry metrics + SignalR push
+- [ ] `phase-6-3-compliance.ps1` exits 0; exit-gate doc; `docs/Redundancy.md` updated with the ServiceLevel matrix
+
+## Adversarial Review — 2026-04-19 (Codex, thread `019da490-3fa0-7340-98b8-cceeca802550`)
+
+1. **Crit · ACCEPT** — No publish-generation fencing enables split-publish advertising both as authoritative. **Change**: coordinator CAS on a monotonic `ConfigGenerationId`; every topology decision is generation-stamped; peers reject state propagated from a lower generation.
+2. **Crit · ACCEPT** — `>1 Primary` at startup covered but runtime containment missing when invalid topology appears later (mid-apply race). **Change**: add runtime `InvalidTopology` state — both nodes self-demote to ServiceLevel 2 (the "detected inconsistency" band, below normal operation) until convergence.
+3. **High · ACCEPT** — `0 = Faulted` collides with OPC UA Part 5 §6.3.34 semantics where 0 means **Maintenance** and 1 means NoData. **Change**: reserve **0** for operator-declared maintenance-mode only; Faulted/unreachable uses **1** (NoData); in-range degraded states occupy 2..199.
+4. **High · ACCEPT** — Matrix collapses distinct operational states onto the same value. **Change**: matrix expanded to Authoritative-Primary=255, Isolated-Primary=230 (peer unreachable — still serving), Primary-Mid-Apply=200, Recovering-Primary=180, Authoritative-Backup=100, Isolated-Backup=80 (primary unreachable — "take over if asked"), Backup-Mid-Apply=50, Recovering-Backup=30.
+5. **High · ACCEPT** — `/healthz` from 6.1 is HTTP-healthy but doesn't guarantee OPC UA data plane. **Change**: add a redundancy-specific probe `UaHealthProbe` — issues a `ReadAsync(ServiceLevel)` against the peer's OPC UA endpoint via a lightweight client session. `/healthz` remains the fast-fail; the UA probe is the authority signal.
+6. **High · ACCEPT** — `ServerUriArray` must include self + peers, not peers only. **Change**: array contains `[self.ApplicationUri, peer.ApplicationUri]` in stable deterministic ordering; compliance test asserts local-plus-peer membership.
+7. **Med · ACCEPT** — No `Faulted → Recovering → Healthy` path. **Change**: add `Recovering` state with min dwell time (60 s default) + positive publish witness (one successful Read on a reference node) before returning to Healthy. Thrash-prevention.
+8. **Med · ACCEPT** — Topology change during in-flight probe undefined. **Change**: every probe task tagged with `ConfigGenerationId` at dispatch; obsolete results discarded; in-flight probes cancelled on topology reload.
+9. **Med · ACCEPT** — Apply-window counter race on exception/cancellation/async ownership. **Change**: apply-window is a named lease keyed to `(ConfigGenerationId, PublishRequestId)` with disposal enforced via `await using`; watchdog detects leased-but-abandoned and force-closes after `ApplyMaxDuration` (default 10 min).
+10. **High · ACCEPT** — Ignition + Kepware + Aveva OI Gateway `ServiceLevel` compliance is unverified. **Change**: risk elevated to High; add Stream F (new) — build an interop matrix: validate against Ignition 8.1/8.3, Kepware KEPServerEX 6.x, Aveva OI Gateway 2020R2 + 2023R1. Document per-client cutover behaviour. Field deployments get a documented compatibility table; clients that ignore ServiceLevel documented as requiring explicit backup-endpoint config.
+11. **Med · ACCEPT** — Galaxy MXAccess re-session on Primary death not in acceptance. **Change**: Stream F adds an end-to-end failover smoke test that boots Galaxy.Proxy on both nodes, kills Primary, asserts Galaxy consumer reconnects to Backup within `(SessionTimeout + KeepAliveInterval × 3)` budget. `docs/Redundancy.md` updated with required session timeouts.
+12. **Med · ACCEPT** — Transparent-mode startup rejection is outage-prone. **Change**: `sp_PublishGeneration` validates `RedundancyMode` pre-publish — unsupported values reject the publish attempt with a clear validation error; runtime never sees an unsupported mode. Last-good config stays active.
+

docs/v2/implementation/phase-6-4-admin-ui-completion.md

@@ -0,0 +1,120 @@
+# Phase 6.4 — Admin UI Completion
+
+> **Status**: DRAFT — Phase 1 Stream E shipped the Admin scaffold + core pages; several feature-completeness items from its completion checklist (`phase-1-configuration-and-admin-scaffold.md` §Stream E) never landed. This phase closes them.
+>
+> **Branch**: `v2/phase-6-4-admin-ui-completion`
+> **Estimated duration**: 2 weeks
+> **Predecessor**: Phase 6.3 (Redundancy runtime) — reuses the `/cluster/{id}` page layout for the new tabs
+> **Successor**: v2 release-readiness capstone (Task #121)
+
+## Phase Objective
+
+Close the Admin UI feature-completeness checklist that Phase 1 Stream E exit gate left open. Each item below is an existing `phase-1-configuration-and-admin-scaffold.md` completion-checklist entry that is currently unchecked.
+
+Gaps to close:
+
+1. **UNS Structure tab drag/move with impact preview** — decision #115 + `admin-ui.md` §"UNS". Current state: list-only render; no drag reorder; no "X lines / Y equipment impacted" preview.
+2. **Equipment CSV import + 5-identifier search** — decision #95 + #117. Current state: basic form; no CSV parser; search indexes only ZTag.
+3. **Draft-generation diff viewer** — enhance existing `DiffViewer.razor` to show generation-diff not just staged-edit diff; highlight ACL grant changes (lands after Phase 6.2).
+4. **`_base` equipment-class Identification fields exposure** — decision #138–139. Columns exist on `Equipment`; no Admin UI field group; no address-space exposure of the OPC 40010 sub-folder.
+
+## Scope — What Changes
+
+| Concern | Change |
+|---------|--------|
+| `Admin/Pages/UnsTab.razor` | Rewrite as a tree component with drag-drop (Blazor-native HTML5 DnD; no third-party dep). Each drag fires a "Compute Impact" call against the draft-generation state + renders a modal preview ("Moving Line 'Oven-2' from 'Packaging' to 'Assembly' will re-home 14 equipment + re-parent 237 tags"). Confirmation commits the draft edit. |
+| `Admin/Services/UnsImpactAnalyzer.cs` | New service. Given a move-operation (line move, area rename, line merge), computes cascade counts by walking the draft-generation `Equipment` + `Tag` tables. Pure-function shape; testable in isolation. |
+| `Admin/Pages/EquipmentTab.razor` | Add CSV-import button → modal with file picker + dry-run preview. Add multi-identifier search bar (ZTag / SAPID / UniqueId / Alias1 / Alias2) per decision #95 — parses any of the five, shows matches across draft + published generations. |
+| `Admin/Services/EquipmentCsvImporter.cs` | New service. Parses CSV with documented header row; validates each row against the `Equipment` schema (required fields + `ExternalIdReservation` freshness); returns `ImportPreview` DTO with per-row accept/reject + reason; commit step wraps in a single EF transaction. |
+| `Admin/Pages/DraftEditor.razor` + `DiffViewer.razor` | Diff viewer expanded: adds sections for ACL grants (from Phase 6.2 `LdapGroupRoleMapping` + `NodeAcl`), redundancy-role changes (from Phase 6.3), equipment-class `_base` Identification fields. Render each section collapsible. |
+| `Admin/Components/IdentificationFields.razor` | New component. Renders the OPC 40010 nullable columns (Manufacturer, Model, SerialNumber, ProductInstanceUri, HardwareRevision, SoftwareRevision, DeviceRevision, YearOfConstruction, MonthOfConstruction) as a labelled field group on the `EquipmentTab` detail view. |
+| `OtOpcUa.Server/OpcUa/DriverNodeManager` — Equipment folder build | When an `Equipment` row has non-null Identification fields, the server adds an `Identification` sub-folder under the Equipment node containing one variable per non-null field. Matches OPC 40010 companion spec. |
+
+## Scope — What Does NOT Change
+
+| Item | Reason |
+|------|--------|
+| Admin UI visual language | Bootstrap 5 / cookie auth / sidebar layout unchanged — consistency with ScadaLink design reference. |
+| LDAP auth flow | Already shipped in Phase 1. Phase 6.4 is additive UI only. |
+| Core abstractions / driver layer | Admin UI changes don't touch drivers. |
+| Equipment-class *template schema validation* | Still deferred (decision #112 — schemas repo not landed). We expose the Identification fields but don't validate against a template hierarchy. |
+| Drag/move to *other clusters* | Out of scope — equipment is cluster-scoped per decision #82. Cross-cluster migration is a different workflow. |
+
+## Entry Gate Checklist
+
+- [ ] Phase 6.2 merged (ACL grants are part of the new diff viewer sections)
+- [ ] Phase 6.3 merged (redundancy-role changes are part of the diff viewer)
+- [ ] `phase-1-configuration-and-admin-scaffold.md` §Stream E completion checklist re-read — confirm these are the remaining items
+- [ ] `admin-ui.md` re-skimmed for screen layouts
+- [ ] Existing `EquipmentTab.razor` / `UnsTab.razor` / `DraftEditor.razor` diff'd against what ships today so the edits are additive not destructive
+- [ ] Dev Galaxy available for OPC 40010 exposure smoke testing
+
+## Task Breakdown
+
+### Stream A — UNS drag/reorder + impact preview (4 days)
+
+1. **A.1** `UnsImpactAnalyzer` service. Inputs: `(DraftGenerationId, MoveOperation)`. Outputs: `ImpactPreview { AffectedEquipmentCount, AffectedTagCount, CascadeWarnings[] }`. Unit tests cover line move / area rename / line merge.
+2. **A.2** HTML5 DnD on a tree component. No JS interop beyond `ondragstart`/`ondragover`/`ondrop` — keeps build + testability simple.
+3. **A.3** Modal preview wired to `UnsImpactAnalyzer` output; "Confirm" commits a draft edit via `DraftService`.
+4. **A.4** Playwright smoke test (or equivalent): drag a line across areas, assert modal shows the right counts, assert draft row reflects the move.
+
+### Stream B — Equipment CSV import + 5-identifier search (4 days)
+
+1. **B.1** `EquipmentCsvImporter` with a documented header row (`ZTag, SAPID, UniqueId, Alias1, Alias2, Name, UnsAreaName, UnsLineName, Manufacturer, Model, SerialNumber, …`). Parser rejects unknown columns + blank required fields + duplicate ZTags.
+2. **B.2** `ImportPreview` UI: per-row accept/reject table. Reject reasons: "ZTag already exists in draft", "ExternalIdReservation conflict with Cluster X", "UnsLineName not found in draft UNS tree", etc. Operator reviews then clicks "Commit" → single EF transaction.
+3. **B.3** Multi-identifier search — bar accepts any of the 5 identifiers, probes each column in parallel, returns first-match-wins + disambiguation list if multiple match.
+4. **B.4** Smoke tests: 100-row CSV with 10 intentional conflicts (5 ZTag dupes, 3 reservation clashes, 2 missing UnsLines); assert preview flags each; assert commit rolls back cleanly when a conflict surfaces post-preview.
+
+### Stream C — Diff viewer enhancements (3 days)
+
+1. **C.1** Refactor `DiffViewer.razor` into a base component + section plugins. Section plugins: `StructuralDiffSection` (UNS tree), `EquipmentDiffSection` (Equipment rows), `TagDiffSection` (Tag rows), `AclDiffSection` (ACL grants — depends on Phase 6.2), `RedundancyDiffSection` (role changes — depends on Phase 6.3), `IdentificationDiffSection` (OPC 40010 fields).
+2. **C.2** Each section renders collapsed by default; counts + top-line summary always visible.
+3. **C.3** Tests: seed two generations with deliberate diffs, assert every section reports the right counts + top-line summary.
+
+### Stream D — OPC 40010 Identification exposure (3 days)
+
+1. **D.1** `IdentificationFields.razor` component — labelled inputs; nullable columns show empty input; required field validation only on commit.
+2. **D.2** `DriverNodeManager` equipment-folder builder — after building the equipment node, inspect the Identification columns; if any non-null, add an `Identification` sub-folder with variable-per-field.
+3. **D.3** Address-space smoke test via Client.CLI: browse an equipment node, assert `Identification` sub-folder present when columns are set, absent when all null.
+
+## Compliance Checks (run at exit gate)
+
+- [ ] **UNS drag/move**: drag a line across areas; modal preview shows correct impacted-equipment + impacted-tag counts.
+- [ ] **Equipment CSV**: 100-row CSV with 10 conflicts imports cleanly (preview flags each, commit rolls back mid-conflict).
+- [ ] **5-identifier search**: querying any of the 5 IDs returns the matching row; ambiguous searches list options.
+- [ ] **Diff viewer**: every section renders for a 2-generation diff with deliberate changes in every category.
+- [ ] **OPC 40010 exposure**: Client.CLI browse shows `Identification` sub-folder when equipment has non-null columns; folder absent when all null.
+- [ ] **ScadaLink visual parity**: operator-equivalence reviewer signs off that the new tabs feel consistent with existing Admin UI pages.
+
+## Risks and Mitigations
+
+| Risk | Likelihood | Impact | Mitigation |
+|------|:----------:|:------:|------------|
+| UNS drag-drop janky on large trees (>500 nodes) | Medium | Medium | Virtualize the tree component; default-collapse nested areas; test with a synthetic 1000-equipment seed |
+| CSV import performance on 10k-row imports | Medium | Medium | Stream-parse rather than load-into-memory; preview renders in batches of 100; commit is chunked-EF-insert with progress bar |
+| Diff viewer becomes unwieldy with many sections | Low | Medium | Each section collapsed by default; top-line summary row always shown; Phase 6.4 caps at 6 sections |
+| OPC 40010 sub-folder accidentally exposes NULL/empty identification columns as empty-string variables | Low | Low | Column null-check in the builder; drop variables whose DB value is null |
+| 5-identifier search pulls full table | Medium | Medium | Indexes on each of ZTag/SAPID/UniqueId/Alias1/Alias2; search query uses a UNION of 5 indexed lookups; falls back to LIKE only on explicit operator opt-in |
+
+## Completion Checklist
+
+- [ ] Stream A: `UnsImpactAnalyzer` + drag-drop tree + modal preview + Playwright smoke
+- [ ] Stream B: `EquipmentCsvImporter` + preview modal + 5-identifier search + conflict-rollback test
+- [ ] Stream C: `DiffViewer` refactor + 6 section plugins + 2-generation diff test
+- [ ] Stream D: `IdentificationFields.razor` + address-space builder change + Client.CLI browse test
+- [ ] Visual-compliance reviewer signoff
+- [ ] Full solution `dotnet test` passes; `phase-6-4-compliance.ps1` exits 0; exit-gate doc
+
+## Adversarial Review — 2026-04-19 (Codex, via `codex-rescue` subagent)
+
+1. **Crit · ACCEPT** — Stale UNS impact preview can overwrite concurrent draft edits. **Change**: each preview carries a `DraftRevisionToken`; `Confirm` compares against the current draft + rejects with a `409 Conflict / refresh-required` modal if any draft edit landed since the preview was generated. Stream A.3 updated.
+2. **High · ACCEPT** — CSV import atomicity is internally contradictory (single EF transaction vs. chunked inserts). **Change**: one explicit model — staged-import table (`EquipmentImportBatch { Id, CreatedAtUtc, RowsStaged, RowsAccepted, RowsRejected }`) receives rows in chunks; final `FinaliseImportBatch` is atomic over `Equipment` + `ExternalIdReservation`. Rollback is "drop the batch row" — the real Equipment table is never partially mutated.
+3. **Crit · ACCEPT** — Identifier contract rewrite mis-cites decisions. **Change**: revert to the `admin-ui.md` + decision #117 canonical set — `ZTag / MachineCode / SAPID / EquipmentId / EquipmentUuid`. CSV header follows that set verbatim. Introduce a separate decision entry for versioned CSV header shape before adding any new column; CSV header row must start with `# OtOpcUaCsv v1` so future shape changes are unambiguous.
+4. **Med · ACCEPT** — Search ordering undefined. **Change**: rank SQL — exact match on any identifier scores 100; prefix match 50; LIKE-fuzzy 20; published > draft tie-breaker; `ORDER BY score DESC, RowVersion DESC`. Typeahead shows which field matched via trailing badge.
+5. **High · ACCEPT** — HTML5 DnD on virtualized tree is aspirational. **Change**: Stream A.2 rewritten — commits to **`MudBlazor.TreeView` + `MudBlazor.DropTarget`** (already a transitive dep via the existing Admin UI). Build a 1000-node synthetic seed in A.1 + validate drag-latency budget before implementing impact preview. If MudBlazor can't hit the budget, fall back to a flat-list reorder UI with Area/Line dropdowns (loss of visual drag affordance but unblocks the feature).
+6. **Med · ACCEPT** — Collapsed-by-default doesn't handle generation-sized diffs. **Change**: each diff section has a hard row cap (1000 by default). Over-cap sections render an aggregate summary + "Load full diff" button that streams via SignalR in 500-row pages. Decision #115 subtree renames surface as a "N equipment re-parented under X → Y" summary instead of row-by-row.
+7. **High · ACCEPT** — OPC 40010 field list doesn't match decision #139. **Change**: field group realigned to `Manufacturer, Model, SerialNumber, HardwareRevision, SoftwareRevision, YearOfConstruction, AssetLocation, ManufacturerUri, DeviceManualUri`. `ProductInstanceUri / DeviceRevision / MonthOfConstruction` dropped from Phase 6.4 — they belong to a future OPC 40010 widening decision.
+8. **High · ACCEPT** — `Identification` subtree unreconciled with ACL hierarchy (Phase 6.2 6-level scope). **Change**: address-space builder creates the Identification sub-folder under the Equipment node **with the same ScopeId as Equipment** — no new scope level. ACL evaluator treats `…/Equipment/Identification/X` as inheriting the `Equipment` scope's grants. Documented in Phase 6.2's `acl-design.md` cross-reference update.
+9. **Low · ACCEPT** — Visual-review gate names nonexistent reviewer role. **Change**: rubric defined — a named "Admin UX reviewer" (role `FleetAdmin` user, not the implementation lead) compares side-by-side screenshots against the `admin-ui.md` §Visual-Design reference panels; signoff artefact is a checked-in screenshot set under `docs/v2/visual-compliance/phase-6-4/`.
+10. **Med · ACCEPT** — Cross-cluster drag/drop lacks loud failure path. **Change**: on drop across cluster boundary, disable the drop target + show a toast "Equipment is cluster-scoped (decision #82). To move across clusters, use the Export → Import workflow on the Cluster detail page." Plus a help link. Tested in Stream A.4.
+

docs/v2/mitsubishi.md

@@ -0,0 +1,451 @@
+# Mitsubishi Electric MELSEC — Modbus TCP quirks
+
+Mitsubishi's MELSEC family speaks Modbus TCP through a patchwork of add-on modules
+and built-in Ethernet ports, not a single unified stack. The module names are
+confusingly similar (`QJ71MB91` is *serial* RTU, `QJ71MT91` is the TCP/IP module
+[9]; `LJ71MT91` is the L-series equivalent; `RJ71EN71` is the iQ-R Ethernet module
+with a MODBUS/TCP *slave* mode bolted on [8]; `FX3U-ENET`, `FX3U-ENET-P502`,
+`FX3U-ENET-ADP`, `FX3GE` built-in, and `FX5U` built-in are all different code
+paths) — and every one of the categories below has at least one trap a textbook
+Modbus client gets wrong: hex-numbered X/Y devices colliding with decimal Modbus
+addresses, a user-defined "device assignment" parameter block that means *no two
+sites are identical*, CDAB-vs-ABCD word order driven by how the ladder built the
+32-bit value, sub-spec FC16 caps on the older QJ71MT91, and an FX3U port-502
+licensing split that makes `FX3U-ENET` and `FX3U-ENET-P502` different SKUs.
+This document catalogues each quirk, cites primary sources, and names the
+ModbusPal integration test we'd write for it (convention from
+`docs/v2/modbus-test-plan.md`: `Mitsubishi_<model>_<behavior>`).
+
+## Models and server/client capability
+
+| Model                  | Family   | Modbus TCP server | Modbus TCP client | Source |
+|------------------------|----------|-------------------|-------------------|--------|
+| `QJ71MT91`             | MELSEC-Q | Yes (slave)       | Yes (master)      | [9]    |
+| `QJ71MB91`             | MELSEC-Q | **Serial only** — RS-232/422/485 RTU, *not TCP* | — | [1][3] |
+| `LJ71MT91`             | MELSEC-L | Yes (slave)       | Yes (master)      | [10]   |
+| `RJ71EN71` / `RnENCPU` | MELSEC iQ-R | Yes (slave)    | Yes (master)      | [8]    |
+| `RJ71C24` / `RJ71C24-R2` | MELSEC iQ-R | RTU (serial)  | RTU (serial)      | [13]   |
+| iQ-R built-in Ethernet | CPU      | Yes (slave)       | Yes (master)      | [7]    |
+| iQ-F `FX5U` built-in Ethernet | CPU | Yes, firmware ≥ 1.060 [11] | Yes | [7][11][12] |
+| `FX3U-ENET`            | FX3U bolt-on | Yes (slave), but **not on port 502** [5] | Yes | [4][5] |
+| `FX3U-ENET-P502`       | FX3U bolt-on | Yes (slave), port 502 enabled | Yes | [5] |
+| `FX3U-ENET-ADP`        | FX3U adapter | **No MODBUS** [5] | No MODBUS     | [5]    |
+| `FX3GE` built-in       | FX3GE CPU | No MODBUS (needs ENET module) [6] | No | [6] |
+| `FX3G` + `FX3U-ENET`   | FX3G | Yes via ENET module | Yes | [6] |
+
+- A common integration mistake is to buy `FX3U-ENET-ADP` expecting MODBUS —
+  that adapter speaks only MC protocol / SLMP. Our driver should surface a clear
+  capability error, not "connection refused", when the operator's device tag
+  says `FX3U-ENET-ADP` [5].
+- Older forum threads assert the FX5U is "client only" [12] — that was true on
+  firmware ≤ 1.040. Firmware 1.060 and later ship the parameter-driven MODBUS
+  TCP server built-in and need no function blocks [11].
+
+## Modbus device assignment (the parameter block)
+
+Unlike a DL260 where the CPU exposes a *fixed* V-memory-to-Modbus mapping, every
+MELSEC MODBUS-TCP module exposes a **Modbus Device Assignment Parameter** block
+that the engineer configures in GX Works2 / GX Configurator-MB / GX Works3.
+Each of the four Modbus tables (Coil, Input, Input Register, Holding Register)
+can be split into up to 16 independent "assignment" entries, each binding a
+contiguous Modbus address range to a MELSEC device head (`M0`, `D0`, `X0`,
+`Y0`, `B0`, `W0`, `SM0`, `SD0`, `R0`, etc.) and a point count [3][7][8][9].
+
+- **There is no canonical "MELSEC Modbus mapping"**. Two sites running the same
+  QJ71MT91 module can expose completely different Modbus layouts. Our driver
+  must treat the mapping as site-data (config-file-driven), not as a device
+  profile constant.
+- **Default values do exist** — both GX Configurator-MB (for Q/L series) and
+  GX Works3 (for iQ-R / iQ-F / FX5) ship a "dedicated pattern" default that is
+  applied when the engineer does not override the assignment. Per the FX5
+  MODBUS Communication manual (JY997D56101) and the QJ71MT91 manual, the FX5
+  dedicated default is [3][7][11]:
+
+  | Modbus table       | Modbus range (0-based) | MELSEC device | Head |
+  |--------------------|------------------------|---------------|------|
+  | Coil (FC01/05/15)  | 0 – 7679               | M             | M0   |
+  | Coil               | 8192 – 8959            | Y             | Y0   |
+  | Input (FC02)       | 0 – 7679               | M             | M0   |
+  | Input              | 8192 – 8959            | X             | X0   |
+  | Input Register (FC04) | 0 – 6143            | D             | D0   |
+  | Holding Register (FC03/06/16) | 0 – 6143    | D             | D0   |
+
+  This matches the widely circulated "FC03 @ 0 = D0" convention that shows up
+  in Ubidots / Ignition / AdvancedHMI integration guides [6][12].
+
+- **X/Y in the default mapping occupy a second, non-zero Modbus range** (8192+
+  on FX5; similar on Q/L/iQ-R). Driver users who expect "X0 = coil 0" will be
+  reading M0 instead. Document this clearly.
+- **Assignment-range collisions silently disable the slave.** The QJ71MT91
+  manual states explicitly that if any two of assignments 1-16 duplicate the
+  head Modbus device number, the slave function is inactive with no clear
+  error — the module just won't respond [9]. The driver probe will look like a
+  simple timeout; the site engineer has to open GX Configurator-MB to diagnose.
+
+Test names:
+`Mitsubishi_FX5U_default_mapping_coil_0_is_M0`,
+`Mitsubishi_FX5U_default_mapping_holding_0_is_D0`,
+`Mitsubishi_QJ71MT91_duplicate_assignment_head_disables_slave`.
+
+## X/Y addressing — hex on MELSEC, decimal on Modbus
+
+**MELSEC X (input) and Y (output) device numbers are hexadecimal on Q / L /
+iQ-R** and **octal** on FX / iQ-F (with a GX Works3 toggle) [14][15].
+
+- On a Q CPU, `X20` means decimal **32**, not 20. On an FX5U in default (octal)
+  mode, `X20` means decimal **16**. GX Works3 exposes a project-level option to
+  display FX5U X/Y in hex to match Q/L/iQ-R convention — the same physical
+  input is then called `X10` [14].
+- The Modbus Device Assignment Parameter block takes the *head device* as a
+  MELSEC-native number, which is interpreted in the CPU's native base
+  (hex for Q/L/iQ-R, octal for FX/iQ-F). After that, **Modbus offsets from
+  the head are plain decimal** — the module does not apply a second hex
+  conversion [3][9].
+- Example (QJ71MT91 on a Q CPU): assignment "Coil 0 = X0, 512 points" exposes
+  physical `X0` through `X1FF` (hex) as coils 0-511. A client reading coil 32
+  gets the bit `X20` (hex) — i.e. the 33rd input, not the value at "input 20"
+  that the operator wrote on the wiring diagram in decimal.
+- **Driver bug source**: if the operator's tag configuration says "read X20" and
+  the driver helpfully converts "20" to decimal 20 → coil offset 20, the
+  returned bit is actually `X14` (hex) — off by twelve. Our config layer must
+  preserve the MELSEC-native base that the site engineer sees in GX Works.
+- Timers/counters (`T`, `C`, `ST`) are always decimal in MELSEC notation.
+  Internal relays (`M`, `B`, `L`), data registers (`D`, `W`, `R`, `ZR`),
+  and special relays/registers (`SM`, `SD`) also decimal. **Only `X` and `Y`
+  (and on Q/L/iQ-R, `B` link relays and `W` link registers) use hex**, and
+  the X/Y decision is itself family-dependent [14][15].
+
+Test names:
+`Mitsubishi_Q_X_address_is_hex_X20_equals_coil_offset_32`,
+`Mitsubishi_FX5U_X_address_is_octal_X20_equals_coil_offset_16`,
+`Mitsubishi_W_link_register_is_hex_W10_equals_holding_offset_16`.
+
+## Word order for 32-bit values
+
+MELSEC stores 32-bit ladder values (`DINT`, `DWORD`, `REAL` / single-precision
+float) across **two consecutive D-registers, low word first** — i.e., `CDAB`
+when viewed as a Modbus register pair [2][6].
+
+```
+D100 (low word)  : 0xCC 0xDD   (big-endian bytes within the word)
+D101 (high word) : 0xAA 0xBB
+```
+
+A Modbus master reading D100/D101 as a `float` with default (ABCD) word order
+gets garbage. Ignition's built-in Modbus driver notes Mitsubishi as a "CDAB
+device" specifically for this reason [2].
+
+- **Q / L / iQ-R / iQ-F all agree** — this is a CPU-level convention, not a
+  module choice. Both the QJ71MT91 manual and the FX5 MODBUS Communication
+  manual describe 32-bit access by "reading the lower 16 bits from the start
+  address and the upper 16 bits from start+1" [6][11].
+- **Byte order within each register is big-endian** (Modbus standard). The
+  module does not byte-swap.
+- **Configurable?** The MODBUS modules themselves do **not** expose a word-
+  order toggle; the behavior is fixed to how the CPU laid out the value in the
+  two D-registers. If the ladder programmer used an `SWAP` instruction or a
+  union-style assignment, the word order can be whatever they made it — but
+  for values produced by the standard `D→DBL` and `FLT`/`FLT2` instructions
+  it is always CDAB [2].
+- **FX5U quirk**: the FX5 MODBUS Communication manual tells the programmer to
+  use the `SWAP` instruction *if* the remote Modbus peer requires
+  little-endian *byte* ordering (BADC) [11]. This is only relevant when the
+  FX5U is the Modbus *client*, but it confirms the FX5U's native wire layout
+  is big-endian-byte / little-endian-word (CDAB) on the server side too.
+- **Rumoured exception**: a handful of MrPLC forum threads report iQ-R
+  RJ71EN71 firmware < 1.05 returning DWORDs in `ABCD` order when accessed via
+  the built-in Ethernet port's MODBUS slave [8]. _Unconfirmed_; treat as a
+  per-site test.
+
+Test names:
+`Mitsubishi_Float32_word_order_is_CDAB`,
+`Mitsubishi_Int32_word_order_is_CDAB`,
+`Mitsubishi_FX5U_SWAP_instruction_changes_byte_order_not_word_order`.
+
+## BCD vs binary encoding
+
+**MELSEC stores integer values in D-registers as plain binary two's-complement**,
+not BCD [16]. This is the opposite of AutomationDirect DirectLOGIC, where
+V-memory defaults to BCD and the ladder must explicitly request binary.
+
+- A ladder `MOV K1234 D100` stores `0x04D2` (1234 decimal) in D100, not
+  `0x1234`. The Modbus master reads `0x04D2` and decodes it as an integer
+  directly — no BCD conversion needed [16].
+- **Timer / counter current values** (`T0` current value, `C0` count) are
+  stored in binary as word devices on Q/L/iQ-R/iQ-F. The ladder preset
+  (`K...`) is also binary [16][17].
+- **Timer / counter preset `K` operand in FX3U / earlier FX**: also binary when
+  loaded from a D-register or a `K` constant. The older A-series CPUs had BCD
+  presets on some timer types, but MELSEC-Q, L, iQ-R, iQ-F, and FX3U all use
+  binary presets by default [17].
+- The FX3U programming manual dedicates `FNC 18 BCD` and `FNC 19 BIN` to
+  explicit conversion — their existence confirms that anything in D-registers
+  that came from a `BCD` instruction output is BCD, but nothing is BCD by
+  default [17].
+- **7-segment display registers** are a common site-specific exception — many
+  ladders pack `BCD D100` into a D-register so the operator panel can drive
+  a display directly. Our driver should not assume; expose a per-tag
+  "encoding = binary | BCD" knob.
+
+Test names:
+`Mitsubishi_D_register_stores_binary_not_BCD`,
+`Mitsubishi_FX3U_timer_current_value_is_binary`.
+
+## Max registers per request
+
+From the FX5 MODBUS Communication manual Chapter 11 [11]:
+
+| FC | Name                       | FX5U (built-in) | QJ71MT91     | iQ-R (RJ71EN71 / built-in) | FX3U-ENET |
+|----|----------------------------|-----------------|--------------|-----------------------------|-----------|
+| 01 | Read Coils                 | 1-2000          | 1-2000 [9]   | 1-2000 [8]                  | 1-2000    |
+| 02 | Read Discrete Inputs       | 1-2000          | 1-2000       | 1-2000                      | 1-2000    |
+| 03 | Read Holding Registers     | **1-125**       | 1-125 [9]    | 1-125 [8]                   | 1-125     |
+| 04 | Read Input Registers       | 1-125           | 1-125        | 1-125                       | 1-125     |
+| 05 | Write Single Coil          | 1               | 1            | 1                           | 1         |
+| 06 | Write Single Register      | 1               | 1            | 1                           | 1         |
+| 0F | Write Multiple Coils       | 1-1968          | 1-1968       | 1-1968                      | 1-1968    |
+| 10 | Write Multiple Registers   | **1-123**       | 1-123        | 1-123                       | 1-123     |
+| 16 | Mask Write Register        | 1               | not supported | 1                          | not supported |
+| 17 | Read/Write Multiple Regs   | R:1-125, W:1-121 | not supported | R:1-125, W:1-121          | not supported |
+
+- **The FX5U / iQ-R native-port limits match the Modbus spec**: 125 for FC03/04,
+  123 for FC16 [11]. No sub-spec caps like DL260's 100-register ceiling.
+- **QJ71MT91 does not support FC16 (0x16, Mask Write Register) or FC17
+  (0x17, Read/Write Multiple)** — requesting them returns exception `01`
+  Illegal Function [9]. FX5U and iQ-R *do* support both.
+- **QJ71MT91 device size**: 64k points (65,536) for each of Coil / Input /
+  Input Register / Holding Register, plus up to 4086k points for Extended
+  File Register via a secondary assignment range [9].
+- **FX3U-ENET / -P502 function code list is a strict subset** of the common
+  eight (FC01/02/03/04/05/06/0F/10). FC16 and FC17 not supported [4].
+
+Test names:
+`Mitsubishi_FX5U_FC03_126_registers_returns_IllegalDataValue`,
+`Mitsubishi_FX5U_FC16_124_registers_returns_IllegalDataValue`,
+`Mitsubishi_QJ71MT91_FC16_MaskWrite_returns_IllegalFunction`,
+`Mitsubishi_QJ71MT91_FC23_ReadWrite_returns_IllegalFunction`.
+
+## Exception codes
+
+MELSEC MODBUS modules return **only the standard Modbus exception codes 01-04**;
+no proprietary exception codes are exposed on the wire [8][9][11]. Module-
+internal diagnostics (buffer-memory error codes like `7380H`) are logged but
+not returned as Modbus exceptions.
+
+| Code | Name                 | MELSEC trigger                                          |
+|------|----------------------|---------------------------------------------------------|
+| 01   | Illegal Function     | FC17 or FC16 on QJ71MT91/FX3U; FC08 (Diagnostics); FC43 |
+| 02   | Illegal Data Address | Modbus address outside any assignment range             |
+| 03   | Illegal Data Value   | Quantity out of per-FC range (see table above); odd coil-byte count |
+| 04   | Server Device Failure | See below                                              |
+
+- **04 (Server Failure) triggers on MELSEC**:
+    - CPU in STOP or PAUSE during a write to an assignment whose "Access from
+      External Device" permission is set to "Disabled in STOP" [9][11].
+      *With the default "always enabled" setting the write succeeds in STOP
+      mode* — another common trap.
+    - CPU errors (parameter error, watchdog) during any access.
+    - Assignment points to a device range that is not configured (e.g. write
+      to `D16384` when CPU D-device size is 12288).
+- **Write to a "System Area" device** (e.g., `SD` special registers that are
+  CPU-reserved read-only) returns `04`, not `02`, on QJ71MT91 and iQ-R — the
+  assignment is valid, the device exists, but the CPU rejects the write [8][9].
+- **FX3U-ENET / -P502** returns `04` on any write attempt while the CPU is in
+  STOP, regardless of permission settings — the older firmware does not
+  implement the "Access from External Device" granularity that Q/L/iQ-R/iQ-F
+  expose [4].
+- **No rumour of proprietary codes 05-0B** from MELSEC; operators sometimes
+  report "exception 0A" but those traces all came from a third-party gateway
+  sitting between the master and the MELSEC module.
+
+Test names:
+`Mitsubishi_QJ71MT91_STOP_mode_write_with_Disabled_permission_returns_ServerFailure`,
+`Mitsubishi_QJ71MT91_STOP_mode_write_with_default_permission_succeeds`,
+`Mitsubishi_SD_system_register_write_returns_ServerFailure`,
+`Mitsubishi_FX3U_STOP_mode_write_always_returns_ServerFailure`.
+
+## Connection behavior
+
+Max simultaneous Modbus TCP clients, per module [7][8][9][11]:
+
+| Model                | Max TCP connections | Port 502 | Keepalive | Source |
+|----------------------|---------------------|----------|-----------|--------|
+| `QJ71MT91`           | 16 (shared with master role) | Yes | No | [9]    |
+| `LJ71MT91`           | 16                  | Yes      | No        | [10]   |
+| iQ-R built-in / `RJ71EN71` | 16            | Yes      | Configurable (KeepAlive = ON in parameter) | [8] |
+| iQ-F `FX5U` built-in | 8                   | Yes      | Configurable | [7][11] |
+| `FX3U-ENET`          | 8 TCP, but **not port 502** | No (port < 1024 blocked) | No | [4][5] |
+| `FX3U-ENET-P502`     | 8, port 502 enabled | Yes      | No        | [5]    |
+
+- **QJ71MT91's 16 is total connections shared between slave-listen and
+  master-initiated sockets** [9]. A site that uses the same module as both
+  master to downstream VFDs and slave to upstream SCADA splits the 16 pool.
+- **FX3U-ENET port-502 gotcha**: if the engineer loads a configuration with
+  port 502 into a non-P502 ENET module, GX Works shows the download as
+  successful; on next power cycle the module enters error state and the
+  MODBUS listener never starts. This is documented on third-party FX3G
+  integration guides [6].
+- **CPU STOP → RUN transition**: does **not** drop Modbus connections on any
+  MELSEC family. Existing sockets stay open; outstanding requests during the
+  transition may see exception 04 for a few scans but then resume [8][9].
+- **CPU reset (power cycle or `SM1255` forced reset)** drops all Modbus
+  connections and the module re-listens after typically 5-10 seconds.
+- **Idle timeout**: QJ71MT91 and iQ-R have a per-connection "Alive-Check"
+  (idle timer) parameter, default 0 (disabled). If enabled, default 10 s
+  probe interval, 3 retries before close [8][9]. FX5U similar defaults.
+- **Keep-alive (TCP-level)**: only iQ-R / iQ-F expose a TCP keep-alive option
+  (parameter "KeepAlive" in the Ethernet settings); QJ71MT91 and FX3U-ENET
+  do not — so NAT/firewall idle drops require driver-side pinging.
+
+Test names:
+`Mitsubishi_QJ71MT91_17th_connection_refused`,
+`Mitsubishi_FX5U_9th_connection_refused`,
+`Mitsubishi_STOP_to_RUN_transition_preserves_socket`,
+`Mitsubishi_CPU_reset_closes_all_sockets`.
+
+## Behavioral oddities
+
+- **Transaction ID echo**: QJ71MT91 and iQ-R reliably echo the MBAP TxId on
+  every response across firmware revisions; no reports of TxId drops under
+  load [8][9]. FX3U-ENET has an older, less-tested TCP stack; at least one
+  MrPLC thread reports out-of-order TxId echoes under heavy polling on
+  firmware < 1.14 [4]. _Unconfirmed_ on current firmware.
+- **Per-connection request serialization**: all MELSEC slaves serialize
+  requests within a single TCP connection — a new request is not processed
+  until the prior response has been sent. Pipelining multiple requests on one
+  socket causes the module to queue them in buffer memory and respond in
+  order, but **the queue depth is 1** on QJ71MT91 (a second in-flight request
+  is held on the TCP receive buffer, not queued) [9]. Driver should treat
+  Mitsubishi slaves as strictly single-flight per socket.
+- **Partial-frame handling**: QJ71MT91 and iQ-R close the socket on malformed
+  MBAP length fields. FX5U resynchronises at the next valid MBAP header
+  within 100 ms but will emit an error to `SD` diagnostics [11]. Driver must
+  reconnect on half-close and replay.
+- **FX3U UDP vs TCP**: `FX3U-ENET` supports both UDP and TCP MODBUS transports;
+  UDP is lossy and reorders under load. Default is TCP. Some legacy SCADA
+  configurations pinned the module to UDP for multicast discovery — do not
+  select UDP unless the site requires it [4].
+- **Known firmware-revision variants**:
+    - QJ71MT91 ≤ firmware 10052000000 (year-month format): FC15 with coil
+      count that forces byte-count to an odd value silently truncates the
+      last coil. Fixed in later revisions [9]. _Operator-reported_.
+    - FX5U firmware < 1.060: no native MODBUS TCP server — only accessible via
+      a predefined-protocol function block hack. Firmware ≥ 1.060 ships
+      parameter-based server. Our capability probe should read `SD203`
+      (firmware version) and flag < 1.060 as unsupported for server mode [11][12].
+    - iQ-R RJ71EN71 early firmware: possible ABCD word order (rumoured,
+      unconfirmed) [8].
+- **SD (special-register) reads during assignment-parameter load**: while
+  the CPU is loading a new MODBUS device assignment parameter (~1-2 s), the
+  slave returns exception 04 Server Failure on every request. Happens after
+  a parameter write from GX Configurator-MB [9].
+- **iQ-R "Station-based block transfer" collision**: if the RJ71EN71 is also
+  running CC-Link IE Control on the same module, a MODBUS/TCP request that
+  arrives during a CCIE cyclic period is delayed to the next scan — visible
+  as jittery response time, not a failure [8].
+
+Test names:
+`Mitsubishi_QJ71MT91_single_flight_per_socket`,
+`Mitsubishi_FX5U_malformed_MBAP_resync_within_100ms`,
+`Mitsubishi_FX3U_TxId_preserved_across_burst`,
+`Mitsubishi_FX5U_firmware_below_1_060_reports_no_server_mode`.
+
+## Model-specific differences for test coverage
+
+Summary of which quirks differ per model, so test-class naming can reflect them:
+
+| Quirk                                    | QJ71MT91 | LJ71MT91 | iQ-R (RJ71EN71 / built-in) | iQ-F (FX5U) | FX3U-ENET(-P502) |
+|------------------------------------------|----------|----------|----------------------------|-------------|------------------|
+| FC16 Mask-Write supported                | No       | No       | Yes                        | Yes         | No               |
+| FC17 Read/Write Multiple supported       | No       | No       | Yes                        | Yes         | No               |
+| Max connections                          | 16       | 16       | 16                         | 8           | 8                |
+| X/Y numbering base                       | hex      | hex      | hex                        | octal (default) | octal        |
+| 32-bit word order                        | CDAB     | CDAB     | CDAB (firmware-dependent rumour of ABCD) | CDAB | CDAB    |
+| Port 502 supported                       | Yes      | Yes      | Yes                        | Yes         | P502 only        |
+| STOP-mode write permission configurable  | Yes      | Yes      | Yes                        | Yes         | No (always blocks) |
+| TCP keep-alive parameter                 | No       | No       | Yes                        | Yes         | No               |
+| Modbus device assignment — max entries   | 16       | 16       | 16                         | 16          | 8                |
+| Server via parameter (no FB)             | Yes      | Yes      | Yes                        | Yes (fw ≥ 1.060) | Yes         |
+
+- **Test file layout**: `Mitsubishi_QJ71MT91_*`, `Mitsubishi_LJ71MT91_*`,
+  `Mitsubishi_iQR_*`, `Mitsubishi_FX5U_*`, `Mitsubishi_FX3U_ENET_*`,
+  `Mitsubishi_FX3U_ENET_P502_*`. iQ-R built-in Ethernet and the RJ71EN71
+  behave identically for MODBUS/TCP slave purposes and can share a file
+  `Mitsubishi_iQR_*`.
+- **Cross-model shared tests** (word order CDAB, binary not BCD, standard
+  exception codes, 125-register FC03 cap) can live in a single
+  `Mitsubishi_Common_*` fixture.
+
+## References
+
+1. Mitsubishi Electric, *MODBUS Interface Module User's Manual — QJ71MB91*
+   (SH-080578ENG), RS-232/422/485 MODBUS RTU serial module for MELSEC-Q —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc/sh080578eng/sh080578engk.pdf
+2. Inductive Automation, *Ignition Modbus Driver — Mitsubishi Q / iQ-R word
+   order*, documents CDAB convention —
+   https://docs.inductiveautomation.com/docs/8.1/ignition-modules/opc-ua/drivers/modbus-v2
+   and forum discussion https://forum.inductiveautomation.com/t/modbus-tcp-device-word-byte-order/65984
+3. Mitsubishi Electric, *Programmable Controller User's Manual QJ71MB91 MODBUS
+   Interface Module*, Chapter 7 "Parameter Setting" describing the Modbus
+   Device Assignment Parameter block (assignments 1-16, head-device
+   configuration) —
+   https://www.lcautomation.com/dbdocument/29156/QJ71MB91%20Users%20manual.pdf
+4. Mitsubishi Electric, *FX3U-ENET User's Manual* (JY997D18101), Chapter on
+   MODBUS/TCP communication; function code support and connection limits —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy997d18101/jy997d18101h.pdf
+5. Venus Automation, *Mitsubishi FX3U-ENET-P502 Module — Open Port 502 for
+   Modbus TCP/IP* —
+   https://venusautomation.com.au/mitsubishi-fx3u-enet-p502-module-open-port-502-for-modbus-tcp-ip/
+   and FX3U-ENET-ADP user manual (JY997D45801), which confirms the -ADP
+   variant does not support MODBUS —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy997d45801/jy997d45801h.pdf
+6. XML Control / Ubidots integration notes, *FX3G Modbus* — port-502 trap,
+   D-register mapping default, word order reference —
+   https://sites.google.com/site/xmlcontrol/archive/fx3g-modbus
+   and https://ubidots.com/blog/mitsubishi-plc-as-modbus-tcp-server/
+7. FA Support Me, *Modbus TCP on Built-in Ethernet port in iQ-F and iQ-R* —
+   confirms 16-connection limit on iQ-R, 8 on iQ-F, parameter-driven
+   configuration via GX Works3 —
+   https://www.fasupportme.com/portal/en/kb/articles/modbus-tcp-on-build-in-ethernet-port-in-iq-f-and-iq-r-en
+8. Mitsubishi Electric, *MELSEC iQ-R Ethernet User's Manual (Application)*
+   (SH-081259ENG) and *MELSEC iQ-RJ71EN71 User's Manual* Chapter on
+   "Communications Using Modbus/TCP" —
+   https://www.allied-automation.com/wp-content/uploads/2015/02/MITSUBISHI_manual_plc_iq-r_ethernet_users.pdf
+   and https://www.manualslib.com/manual/1533351/Mitsubishi-Electric-Melsec-Iq-Rj71en71.html?page=109
+9. Mitsubishi Electric, *MODBUS/TCP Interface Module User's Manual — QJ71MT91*
+   (SH-080446ENG), exception codes page 248, device assignment parameter
+   pages 116-124, duplicate-assignment-disables-slave note —
+   https://dl.mitsubishielectric.com/dl/fa/document/manual/plc/sh080446eng/sh080446engj.pdf
+10. Mitsubishi Electric, *MELSEC-L Network Features* — LJ71MT91 documented as
+    L-series equivalent of QJ71MT91 with identical MODBUS/TCP behavior —
+    https://us.mitsubishielectric.com/fa/en/products/cnt/programmable-controllers/melsec-l-series/network/features/
+11. Mitsubishi Electric, *MELSEC iQ-F FX5 User's Manual (MODBUS Communication)*
+    (JY997D56101), Chapter 11 "Modbus/TCP Communication Specifications" —
+    function code max-quantity table, frame specification, device assignment
+    defaults —
+    https://dl.mitsubishielectric.com/dl/fa/document/manual/plcf/jy997d56101/jy997d56101h.pdf
+12. MrPLC forum, *FX5U Modbus-TCP Server (Slave)*, firmware ≥ 1.60 enables
+    native server via parameter; earlier firmware required function block —
+    https://mrplc.com/forums/topic/31883-fx5u-modbus-tcp-server-slave/
+    and Industrial Monitor Direct's "FX5U MODBUS TCP Server Workaround"
+    article (reflects older firmware behavior) —
+    https://industrialmonitordirect.com/blogs/knowledgebase/mitsubishi-fx5u-modbus-tcp-server-configuration-workaround
+13. Mitsubishi Electric, *MELSEC iQ-R MODBUS and MODBUS/TCP Reference Manual —
+    RJ71C24 / RJ71C24-R2* (BCN-P5999-1060) — RJ71C24 is serial RTU only,
+    not TCP —
+    https://dl.mitsubishielectric.com/dl/fa/document/manual/plc/bcn-p5999-1060/bcnp59991060b.pdf
+14. HMS Industrial Networks, *eWON and Mitsubishi FX5U PLC* (KB-0264-00) —
+    documents that FX5U X/Y are octal in GX Works3 but hex when viewed as a
+    Q-series PLC through eWON; the project-level hex/octal toggle —
+    https://hmsnetworks.blob.core.windows.net/www/docs/librariesprovider10/downloads-monitored/manuals/knowledge-base/kb-0264-00-en-ewon-and-mitsubishi-fx5u-plc.pdf
+15. Fernhill Software, *Mitsubishi Melsec PLC Data Address* — documents
+    hex-vs-octal device numbering split across MELSEC families —
+    https://www.fernhillsoftware.com/help/drivers/mitsubishi-melsec/data-address-format.html
+16. Inductive Automation support, *Understanding Mitsubishi PLCs* — D registers
+    store signed 16-bit binary, not BCD; DINT combines two consecutive D
+    registers —
+    https://support.inductiveautomation.com/hc/en-us/articles/16517576753165-Understanding-Mitsubishi-PLCs
+17. Mitsubishi Electric, *FXCPU Structured Programming Manual [Device &
+    Common]* (JY997D26001) — FNC 18 BCD and FNC 19 BIN explicit-conversion
+    instructions confirm binary-by-default storage —
+    https://dl.mitsubishielectric.com/dl/fa/document/manual/plc_fx/jy997d26001/jy997d26001l.pdf

docs/v2/s7.md

@@ -0,0 +1,485 @@
+# Siemens SIMATIC S7 (S7-1200 / S7-1500 / S7-300 / S7-400 / ET 200SP) — Modbus TCP quirks
+
+Siemens S7 PLCs do *not* speak Modbus TCP natively at the OS/firmware level. Every
+S7 Modbus-TCP-server deployment is either (a) the **`MB_SERVER`** library block
+running on the CPU's PROFINET port (S7-1200 / S7-1500 / CPU 1510SP-series
+ET 200SP), or (b) the **`MODBUSCP`** function block running on a separate
+communication processor (**CP 343-1 / CP 343-1 Lean** on S7-300, **CP 443-1** on
+S7-400), or (c) the **`MODBUSPN`** block on an S7-1500 PN port via a licensed
+library. That means the quirks a Modbus client has to cope with are as much
+"this is how the user's PLC programmer wired the library block up" as "this is
+how the firmware behaves" — the byte-order and coil-mapping rules aren't
+hard-wired into silicon like they are on a DL260. This document catalogues the
+behaviours a driver has to handle across the supported model/CP variants, cites
+primary sources, and names the ModbusPal integration test we'd write for each
+(convention from `docs/v2/modbus-test-plan.md`: `S7_<model>_<behavior>`).
+
+## Model / CP Capability Matrix
+
+| PLC family          | Modbus TCP server mechanism        | Modbus TCP client mechanism       | License required?     | Typical port 502 source                                   |
+|---------------------|------------------------------------|------------------------------------|-----------------------|-----------------------------------------------------------|
+| S7-1200 (V4.0+)     | `MB_SERVER` on integrated PN port  | `MB_CLIENT`                       | No (in TIA Portal)    | CPU's onboard Ethernet [1][2]                              |
+| S7-1500 (all)       | `MB_SERVER` on integrated PN port  | `MB_CLIENT`                       | No (in TIA Portal)    | CPU's onboard Ethernet [1][3]                              |
+| S7-1500 + CP 1543-1 | `MB_SERVER` on CP's IP             | `MB_CLIENT`                       | No                    | Separate CP IP address [1]                                 |
+| ET 200SP CPU (1510SP, 1512SP) | `MB_SERVER` on PN port   | `MB_CLIENT`                       | No                    | CPU's onboard Ethernet [3]                                 |
+| S7-300 + CP 343-1 / CP 343-1 Lean | `MODBUSCP` (FB `MODBUSCP`, instance DB per connection) | Same FB, client mode | **Yes — 2XV9450-1MB00** per CP | CP's Ethernet port [4][5] |
+| S7-400 + CP 443-1   | `MODBUSCP`                         | `MODBUSCP` client mode            | **Yes — 2XV9450-1MB00** per CP | CP's Ethernet port [4] |
+| S7-400H + CP 443-1 (redundant H) | `MODBUSCP_REDUNDANT` / paired FBs | Not typical | Yes | Paired CPs in H-system [6] |
+| S7-300 / S7-400 CPU PN (e.g. CPU 315-2 PN/DP) | `MODBUSPN` library | `MODBUSPN` client mode | **Yes** — Modbus-TCP PN CPU lib | CPU's PN port [7] |
+| "CP 343-1 Lean"     | **Server only** (no client mode supported by Lean) | — | Yes, but with restrictions | CP's Ethernet port [4][5] |
+
+- **CP 343-1 Lean is server-only.** It can host `MODBUSCP` in server mode only;
+  client calls return an immediate error. A surprising number of "Lean + client
+  doesn't work" forum posts trace back to this [5].
+- **Pure OPC UA / PROFINET CPs (CP 1542SP-1, CP 1543-1)** support Modbus TCP on
+  S7-1500 via the same `MB_SERVER`/`MB_CLIENT` instructions by passing the
+  CP's `hw_identifier`. There is no separate "Modbus CP" license needed on
+  S7-1500, unlike S7-300/400 [1].
+- **No S7 Modbus server supports function codes 20/21 (file records),
+  22 (mask write), 23 (read-write multiple), or 43 (device identification).**
+  Sending any of these returns exception `01` (Illegal Function) on every S7
+  variant [1][4]. Our driver must not negotiate FC23 as a "bulk-read optimization"
+  when the profile is S7.
+
+Test names:
+`S7_1200_MBSERVER_Loads_OB1_Cyclic`,
+`S7_CP343_Lean_Client_Mode_Rejected`,
+`S7_All_FC23_Returns_IllegalFunction`.
+
+## Address / DB Mapping
+
+S7 Modbus servers **do not auto-expose PLC memory** — the PLC programmer has to
+wire one area per Modbus table to a DB or process-image region. This is the
+single biggest difference vs. DL205/Modicon/etc., where the memory map is
+fixed at the factory. Our driver must therefore be tolerant of "the same
+`40001` means completely different things on two S7-1200s on the same site."
+
+### S7-1200 / S7-1500 `MB_SERVER`
+
+The `MB_SERVER` instance exposes four Modbus tables to each connected client;
+each table's backing storage is a per-block parameter [1][8]:
+
+| Modbus table        | FCs         | Backing parameter           | Default / typical backing   |
+|---------------------|-------------|-----------------------------|-----------------------------|
+| Coils (0x)          | FC01, FC05, FC15 | *implicit* — Q process image | `%Q0.0`–`%Q1023.7` (→ coil addresses 0–8191) [1][9] |
+| Discrete Inputs (1x)| FC02        | *implicit* — I process image | `%I0.0`–`%I1023.7` (→ discrete addresses 0–8191) [1][9] |
+| Input Registers (3x)| FC04        | *implicit* — M memory or DB (version-dependent) | Some firmware routes FC04 through the same MB_HOLD_REG buffer [1][8] |
+| Holding Registers (4x)| FC03, FC06, FC16 | `MB_HOLD_REG` pointer  | User DB (e.g. `DB10.DBW0`) or `%MW` area [1][2][8] |
+
+- **`MB_HOLD_REG` is a pointer (VARIANT / ANY) into a user-defined DB** whose
+  first byte is holding-register 0 (`40001` in 1-based Modicon form). Byte
+  offset 2 is register 1, byte offset 4 is register 2, etc. [1][2].
+- **The DB *must* have "Optimized block access" UNCHECKED.** Optimized DBs let
+  the compiler reorder fields for alignment; Modbus requires fixed byte
+  offsets. With optimized access on, the compiler accepts the project but
+  `MB_SERVER` returns STATUS `0x8383` (misaligned access) or silently reads
+  zeros [8][10][11]. This is the #1 support-forum complaint.
+- **FC01/FC02/FC05/FC15 hit the Q and I process images directly — not the
+  `MB_HOLD_REG` DB.** Coil address 0 = `%Q0.0`, coil 1 = `%Q0.1`, coil 8 =
+  `%Q1.0`. The S7-1200 system manual publishes this mapping as `00001 → Q0.0`
+  through `09999 → Q1023.7` and `10001 → I0.0` through `19999 → I1023.7` in
+  1-based form; on the wire (0-based) that's coils 0-8191 and discrete inputs
+  0-8191 [9].
+- **`%M` markers are NOT automatically exposed.** To expose `%M` over Modbus
+  the programmer must either (a) copy `%M` to the `MB_HOLD_REG` DB each scan,
+  or (b) define an Array\[0..n\] of Bool inside that DB and copy bits in/out
+  of `%M`. Siemens has no "MB_COIL_REG" parameter analogous to
+  `MB_HOLD_REG` — this confuses users migrating from Schneider [9][12].
+- **Bit ordering within a Modbus holding register sourced from an `Array of
+  Bool`**: S7 stores bool\[0\] at `DBX0.0` which is bit 0 of byte 0 which is
+  the **low byte, low bit** of Modbus register `40001`. A naive client that
+  reads register `40001` and masks `0x0001` gets bool\[0\]. A client that
+  masks `0x8000` gets bool\[15\] because the high byte of the Modbus register
+  is the *second* byte of the DB. Siemens programmers routinely get this
+  wrong in the DB-via-DBX form; `Array[0..n] of Bool` is the recommended
+  layout because it aligns naturally [12][13].
+
+### S7-300/400 + CP 343-1 / CP 443-1 `MODBUSCP`
+
+Different paradigm: per-connection **parameter DB** (template
+`MODBUS_PARAM_CP`) declares a table of up to 8 register-area mappings. Each
+mapping is a tuple `(data_type, DB#, start_offset, length)` where `data_type`
+picks the Modbus table [4]:
+
+- `B#16#1` = Coils
+- `B#16#2` = Discrete Inputs
+- `B#16#3` = Holding Registers
+- `B#16#4` = Input Registers
+
+The `holding_register_start` and analogous `coils_start` parameters declare
+**which Modbus address range** the CP will serve, and the DB pointers say
+where in S7 memory that range lives [4][14]. Unlike `MB_SERVER`, the CP does
+not reach into `%Q`/`%I` directly — *everything* goes through a DB. If an
+address outside the declared ranges is requested, the CP returns exception
+`02` (Illegal Data Address) [4].
+
+Test names:
+`S7_1200_FC03_Reg0_Reads_DB10_DBW0`,
+`S7_1200_Optimized_DB_Returns_0x8383_MisalignedAccess`,
+`S7_1200_FC01_Coil0_Reads_Q0_0`,
+`S7_CP343_FC03_Outside_ParamBlock_Range_Returns_IllegalDataAddress`.
+
+## Data Types and Byte Order
+
+Siemens CPUs store scalars **big-endian** internally ("Motorola format"), which
+is the same byte order Modbus specifies inside each register. So for 16-bit
+values (`Int`, `Word`, `UInt`) the on-the-wire layout is straightforward
+`AB` — high byte of the PLC value in the high byte of the Modbus register
+[15][16]. No byte-swap trap for 16-bit types.
+
+The trap is 32-bit types (`DInt`, `DWord`, `Real`). Here's what actually
+happens across the S7 family:
+
+### S7-1200 / S7-1500 `MB_SERVER`
+
+- **The backing DB stores 32-bit values in big-endian byte order, high word
+  first** — i.e. `ABCD` when viewed as two consecutive Modbus registers. A
+  `Real` at `DB10.DBD0` with value `0x12345678` reads over Modbus as
+  register 0 = `0x1234`, register 1 = `0x5678` [15][16][17].
+- **This is `ABCD`, *not* `CDAB`.** Clients that hard-code CDAB (common default
+  for meters and VFDs) will get wildly wrong floats. Configure the S7 profile
+  with `WordOrder = ABCD` (aka "big-endian word + big-endian byte" aka
+  "high-word first") [15][17].
+- **`MB_SERVER` does not swap.** It's a direct memcpy from the DB bytes to
+  the Modbus payload. Whatever byte order the ladder programmer stored into
+  the DB is what the client receives [17]. This means a programmer who used
+  `MOVE_BLK` from two separate `Word`s into `DBD` with the "wrong" order can
+  produce `CDAB` without realising.
+- **`Real` is IEEE 754 single-precision** — unambiguous, no BCD trap like on
+  DL series [15].
+- **Strings**: S7 `String[n]` has a 2-byte header (max length, current length)
+  *before* the character bytes. A client reading a string over Modbus gets
+  the header in the first register and then the characters two-per-register
+  in high-byte-first order. `WString` is UTF-16 and the header is 4 bytes
+  [18]. Our driver's string decoder must expose the "skip header" option for
+  S7 profile.
+
+### S7-300/400 `MODBUSCP` (CP 343-1 / CP 443-1)
+
+- The CP writes the exact DB bytes onto the wire — again `ABCD` if the DB
+  stores `DInt`/`Real` in native Siemens order [4].
+- **`MODBUSCP` has no `data_type` byte-swap knob.** (The `data_type` parameter
+  names the Modbus table, not the byte order — see the Address Mapping
+  section.) If the other end of the link expects `CDAB`, the programmer has
+  to swap words in ladder before writing the DB [4][14].
+
+### Operator-reported oddity
+
+- Some S7 drivers (Kepware's "Siemens TCP/IP Ethernet" driver, Ignition's
+  "Siemens S7" driver) expose a per-tag `Float Byte Order` with options
+  `ABCD`/`CDAB`/`BADC`/`DCBA` because end-users have encountered every
+  permutation in the field — not because the PLC natively swaps, but because
+  ladder programmers have historically stored floats every which way [19].
+  Our S7 Modbus profile should default to `ABCD` but expose a per-tag
+  override.
+- **Unconfirmed rumour**: that S7-1500 firmware V2.0+ reverses float byte
+  order for `MB_CLIENT` only. Not reproduced; the Siemens forum thread that
+  launched it was a user error (the remote server was the swapper, not the
+  S7) [20]. Treat as false until proven.
+
+Test names:
+`S7_1200_Real_WordOrder_ABCD_Default`,
+`S7_1200_DInt_HighWord_First_At_DBD0`,
+`S7_1200_String_Header_First_Two_Bytes`,
+`S7_CP343_No_Internal_ByteSwap`.
+
+## Coil / Discrete Input Mapping
+
+On `MB_SERVER` the mapping from coil address → S7 bit is fixed at the
+process-image level [1][9][12]:
+
+| Modbus coil / discrete input addr | S7 address    | Notes                               |
+|-----------------------------------|---------------|-------------------------------------|
+| Coil 0 (FC01/05/15)               | `%Q0.0`       | bit 0 of output byte 0              |
+| Coil 7                            | `%Q0.7`       | bit 7 of output byte 0              |
+| Coil 8                            | `%Q1.0`       | bit 0 of output byte 1              |
+| Coil 8191 (max)                   | `%Q1023.7`    | highest exposed output bit          |
+| Discrete input 0 (FC02)           | `%I0.0`       | bit 0 of input byte 0               |
+| Discrete input 8191               | `%I1023.7`    | highest exposed input bit           |
+
+Formulas:
+
+```
+coil_addr   = byte_index * 8 + bit_index    (e.g. %Q5.3 → coil 43)
+discr_addr  = byte_index * 8 + bit_index    (e.g. %I10.2 → disc 82)
+```
+
+- **1-based Modicon form adds 1:** coil 0 (wire) = `00001` (Modicon), etc.
+  Our driver sends the 0-based PDU form, so `%Q0.0` writes to wire address 0.
+- **Writing FC05/FC15 to `%Q` is accepted even while the CPU is in STOP** —
+  the PLC's process image doesn't care about the user program state. But the
+  output won't propagate to the physical module until RUN (see STOP section
+  below) [1][21].
+- **`%M` markers require a DB-backed `Array of Bool`** as described in the
+  Address Mapping section. Our driver can't assume "coil N = MN.0" like it
+  can on Modicon — on S7 it's always Q/I unless the programmer built a
+  mapping DB [12].
+- **Bit-inside-holding-register**: for `Array of Bool` inside the
+  `MB_HOLD_REG` DB, bool[0] is bit 0 of byte 0 → **low byte, low bit** of
+  Modbus register 40001. Most third-party clients probe this in the low
+  byte, so the common case works; the less-common case (bool[8]) is bit 0 of
+  byte 1 → **high byte, low bit** of Modbus register 40001. Clients that
+  test only bool[0] will pass and miss the mis-alignment on bool[8] [12][13].
+
+Test names:
+`S7_1200_Coil_0_Is_Q0_0`,
+`S7_1200_Coil_8_Is_Q1_0`,
+`S7_1200_Discrete_Input_7_Is_I0_7`,
+`S7_1200_Coil_Write_In_STOP_Accepted_But_Output_Frozen`.
+
+## Function Code Support & Max Registers Per Request
+
+| FC | Name                       | S7-1200 / S7-1500 MB_SERVER | CP 343-1 / CP 443-1 MODBUSCP | Max qty per request            |
+|----|----------------------------|-----------------------------|------------------------------|--------------------------------|
+| 01 | Read Coils                 | Yes                         | Yes                          | 2000 bits (spec)               |
+| 02 | Read Discrete Inputs       | Yes                         | Yes                          | 2000 bits (spec)               |
+| 03 | Read Holding Registers     | Yes                         | Yes                          | **125** (spec max)             |
+| 04 | Read Input Registers       | Yes                         | Yes                          | **125**                        |
+| 05 | Write Single Coil          | Yes                         | Yes                          | 1                              |
+| 06 | Write Single Register      | Yes                         | Yes                          | 1                              |
+| 15 | Write Multiple Coils       | Yes                         | Yes                          | 1968 bits (spec) — *see note*  |
+| 16 | Write Multiple Registers   | Yes                         | Yes                          | **123** (spec max for TCP)     |
+| 07 | Read Exception Status      | No (RTU only)               | No                           | —                              |
+| 17 | Report Server ID           | No                          | No                           | —                              |
+| 20/21 | Read/Write File Record  | No                          | No                           | —                              |
+| 22 | Mask Write Register        | No                          | No                           | —                              |
+| 23 | Read/Write Multiple        | No                          | No                           | —                              |
+| 43 | Read Device Identification | No                          | No                           | —                              |
+
+- **S7-1200/1500 honour the full spec maxima** for FC03/04 (125) and FC16
+  (123) [1][22]. No sub-spec cap like DL260's 100-register FC16 limit.
+- **FC15 (Write Multiple Coils) on `MB_SERVER`** writes into `%Q`, which maxes
+  out at 1024 bytes = 8192 bits, but the spec's 1968-bit per-request limit
+  caps any single call first [1][9].
+- **`MB_HOLD_REG` buffer size is bounded by DB size** — max DB size on
+  S7-1200 is 64 KB, on S7-1500 is much larger (several MB depending on CPU),
+  so the practical `MB_HOLD_REG` limit is 32767 16-bit registers on S7-1200
+  and effectively unbounded on S7-1500 [22][23]. The *per-request* limit is
+  still 125.
+- **Read past the end of `MB_HOLD_REG`** returns exception `02` (Illegal
+  Data Address) at the start of the overflow register, not a partial read
+  [1][8].
+- **Request larger than spec max** (e.g. FC03 quantity 126) returns exception
+  `03` (Illegal Data Value). Verified on S7-1200 V4.2 [1][24].
+- **CP 343-1 `MODBUSCP` per-request maxima are spec** (125/125/123/1968/2000),
+  matching the standard [4]. The CP's `MODBUS_PARAM_CP` caps the total
+  *exposed* range, not the per-call quantity.
+
+Test names:
+`S7_1200_FC03_126_Registers_Returns_IllegalDataValue`,
+`S7_1200_FC16_124_Registers_Returns_IllegalDataValue`,
+`S7_1200_FC03_Past_MB_HOLD_REG_End_Returns_IllegalDataAddress`,
+`S7_1200_FC17_ReportServerId_Returns_IllegalFunction`.
+
+## Exception Codes
+
+S7 Modbus servers return only the four standard exception codes [1][4]:
+
+| Code | Name                  | Triggered by                                                         |
+|------|-----------------------|----------------------------------------------------------------------|
+| 01   | Illegal Function      | FC not in the supported list (17, 20-23, 43, any undefined FC)       |
+| 02   | Illegal Data Address  | Register outside `MB_HOLD_REG` / outside `MODBUSCP` param-block range |
+| 03   | Illegal Data Value    | Quantity exceeds spec (FC03/04 > 125, FC16 > 123, FC01/02 > 2000, FC15 > 1968) |
+| 04   | Server Failure        | Runtime error inside MB_SERVER (DB access fault, corrupt DB header, MB_SERVER disabled mid-request) [1][24] |
+
+- **No proprietary exception codes (05/06/0A/0B) are used** on any S7
+  Modbus server [1][4]. Our driver's status-code mapper can treat these as
+  "never observed" on the S7 profile.
+- **CPU in STOP → `MB_SERVER` keeps running if it's in OB1 of the firmware's
+  communication task, but OB1 itself is not scanned.** In practice:
+  - Holding-register *reads* (FC03) continue to return the last DB values
+    frozen at the moment the CPU entered STOP. The `MB_SERVER` block is in
+    OB1 so it isn't re-invoked; however the TCP stack keeps the socket open
+    and returns cached data on subsequent polls [1][21]. **Unconfirmed**
+    whether this is cached in the CP or in the CPU's communication processor;
+    behaviour varies between firmware 4.0 and 4.5 [21].
+  - Holding-register *writes* (FC06/FC16) during STOP return exception `04`
+    (Server Failure) on S7-1200 V4.2+, and return success-but-discarded on
+    older firmware [1][24]. Our driver should treat FC06/FC16 during STOP as
+    non-deterministic and not rely on the response code.
+  - Coil *writes* (FC05/FC15) to `%Q` are *accepted* by the process image
+    during STOP, but the physical output freezes at its last RUN-mode value
+    (or the configured STOP-mode substitute value) until RUN resumes [1][21].
+- **Writing a read-only address via FC06/FC16**: returns `02` (Illegal Data
+  Address), not `04`. S7 does not have "write-protected" holding registers —
+  the programmer either exposes a DB for read-write or doesn't expose it at
+  all [1][12].
+
+STATUS codes (returned in the `STATUS` output of the block, not on the wire):
+
+- `0x0000` — no error.
+- `0x7001` — first call, connection being established.
+- `0x7002` — subsequent cyclic call, connection in progress.
+- `0x8383` — data access error (optimized DB, DB too small, or type mismatch)
+  [10][24].
+- `0x8188` — invalid parameter combination (e.g. MB_MODE out of range) [24].
+- `0x80C8` — mismatched UNIT_ID between MB_CLIENT and `MB_SERVER` [25].
+
+Test names:
+`S7_1200_FC03_Outside_HoldReg_Returns_IllegalDataAddress`,
+`S7_1200_FC16_In_STOP_Returns_ServerFailure`,
+`S7_1200_FC03_In_STOP_Returns_Cached_Values`,
+`S7_1200_No_Proprietary_ExceptionCodes_0x05_0x06_0x0A_0x0B`.
+
+## Connection Behavior
+
+- **Max simultaneous Modbus TCP connections**:
+  - **S7-1200**: shares a pool of 8 open-communication connections across
+    all TCP/UDP/Modbus use. On a CPU 1211C you get 8 total; on 1215C/1217C
+    still 8 shared among PG/HMI/OUC/Modbus. Each `MB_SERVER` instance
+    reserves one. A typical site with a PG + 1 HMI + 2 Modbus clients uses
+    4 of the 8 [1][26].
+  - **S7-1500**: up to **8 concurrent Modbus TCP server connections** per
+    `MB_SERVER` port, across multiple `MB_SERVER` instance DBs each with a
+    unique port. Total open-communication resources depend on CPU (e.g.
+    CPU 1515-2 PN supports 128 OUC connections total; Modbus is a subset)
+    [1][27].
+  - **CP 343-1 Lean**: up to **8** simultaneous Modbus TCP connections on
+    port 502 [4][5]. Exceeding this refuses at TCP accept.
+  - **CP 443-1 Advanced**: up to **16** simultaneous Modbus TCP connections
+    [4].
+- **Multi-connection model on `MB_SERVER`**: one instance DB per connection.
+  An instance DB listening on port 502 serves exactly one connection at a
+  time; to serve N simultaneous clients you need N instance DBs each with a
+  unique port (502/503/504...). **This is a real trap** — most users expect
+  port 502 to multiplex [27][28]. Our driver must not assume port 502 is the
+  only listener.
+- **Keep-alive**: S7-1500's TCP stack does send TCP keepalives (default
+  every ~30 s) but the interval is not exposed as a configurable. S7-1200 is
+  the same. CP 343-1 keepalives are configured via HW Config → CP properties
+  → Options → "Send keepalive" (default **off** on older firmware, default
+  **on** on firmware V3.0+) [1][29]. Driver-side keepalive is still
+  advisable for S7-300/CP 343-1 on old firmware.
+- **Idle-timeout close**: `MB_SERVER` does *not* close idle sockets on its
+  own. However, the TCP stack on S7-1500 will close a socket that fails
+  three consecutive keepalive probes (~2 minutes). Forum reports describe
+  `MB_SERVER` connections "dying overnight" on S7-1500 when an HMI stops
+  polling — the fix is to enable driver-side periodic reads or driver-side
+  TCP keepalive [29][30].
+- **Reconnect after power cycle**: MB_SERVER starts listening ~1-2 seconds
+  after the CPU reaches RUN. If the client reconnects during STARTUP OB
+  (OB100), the connection is refused until OB1 runs the block at least once.
+  Our driver should back off and retry on `ECONNREFUSED` for the first 5
+  seconds after a power-cycle detection [1][24].
+- **Unit Identifier**: `MB_SERVER` accepts **any** Unit ID by default — there
+  is no configurable filter; the PLC ignores the Unit ID field entirely.
+  `MB_CLIENT` defaults to Unit ID = 255 as "ignore" [25][31]. Some
+  third-party Modbus-TCP gateways *require* a specific Unit ID; sending
+  anything to S7 is safe. **CP 343-1 `MODBUSCP`** also accepts any Unit ID
+  in server mode, but the parameter DB exposes a `single_write` / `unit_id`
+  field on newer firmware to allow filtering [4].
+
+Test names:
+`S7_1200_9th_TCP_Connection_Refused_On_8_Conn_Pool`,
+`S7_1500_Port_503_Required_For_Second_Instance`,
+`S7_1200_Reconnect_After_Power_Cycle_Succeeds_Within_5s`,
+`S7_1200_Unit_ID_Ignored_Any_Accepted`.
+
+## Behavioral Oddities
+
+- **Transaction ID echo** is reliable on all S7 variants. `MB_SERVER` copies
+  the MBAP TxId verbatim. No known firmware that drops TxId under load [1][31].
+- **Request serialization**: a single `MB_SERVER` instance serializes
+  requests from its one connected client — the block processes one PDU per
+  call and calls happen once per OB1 scan. OB1 scan time of 5-50 ms puts an
+  upper bound on throughput at ~20-200 requests/sec per connection [1][30].
+  Multiple `MB_SERVER` instances (one per port) run in parallel because OB1
+  calls them sequentially within the same scan.
+- **OB1 scan coupling**: `MB_SERVER` must be called cyclically from OB1 (or
+  another cyclic OB). If the programmer puts it in a conditional branch
+  that doesn't fire every scan, requests time out. The STATUS `0x7002`
+  "in progress" is *expected* between calls, not an error [1][24].
+- **Optimized DB backing `MB_HOLD_REG`** — already covered in Address
+  Mapping; STATUS becomes `0x8383`. This is the most common deployment bug
+  on S7-1500 projects migrated from older S7-1200 examples [10][11].
+- **CPU STOP behaviour** — covered in Exception Codes section. The short
+  version: reads may return stale data without error; writes return exception
+  04 on modern firmware.
+- **Partial-frame disconnect**: S7-1200/1500 TCP stack closes the socket on
+  any MBAP header where the `Length` field doesn't match the PDU length.
+  Driver must detect half-close and reconnect [1][29].
+- **MBAP `Protocol ID` must be 0**. Any non-zero value causes the CP/CPU to
+  drop the frame silently (no response, no RST) on S7-1500 firmware V2.0
+  through V2.9; firmware V3.0+ sends an RST [1][30]. *Unconfirmed* whether
+  V3.1 still sends RST or returns to silent drop.
+- **FC01/FC02 access outside `%Q`/`%I` range**: on S7-1200, requesting
+  coil address 8192 (= `%Q1024.0`) returns exception `02` (Illegal Data
+  Address) [1][9]. The 8192-bit hard cap is a process-image size limit on
+  the CPU, not a Modbus protocol limit.
+- **`MB_CLIENT` UNIT_ID mismatch with remote `MB_SERVER`** produces STATUS
+  `0x80C8` on the client side, and the server silently discards the frame
+  (no response on the wire) [25]. This matters for Modbus-TCP-to-RTU
+  gateway scenarios where the Unit ID picks the RTU slave.
+- **Non-IEEE REAL / BCD**: S7 does *not* use BCD like DirectLOGIC. `Real` is
+  always IEEE 754 single-precision. `LReal` (8-byte double) occupies 4
+  Modbus registers in `ABCDEFGH` order (big-endian byte, big-endian word)
+  [15][18].
+- **`MODBUSCP` single-write** on CP 343-1: a parameter `single_write` in the
+  param DB controls whether FC06 on a register in the "holding register"
+  area triggers a callback to the user program vs. updates the DB directly.
+  Default is direct update. If a ladder programmer enables the callback
+  without implementing the callback OB, FC06 writes hang for 5 seconds then
+  return exception `04` [4].
+
+Test names:
+`S7_1200_TxId_Preserved_Across_Burst_Of_50_Requests`,
+`S7_1200_MBSERVER_Throughput_Capped_By_OB1_Scan`,
+`S7_1200_MBAP_ProtocolID_NonZero_Frame_Dropped`,
+`S7_1200_Partial_MBAP_Causes_Half_Close`.
+
+## Model-specific Differences Worth Separate Test Coverage
+
+- **S7-1200 V4.0 vs V4.4+**: Older firmware does not support `WString` over
+  `MB_HOLD_REG` and returns `0x8383` if the DB contains one [18][24]. Test
+  both firmware bands separately.
+- **S7-1500 vs S7-1200**: S7-1500 supports multiple `MB_SERVER` instances on
+  the *same* CPU with different ports cleanly; S7-1200 can too but its
+  8-connection pool is shared tighter [1][27]. Throughput per-connection is
+  ~5× faster on S7-1500 because the comms task runs on a dedicated core.
+- **S7-300 + CP 343-1 vs S7-1200/1500**: parameter-block mapping (not
+  `MB_HOLD_REG` pointer), per-connection license, no `%Q`/`%I` direct
+  access for coils (everything goes through a DB), different STATUS codes
+  (`DONE`/`ERROR`/`STATUS` word pairs vs. the single STATUS word) [4][14].
+  Driver-side it's a different profile.
+- **CP 343-1 Lean vs CP 343-1 Advanced**: Lean is server-only; Advanced is
+  client + server. Lean's max connections = 8; Advanced = 16 [4][5].
+- **CP 443-1 in S7-400H**: uses `MODBUSCP_REDUNDANT` which presents two
+  Ethernet endpoints that fail over. Our driver's redundancy support should
+  recognize the S7-400H profile as "two IP addresses, same server state,
+  advertise via `ServerUriArray`" [6].
+- **ET 200SP CPU (1510SP / 1512SP)**: behaves as S7-1500 from `MB_SERVER`
+  perspective. No known deltas [3].
+
+## References
+
+1. Siemens Industry Online Support, *Modbus/TCP Communication between SIMATIC S7-1500 / S7-1200 and Modbus/TCP Controllers with Instructions `MB_CLIENT` and `MB_SERVER`*, Entry ID 102020340, V6 (Feb 2021). https://cache.industry.siemens.com/dl/files/340/102020340/att_118119/v6/net_modbus_tcp_s7-1500_s7-1200_en.pdf
+2. Siemens TIA Portal Online Docs, *MB_SERVER instruction*. https://docs.tia.siemens.cloud/r/simatic_s7_1200_manual_collection_eses_20/communication-processor-and-modbus-tcp/modbus-communication/modbus-tcp/modbus-tcp-instructions/mb_server-communicate-using-profinet-as-modbus-tcp-server-instruction
+3. Siemens, *SIMATIC S7-1500 Communication Function Manual* (covers ET 200SP CPU). http://public.eandm.com/Public_Docs/s71500_communication_function_manual_en-US_en-US.pdf
+4. Siemens Industry Online Support, *SIMATIC Modbus/TCP communication using CP 343-1 and CP 443-1 — Programming Manual*, Entry ID 103447617. https://cache.industry.siemens.com/dl/files/617/103447617/att_106971/v1/simatic_modbus_tcp_cp_en-US_en-US.pdf
+5. Siemens Industry Online Support FAQ *"Which technical data applies for the SIMATIC Modbus/TCP software for CP 343-1 / CP 443-1?"*, Entry ID 104946406. https://www.industry-mobile-support.siemens-info.com/en/article/detail/104946406
+6. Siemens Industry Online Support, *Redundant Modbus/TCP communication via CP 443-1 in S7-400H systems*, Entry ID 109739212. https://cache.industry.siemens.com/dl/files/212/109739212/att_887886/v1/SIMATIC_modbus_tcp_cp_red_e_en-US.pdf
+7. Siemens Industry Online Support, *SIMATIC MODBUS (TCP) PN CPU Library — Programming and Operating Manual 06/2014*, Entry ID 75330636. https://support.industry.siemens.com/cs/attachments/75330636/ModbusTCPPNCPUen.pdf
+8. DMC Inc., *Using an S7-1200 PLC as a Modbus TCP Slave*. https://www.dmcinfo.com/blog/27313/using-an-s7-1200-plc-as-a-modbus-tcp-slave/
+9. Siemens, *SIMATIC S7-1200 System Manual* (V4.x), "MB_SERVER" pages 736-742. https://www.manualslib.com/manual/1453610/Siemens-S7-1200.html?page=736
+10. lamaPLC, *Simatic Modbus S7 error- and statuscodes*. https://www.lamaplc.com/doku.php?id=simatic:errorcodes
+11. ScadaProtocols, *How to Configure Modbus TCP on Siemens S7-1200 (TIA Portal Step-by-Step)*. https://scadaprotocols.com/modbus-tcp-siemens-s7-1200-tia-portal/
+12. Industrial Monitor Direct, *Reading and Writing Memory Bits via Modbus TCP on S7-1200*. https://industrialmonitordirect.com/blogs/knowledgebase/reading-and-writing-memory-bits-via-modbus-tcp-on-s7-1200
+13. PLCtalk forum *"Siemens S7-1200 modbus understanding"*. https://www.plctalk.net/forums/threads/siemens-s7-1200-modbus-understanding.104119/
+14. Siemens SIMATIC S7 Manual, "Function block MODBUSCP — Functionality" (ManualsLib p29). https://www.manualslib.com/manual/1580661/Siemens-Simatic-S7.html?page=29
+15. Chipkin, *How Real (Floating Point) and 32-bit Data is Encoded in Modbus*. https://store.chipkin.com/articles/how-real-floating-point-and-32-bit-data-is-encoded-in-modbus-rtu-messages
+16. Siemens Industry Online Support forum, *MODBUS DATA conversion in S7-1200 CPU*, Entry ID 97287. https://support.industry.siemens.com/forum/WW/en/posts/modbus-data-converson-in-s7-1200-cpu/97287
+17. Industrial Monitor Direct, *Siemens S7-1500 MB_SERVER Modbus TCP Configuration Guide*. https://industrialmonitordirect.com/de/blogs/knowledgebase/siemens-s7-1500-mb-server-modbus-tcp-configuration-guide
+18. Siemens TIA Portal, *Data types in SIMATIC S7-1200/1500 — String/WString header layout* (system manual, "Elementary Data Types").
+19. Kepware / PTC, *Siemens TCP/IP Ethernet Driver Help*, "Byte / Word Order" tag property. https://www.opcturkey.com/uploads/siemens-tcp-ip-ethernet-manual.pdf
+20. Siemens SiePortal forum, *Transfer float out of words*, Entry ID 187811. https://sieportal.siemens.com/en-ww/support/forum/posts/transfer-float-out-of-words/187811 _(operator-reported "S7 swaps float" claim — traced to remote-device issue; **unconfirmed**.)_
+21. Siemens SiePortal forum, *S7-1200 communication with Modbus TCP*, Entry ID 133086. https://support.industry.siemens.com/forum/WW/en/posts/s7-1200-communication-with-modbus-tcp/133086
+22. Siemens SiePortal forum, *S7-1500 MB Server Holding Register Max Word*, Entry ID 224636. https://support.industry.siemens.com/forum/WW/en/posts/s7-1500-mb-server-holding-register-max-word/224636
+23. Siemens, *SIMATIC S7-1500 Technical Specifications* — CPU-specific DB size limits in each CPU manual's "Memory" table.
+24. Siemens TIA Portal Online Docs, *Error messages (S7-1200, S7-1500) — Modbus instructions*. https://docs.tia.siemens.cloud/r/en-us/v20/modbus-rtu-s7-1200-s7-1500/error-messages-s7-1200-s7-1500
+25. Industrial Monitor Direct, *Fix Siemens S7-1500 MB_Client UnitID Error 80C8*. https://industrialmonitordirect.com/blogs/knowledgebase/troubleshooting-mb-client-on-s7-1500-cpu-1515sp-modbus-tcp
+26. Siemens SiePortal forum, *How many TCP connections can the S7-1200 make?*, Entry ID 275570. https://support.industry.siemens.com/forum/WW/en/posts/how-many-tcp-connections-can-the-s7-1200-make/275570
+27. Siemens SiePortal forum, *Simultaneous connections of Modbus TCP*, Entry ID 189626. https://support.industry.siemens.com/forum/ww/en/posts/simultaneous-connections-of-modbus-tcp/189626
+28. Siemens SiePortal forum, *How many Modbus TCP IP clients can read simultaneously from S7-1517*, Entry ID 261569. https://support.industry.siemens.com/forum/WW/en/posts/how-many-modbus-tcp-ip-client-can-read-simultaneously-in-s7-1517/261569
+29. Industrial Monitor Direct, *Troubleshooting Intermittent Modbus TCP Connections on S7-1500 PLC*. https://industrialmonitordirect.com/blogs/knowledgebase/troubleshooting-intermittent-modbus-tcp-connections-on-s7-1500-plc
+30. PLCtalk forum *"S7-1500 modbus tcp speed?"*. https://www.plctalk.net/forums/threads/s7-1500-modbus-tcp-speed.114046/
+31. Siemens SiePortal forum, *MB_Unit_ID parameter in Modbus TCP*, Entry ID 156635. https://support.industry.siemens.com/forum/WW/en/posts/mb-unit-id-parameter-in-modbus-tcp/156635

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

@@ -0,0 +1,161 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Mitsubishi MELSEC PLC family selector for address-translation helpers. The Q/L/iQ-R
+///     families write bit-device addresses (X, Y) in <b>hexadecimal</b> in GX Works and the
+///     CPU manuals; the FX and iQ-F families write them in <b>octal</b> (same convention as
+///     AutomationDirect DirectLOGIC). Mixing the two up is the #1 MELSEC driver bug source —
+///     an operator typing <c>X20</c> into a Q-series tag config means decimal 32, but the
+///     same string on an FX3U means decimal 16, so the helper must know the family to route
+///     correctly.
+/// </summary>
+public enum MelsecFamily
+{
+    /// <summary>
+    ///     MELSEC-Q / MELSEC-L / MELSEC iQ-R. X and Y device numbers are interpreted as
+    ///     <b>hexadecimal</b>; <c>X20</c> means decimal 32.
+    /// </summary>
+    Q_L_iQR,
+
+    /// <summary>
+    ///     MELSEC-F (FX3U / FX3GE / FX3G) and MELSEC iQ-F (FX5U). X and Y device numbers
+    ///     are interpreted as <b>octal</b> (same as DirectLOGIC); <c>X20</c> means decimal 16.
+    ///     iQ-F has a GX Works3 project toggle that can flip to decimal — if a site uses
+    ///     that, configure the tag's Address directly as a decimal PDU address and do not
+    ///     route through this helper.
+    /// </summary>
+    F_iQF,
+}
+
+/// <summary>
+///     Mitsubishi MELSEC address-translation helpers for the QJ71MT91 / LJ71MT91 / RJ71EN71 /
+///     iQ-R built-in / iQ-F / FX3U-ENET-P502 Modbus modules. MELSEC does NOT hard-wire
+///     Modbus-to-device mappings like DL260 does — every site configures its own "Modbus
+///     Device Assignment Parameter" block of up to 16 entries. The helpers here cover only
+///     the <b>address-notation</b> portion of the translation (hex X20 vs octal X20 + adding
+///     the bank base); the caller is still responsible for knowing the assignment-block
+///     offset for their site.
+/// </summary>
+/// <remarks>
+///     See <c>docs/v2/mitsubishi.md</c> §device-assignment + §X-Y-hex-trap for the full
+///     matrix and primary-source citations.
+/// </remarks>
+public static class MelsecAddress
+{
+    /// <summary>
+    ///     Translate a MELSEC X-input address (e.g. <c>"X0"</c>, <c>"X10"</c>) to a 0-based
+    ///     Modbus discrete-input address, given the PLC family's address notation (hex or
+    ///     octal) and the Modbus Device Assignment block's X-range base.
+    /// </summary>
+    /// <param name="xAddress">MELSEC X address. <c>X</c> prefix optional, case-insensitive.</param>
+    /// <param name="family">The PLC family — determines whether the trailing digits are hex or octal.</param>
+    /// <param name="xBankBase">
+    ///     0-based Modbus DI address the assignment-block has configured X0 to land at.
+    ///     Typical default on QJ71MT91 sample projects: 0. Pass the site-specific value.
+    /// </param>
+    public static ushort XInputToDiscrete(string xAddress, MelsecFamily family, ushort xBankBase = 0) =>
+        AddFamilyOffset(xBankBase, StripPrefix(xAddress, 'X'), family);
+
+    /// <summary>
+    ///     Translate a MELSEC Y-output address to a 0-based Modbus coil address. Same rules
+    ///     as <see cref="XInputToDiscrete"/> for hex/octal parsing.
+    /// </summary>
+    public static ushort YOutputToCoil(string yAddress, MelsecFamily family, ushort yBankBase = 0) =>
+        AddFamilyOffset(yBankBase, StripPrefix(yAddress, 'Y'), family);
+
+    /// <summary>
+    ///     Translate a MELSEC M-relay address (internal relay) to a 0-based Modbus coil
+    ///     address. M-addresses are <b>decimal</b> on every MELSEC family — unlike X/Y which
+    ///     are hex on Q/L/iQ-R. Includes the bank base that the assignment-block configured.
+    /// </summary>
+    public static ushort MRelayToCoil(string mAddress, ushort mBankBase = 0)
+    {
+        var digits = StripPrefix(mAddress, 'M');
+        if (!ushort.TryParse(digits, out var offset))
+            throw new ArgumentException(
+                $"M-relay address '{mAddress}' is not a valid decimal integer", nameof(mAddress));
+        var result = mBankBase + offset;
+        if (result > ushort.MaxValue)
+            throw new OverflowException($"M-relay {mAddress} + base {mBankBase} exceeds 0xFFFF");
+        return (ushort)result;
+    }
+
+    /// <summary>
+    ///     Translate a MELSEC D-register address (data register) to a 0-based Modbus holding
+    ///     register address. D-addresses are <b>decimal</b>. Default assignment convention is
+    ///     D0 → HR 0 (pass <paramref name="dBankBase"/> = 0); sites with shifted layouts pass
+    ///     their configured base.
+    /// </summary>
+    public static ushort DRegisterToHolding(string dAddress, ushort dBankBase = 0)
+    {
+        var digits = StripPrefix(dAddress, 'D');
+        if (!ushort.TryParse(digits, out var offset))
+            throw new ArgumentException(
+                $"D-register address '{dAddress}' is not a valid decimal integer", nameof(dAddress));
+        var result = dBankBase + offset;
+        if (result > ushort.MaxValue)
+            throw new OverflowException($"D-register {dAddress} + base {dBankBase} exceeds 0xFFFF");
+        return (ushort)result;
+    }
+
+    private static string StripPrefix(string address, char expectedPrefix)
+    {
+        if (string.IsNullOrWhiteSpace(address))
+            throw new ArgumentException("Address must not be empty", nameof(address));
+        var s = address.Trim();
+        if (s.Length > 0 && char.ToUpperInvariant(s[0]) == char.ToUpperInvariant(expectedPrefix))
+            s = s.Substring(1);
+        if (s.Length == 0)
+            throw new ArgumentException($"Address '{address}' has no digits after prefix", nameof(address));
+        return s;
+    }
+
+    private static ushort AddFamilyOffset(ushort baseAddr, string digits, MelsecFamily family)
+    {
+        uint offset = family switch
+        {
+            MelsecFamily.Q_L_iQR => ParseHex(digits),
+            MelsecFamily.F_iQF => ParseOctal(digits),
+            _ => throw new ArgumentOutOfRangeException(nameof(family), family, "Unknown MELSEC family"),
+        };
+        var result = baseAddr + offset;
+        if (result > ushort.MaxValue)
+            throw new OverflowException($"Address {baseAddr}+{offset} exceeds 0xFFFF");
+        return (ushort)result;
+    }
+
+    private static uint ParseHex(string digits)
+    {
+        uint result = 0;
+        foreach (var ch in digits)
+        {
+            uint nibble;
+            if (ch >= '0' && ch <= '9') nibble = (uint)(ch - '0');
+            else if (ch >= 'A' && ch <= 'F') nibble = (uint)(ch - 'A' + 10);
+            else if (ch >= 'a' && ch <= 'f') nibble = (uint)(ch - 'a' + 10);
+            else throw new ArgumentException(
+                $"Address contains non-hex digit '{ch}' — Q/L/iQ-R X/Y addresses are hexadecimal",
+                nameof(digits));
+            result = result * 16 + nibble;
+            if (result > ushort.MaxValue)
+                throw new OverflowException($"Hex address exceeds 0xFFFF");
+        }
+        return result;
+    }
+
+    private static uint ParseOctal(string digits)
+    {
+        uint result = 0;
+        foreach (var ch in digits)
+        {
+            if (ch < '0' || ch > '7')
+                throw new ArgumentException(
+                    $"Address contains non-octal digit '{ch}' — FX/iQ-F X/Y addresses are octal (0-7)",
+                    nameof(digits));
+            result = result * 8 + (uint)(ch - '0');
+            if (result > ushort.MaxValue)
+                throw new OverflowException($"Octal address exceeds 0xFFFF");
+        }
+        return result;
+    }
+}

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

@@ -0,0 +1,180 @@
+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>. Convenience
+    ///     shortcut for a single-endpoint deployment — equivalent to setting
+    ///     <see cref="EndpointUrls"/> to a list with this one URL. When both are provided,
+    ///     the list wins and <see cref="EndpointUrl"/> is ignored.
+    /// </summary>
+    public string EndpointUrl { get; init; } = "opc.tcp://localhost:4840";
+
+    /// <summary>
+    ///     Ordered list of candidate endpoint URLs for failover. The driver tries each in
+    ///     order at <see cref="OpcUaClientDriver.InitializeAsync"/> and on session drop;
+    ///     the first URL that successfully connects wins. Typical use-case: an OPC UA server
+    ///     pair running in hot-standby (primary 4840 + backup 4841) where either can serve
+    ///     the same address space. Leave unset (or empty) to use <see cref="EndpointUrl"/>
+    ///     as a single-URL shortcut.
+    /// </summary>
+    public IReadOnlyList<string> EndpointUrls { get; init; } = [];
+
+    /// <summary>
+    ///     Per-endpoint connect-attempt timeout during the failover sweep. Short enough that
+    ///     cycling through several dead servers doesn't blow the overall init budget, long
+    ///     enough to tolerate a slow TLS handshake on a healthy server. Applied independently
+    ///     of <see cref="Timeout"/> which governs steady-state operations.
+    /// </summary>
+    public TimeSpan PerEndpointConnectTimeout { get; init; } = TimeSpan.FromSeconds(3);
+
+    /// <summary>
+    ///     Security policy to require when selecting an endpoint. Either a
+    ///     <see cref="OpcUaSecurityPolicy"/> enum constant or a free-form string (for
+    ///     forward-compatibility with future OPC UA policies not yet in the enum).
+    ///     Matched against <c>EndpointDescription.SecurityPolicyUri</c> suffix — the driver
+    ///     connects to the first endpoint whose policy name matches AND whose mode matches
+    ///     <see cref="SecurityMode"/>. When set to <see cref="OpcUaSecurityPolicy.None"/>
+    ///     the driver picks any unsecured endpoint regardless of policy string.
+    /// </summary>
+    public OpcUaSecurityPolicy SecurityPolicy { get; init; } = OpcUaSecurityPolicy.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>
+    ///     Filesystem path to the user-identity certificate (PFX/PEM). Required when
+    ///     <see cref="AuthType"/> is <see cref="OpcUaAuthType.Certificate"/>. The driver
+    ///     loads the cert + private key, which the remote server validates against its
+    ///     <c>TrustedUserCertificates</c> store to authenticate the session's user token.
+    ///     Leave unset to use the driver's application-instance certificate as the user
+    ///     token (not typical — most deployments have a separate user cert).
+    /// </summary>
+    public string? UserCertificatePath { get; init; }
+
+    /// <summary>
+    ///     Optional password that unlocks <see cref="UserCertificatePath"/> when the PFX is
+    ///     protected. PEM files generally have their password on the adjacent key file; this
+    ///     knob only applies to password-locked PFX.
+    /// </summary>
+    public string? UserCertificatePassword { 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>
+///     OPC UA security policies recognized by the driver. Maps to the standard
+///     <c>http://opcfoundation.org/UA/SecurityPolicy#</c> URI suffixes the SDK uses for
+///     endpoint matching.
+/// </summary>
+/// <remarks>
+///     <see cref="Basic128Rsa15"/> and <see cref="Basic256"/> are <b>deprecated</b> per OPC UA
+///     spec v1.04 — they remain in the enum only for brownfield interop with older servers.
+///     Prefer <see cref="Basic256Sha256"/>, <see cref="Aes128_Sha256_RsaOaep"/>, or
+///     <see cref="Aes256_Sha256_RsaPss"/> for new deployments.
+/// </remarks>
+public enum OpcUaSecurityPolicy
+{
+    /// <summary>No security. Unsigned, unencrypted wire.</summary>
+    None,
+    /// <summary>Deprecated (OPC UA 1.04). Retained for legacy server interop.</summary>
+    Basic128Rsa15,
+    /// <summary>Deprecated (OPC UA 1.04). Retained for legacy server interop.</summary>
+    Basic256,
+    /// <summary>Recommended baseline for current deployments.</summary>
+    Basic256Sha256,
+    /// <summary>Current OPC UA policy; AES-128 + SHA-256 + RSA-OAEP.</summary>
+    Aes128_Sha256_RsaOaep,
+    /// <summary>Current OPC UA policy; AES-256 + SHA-256 + RSA-PSS.</summary>
+    Aes256_Sha256_RsaPss,
+}
+
+/// <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.Modbus.IntegrationTests/Mitsubishi/MitsubishiProfile.cs

@@ -0,0 +1,43 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.Mitsubishi;
+
+/// <summary>
+///     Tag map for the Mitsubishi MELSEC device class with a representative Modbus Device
+///     Assignment block mapping D0..D1023 → HR[0..1023]. Mirrors the behaviors in
+///     <c>mitsubishi.json</c> pymodbus profile and <c>docs/v2/mitsubishi.md</c>.
+/// </summary>
+/// <remarks>
+///     MELSEC Modbus sites all have *different* device-assignment parameter blocks; this profile
+///     models the conventional default. Per-model differences (FX5U needs firmware ≥ 1.060 for
+///     Modbus server; QJ71MT91 lacks FC22/FC23; FX/iQ-F use octal X/Y while Q/L/iQ-R use hex)
+///     are handled in <see cref="MelsecAddress"/> (PR 59) and the per-model test files.
+/// </remarks>
+public static class MitsubishiProfile
+{
+    /// <summary>
+    ///     Scratch HR the smoke test writes + reads. Address 200 mirrors the
+    ///     dl205/s7_1500/standard scratch range so one smoke test pattern works across every
+    ///     device profile the simulator supports.
+    /// </summary>
+    public const ushort SmokeHoldingRegister = 200;
+
+    /// <summary>Value the smoke test writes then reads back.</summary>
+    public const short SmokeHoldingValue = 7890;
+
+    public static ModbusDriverOptions BuildOptions(string host, int port) => new()
+    {
+        Host = host,
+        Port = port,
+        UnitId = 1,
+        Timeout = TimeSpan.FromSeconds(2),
+        Tags =
+        [
+            new ModbusTagDefinition(
+                Name: "Smoke_HReg200",
+                Region: ModbusRegion.HoldingRegisters,
+                Address: SmokeHoldingRegister,
+                DataType: ModbusDataType.Int16,
+                Writable: true),
+        ],
+        Probe = new ModbusProbeOptions { Enabled = false },
+    };
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Mitsubishi/MitsubishiQuirkTests.cs

@@ -0,0 +1,179 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.Mitsubishi;
+
+/// <summary>
+///     Verifies the MELSEC-family Modbus quirks against the <c>mitsubishi.json</c> pymodbus
+///     profile: CDAB word order default, binary-not-BCD D-register encoding, hex X-input
+///     parsing (Q/L/iQ-R), D0 fingerprint, M-relay coil mapping with bank base.
+/// </summary>
+/// <remarks>
+///     Groups all quirks in one test class instead of per-behavior classes (unlike the DL205
+///     set) because MELSEC's per-model differentiation is handled by the
+///     <see cref="MelsecFamily"/> enum on the helper + <c>MODBUS_SIM_PROFILE</c> env var on
+///     the fixture, rather than per-PR test classes.
+/// </remarks>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "Mitsubishi")]
+public sealed class MitsubishiQuirkTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task Mitsubishi_D0_fingerprint_reads_0x1234()
+    {
+        if (!ShouldRun()) return;
+        await using var driver = await NewDriverAsync(
+            new ModbusTagDefinition("D0_Fingerprint",
+                ModbusRegion.HoldingRegisters,
+                Address: MelsecAddress.DRegisterToHolding("D0"),
+                DataType: ModbusDataType.UInt16, Writable: false));
+
+        var r = await driver.ReadAsync(["D0_Fingerprint"], TestContext.Current.CancellationToken);
+        r[0].StatusCode.ShouldBe(0u);
+        r[0].Value.ShouldBe((ushort)0x1234);
+    }
+
+    [Fact]
+    public async Task Mitsubishi_Float32_CDAB_decodes_1_5f_from_D100()
+    {
+        if (!ShouldRun()) return;
+        // MELSEC Q/L/iQ-R/iQ-F all store 32-bit values with CDAB word order (low word at
+        // lower D-register address). HR[100..101] = [0, 0x3FC0] decodes as 1.5f under
+        // WordSwap but as a denormal under BigEndian.
+        var addr = MelsecAddress.DRegisterToHolding("D100");
+        await using var driver = await NewDriverAsync(
+            new ModbusTagDefinition("D100_Float_CDAB",
+                ModbusRegion.HoldingRegisters, Address: addr,
+                DataType: ModbusDataType.Float32, Writable: false,
+                ByteOrder: ModbusByteOrder.WordSwap),
+            new ModbusTagDefinition("D100_Float_ABCD_control",
+                ModbusRegion.HoldingRegisters, Address: addr,
+                DataType: ModbusDataType.Float32, Writable: false,
+                ByteOrder: ModbusByteOrder.BigEndian));
+
+        var r = await driver.ReadAsync(
+            ["D100_Float_CDAB", "D100_Float_ABCD_control"],
+            TestContext.Current.CancellationToken);
+        r[0].Value.ShouldBe(1.5f, "MELSEC stores Float32 CDAB; WordSwap decode returns 1.5f");
+        r[1].Value.ShouldNotBe(1.5f, "same wire with BigEndian must decode to a different value");
+    }
+
+    [Fact]
+    public async Task Mitsubishi_D10_is_binary_not_BCD()
+    {
+        if (!ShouldRun()) return;
+        // Counter-to-DL205: MELSEC D-registers are binary by default. D10 = 1234 decimal =
+        // 0x04D2. Reading as Int16 returns 1234; reading as Bcd16 would throw (nibble 0xD is
+        // non-BCD) — the integration test proves the Int16 decode wins.
+        await using var driver = await NewDriverAsync(
+            new ModbusTagDefinition("D10_Binary",
+                ModbusRegion.HoldingRegisters,
+                Address: MelsecAddress.DRegisterToHolding("D10"),
+                DataType: ModbusDataType.Int16, Writable: false));
+
+        var r = await driver.ReadAsync(["D10_Binary"], TestContext.Current.CancellationToken);
+        r[0].StatusCode.ShouldBe(0u);
+        r[0].Value.ShouldBe((short)1234, "MELSEC stores numeric D-register values in binary; 0x04D2 = 1234");
+    }
+
+    [Fact]
+    public async Task Mitsubishi_D10_as_BCD_throws_because_nibble_is_non_decimal()
+    {
+        if (!ShouldRun()) return;
+        // If a site configured D10 with Bcd16 data type but the ladder writes binary, the
+        // BCD decoder MUST reject the garbage rather than silently returning wrong decimal.
+        // 0x04D2 contains nibble 0xD which fails BCD validation.
+        await using var driver = await NewDriverAsync(
+            new ModbusTagDefinition("D10_WrongBcd",
+                ModbusRegion.HoldingRegisters,
+                Address: MelsecAddress.DRegisterToHolding("D10"),
+                DataType: ModbusDataType.Bcd16, Writable: false));
+
+        var r = await driver.ReadAsync(["D10_WrongBcd"], TestContext.Current.CancellationToken);
+        // ReadAsync catches the InvalidDataException from DecodeBcd and surfaces it as
+        // BadCommunicationError (PR 52 mapping). Non-zero status = caller sees a real
+        // problem and can check their tag config instead of getting silently-wrong numbers.
+        r[0].StatusCode.ShouldNotBe(0u, "BCD decode of binary 0x04D2 must fail loudly because nibble D is non-BCD");
+    }
+
+    [Fact]
+    public async Task Mitsubishi_QLiQR_X210_hex_maps_to_DI_528_reads_ON()
+    {
+        if (!ShouldRun()) return;
+        // MELSEC-Q / L / iQ-R: X addresses are hex. X210 = 0x210 = 528 decimal.
+        // mitsubishi.json seeds cell 33 (DI 528..543) with value 9 = bit 0 + bit 3 set.
+        // X210 → DI 528 → cell 33 bit 0 = 1 (ON).
+        var addr = MelsecAddress.XInputToDiscrete("X210", MelsecFamily.Q_L_iQR);
+        addr.ShouldBe((ushort)528);
+
+        await using var driver = await NewDriverAsync(
+            new ModbusTagDefinition("X210_hex",
+                ModbusRegion.DiscreteInputs, Address: addr,
+                DataType: ModbusDataType.Bool, Writable: false));
+
+        var r = await driver.ReadAsync(["X210_hex"], TestContext.Current.CancellationToken);
+        r[0].StatusCode.ShouldBe(0u);
+        r[0].Value.ShouldBe(true);
+    }
+
+    [Fact]
+    public void Mitsubishi_family_trap_X20_differs_on_Q_vs_FX()
+    {
+        // Not a live-sim test — a unit-level proof that the MELSEC family selector gates the
+        // address correctly. Included in the integration suite so anyone running the MELSEC
+        // tests sees the trap called out explicitly.
+        MelsecAddress.XInputToDiscrete("X20", MelsecFamily.Q_L_iQR).ShouldBe((ushort)32);
+        MelsecAddress.XInputToDiscrete("X20", MelsecFamily.F_iQF).ShouldBe((ushort)16);
+    }
+
+    [Fact]
+    public async Task Mitsubishi_M512_maps_to_coil_512_reads_ON()
+    {
+        if (!ShouldRun()) return;
+        // mitsubishi.json seeds cell 32 (coil 512..527) with value 5 = bit 0 + bit 2 set.
+        // M512 → coil 512 → cell 32 bit 0 = 1 (ON).
+        var addr = MelsecAddress.MRelayToCoil("M512");
+        addr.ShouldBe((ushort)512);
+
+        await using var driver = await NewDriverAsync(
+            new ModbusTagDefinition("M512",
+                ModbusRegion.Coils, Address: addr,
+                DataType: ModbusDataType.Bool, Writable: false));
+
+        var r = await driver.ReadAsync(["M512"], TestContext.Current.CancellationToken);
+        r[0].StatusCode.ShouldBe(0u);
+        r[0].Value.ShouldBe(true);
+    }
+
+    // --- helpers ---
+
+    private bool ShouldRun()
+    {
+        if (sim.SkipReason is not null) { Assert.Skip(sim.SkipReason); return false; }
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "mitsubishi",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != mitsubishi — skipping.");
+            return false;
+        }
+        return true;
+    }
+
+    private async Task<ModbusDriver> NewDriverAsync(params ModbusTagDefinition[] tags)
+    {
+        var drv = new ModbusDriver(
+            new ModbusDriverOptions
+            {
+                Host = sim.Host,
+                Port = sim.Port,
+                UnitId = 1,
+                Timeout = TimeSpan.FromSeconds(2),
+                Tags = tags,
+                Probe = new ModbusProbeOptions { Enabled = false },
+            },
+            driverInstanceId: "melsec-quirk");
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+        return drv;
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Mitsubishi/MitsubishiSmokeTests.cs

@@ -0,0 +1,45 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.Mitsubishi;
+
+/// <summary>
+///     End-to-end smoke against the MELSEC <c>mitsubishi.json</c> pymodbus profile (or a real
+///     MELSEC QJ71MT91 / iQ-R / FX5U when <c>MODBUS_SIM_ENDPOINT</c> points at one). Drives
+///     the full <see cref="ModbusDriver"/> + real <see cref="ModbusTcpTransport"/> stack.
+///     Success proves the driver initializes against the MELSEC sim, writes a known value,
+///     and reads it back — the baseline every Mitsubishi-specific test (PR 59+) builds on.
+/// </summary>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "Mitsubishi")]
+public sealed class MitsubishiSmokeTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task Mitsubishi_roundtrip_write_then_read_of_holding_register()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "mitsubishi",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != mitsubishi — skipping.");
+        }
+
+        var options = MitsubishiProfile.BuildOptions(sim.Host, sim.Port);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "melsec-smoke");
+        await driver.InitializeAsync(driverConfigJson: "{}", TestContext.Current.CancellationToken);
+
+        var writeResults = await driver.WriteAsync(
+            [new(FullReference: "Smoke_HReg200", Value: (short)MitsubishiProfile.SmokeHoldingValue)],
+            TestContext.Current.CancellationToken);
+        writeResults.Count.ShouldBe(1);
+        writeResults[0].StatusCode.ShouldBe(0u, "write must succeed against the MELSEC pymodbus profile");
+
+        var readResults = await driver.ReadAsync(
+            ["Smoke_HReg200"],
+            TestContext.Current.CancellationToken);
+        readResults.Count.ShouldBe(1);
+        readResults[0].StatusCode.ShouldBe(0u);
+        readResults[0].Value.ShouldBe((short)MitsubishiProfile.SmokeHoldingValue);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Pymodbus/mitsubishi.json

@@ -0,0 +1,83 @@
+{
+  "_comment": "mitsubishi.json -- Mitsubishi MELSEC Modbus TCP quirk simulator covering QJ71MT91, iQ-R, iQ-F/FX5U, and FX3U-ENET-P502 behaviors documented in docs/v2/mitsubishi.md. MELSEC CPUs store multi-word values in CDAB order (opposite of S7 ABCD, same family as DL260). The Modbus-module 'Modbus Device Assignment Parameter' block is per-site, so this profile models one *representative* assignment mapping D-register D0..D1023 -> HR 0..1023, M-relay M0..M511 -> coil 0..511, X-input X0..X15 -> DI 0..15 (X-addresses are HEX on Q/L/iQ-R, so X10 = decimal 16; on FX/iQ-F they're OCTAL like DL260). pymodbus bit-address semantics are the same as dl205.json and s7_1500.json (FC01/02/05/15 address N maps to cell index N/16).",
+
+  "server_list": {
+    "srv": {
+      "comm": "tcp",
+      "host": "0.0.0.0",
+      "port": 5020,
+      "framer": "socket",
+      "device_id": 1
+    }
+  },
+
+  "device_list": {
+    "dev": {
+      "setup": {
+        "co size":    4096,
+        "di size":    4096,
+        "hr size":    4096,
+        "ir size":    1024,
+        "shared blocks": true,
+        "type exception": false,
+        "defaults": {
+          "value":  {"bits": 0, "uint16": 0, "uint32": 0, "float32": 0.0, "string": " "},
+          "action": {"bits": null, "uint16": null, "uint32": null, "float32": null, "string": null}
+        }
+      },
+      "invalid": [],
+      "write": [
+        [0, 0],
+        [10, 10],
+        [100, 101],
+        [200, 209],
+        [300, 301],
+        [500, 500]
+      ],
+
+      "uint16": [
+        {"_quirk": "D0 fingerprint marker. MELSEC D0 is the first data register; Modbus Device Assignment typically maps D0..D1023 -> HR 0..1023. 0x1234 is the fingerprint operators set in GX Works to prove the mapping parameter block is in effect.",
+         "addr": 0, "value": 4660},
+
+        {"_quirk": "Scratch HR range 200..209 -- mirrors the dl205/s7_1500/standard scratch range so smoke tests (MitsubishiProfile.SmokeHoldingRegister=200) round-trip identically against any profile.",
+         "addr": 200, "value": 0},
+        {"addr": 201, "value": 0},
+        {"addr": 202, "value": 0},
+        {"addr": 203, "value": 0},
+        {"addr": 204, "value": 0},
+        {"addr": 205, "value": 0},
+        {"addr": 206, "value": 0},
+        {"addr": 207, "value": 0},
+        {"addr": 208, "value": 0},
+        {"addr": 209, "value": 0},
+
+        {"_quirk": "Float32 1.5f in CDAB word order (MELSEC Q/L/iQ-R/iQ-F default, same as DL260). HR[100]=0x0000=0 low word, HR[101]=0x3FC0=16320 high word. Decode with ByteOrder.WordSwap returns 1.5f; BigEndian decode returns a denormal.",
+         "addr": 100, "value": 0},
+        {"addr": 101, "value": 16320},
+
+        {"_quirk": "Int32 0x12345678 in CDAB word order. HR[300]=0x5678=22136 low word, HR[301]=0x1234=4660 high word. Contrasts with the S7 profile's ABCD encoding at the same address.",
+         "addr": 300, "value": 22136},
+        {"addr": 301, "value": 4660},
+
+        {"_quirk": "D10 = decimal 1234 stored as BINARY (NOT BCD like DL205). 0x04D2 = 1234 decimal. Caller reading with Bcd16 data type would decode this as binary 1234's BCD nibbles which are non-BCD and throw InvalidDataException -- proves MELSEC is binary-by-default, opposite of DL205's BCD-by-default quirk.",
+         "addr": 10, "value": 1234},
+
+        {"_quirk": "Modbus Device Assignment boundary marker. HR[500] represents the last register in an assigned D-range D500. Beyond this (HR[501..4095]) would be Illegal Data Address on a real QJ71MT91 with this specific parameter block; pymodbus returns default 0 because its shared cell array has space -- real-PLC parity is documented in docs/v2/mitsubishi.md §device-assignment, not enforced here.",
+         "addr": 500, "value": 500}
+      ],
+
+      "bits": [
+        {"_quirk": "M-relay marker cell at cell 32 = Modbus coil 512 = MELSEC M512 (coils 0..15 collide with the D0 uint16 marker cell, so we place the M marker above that). Cell 32 bit 0 = 1 and bit 2 = 1 (value = 0b101 = 5) = M512=ON, M513=OFF, M514=ON. Matches the Y0/Y2 marker pattern in dl205 and s7_1500 profiles.",
+         "addr": 32, "value": 5},
+
+        {"_quirk": "X-input marker cell at cell 33 = Modbus DI 528 (= MELSEC X210 hex on Q/L/iQ-R). Cell 33 bit 0 = 1 and bit 3 = 1 (value = 0x9 = 9). Chosen above cell 1 so it doesn't collide with any uint16 D-register. Proves the hex-parsing X-input helper on Q/L/iQ-R family; FX/iQ-F families use octal X-addresses tested separately.",
+         "addr": 33, "value": 9}
+      ],
+
+      "uint32":  [],
+      "float32": [],
+      "string":  [],
+      "repeat":  []
+    }
+  }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Pymodbus/s7_1500.json

@@ -0,0 +1,77 @@
+{
+  "_comment": "s7_1500.json -- Siemens SIMATIC S7-1500 + MB_SERVER quirk simulator. Models docs/v2/s7.md behaviors as concrete register values. Unlike DL260 (CDAB word order default) or Mitsubishi (CDAB default), S7 MB_SERVER uses ABCD word order by default because Siemens native CPU types are big-endian top-to-bottom both within the register pair and byte pair. This profile exists so the driver's S7 profile default ByteOrder.BigEndian can be validated end-to-end. pymodbus bit-address semantics are the same as dl205.json (FC01/02/05/15 address X maps to cell index X/16); seed bits at the appropriate cell-indexed positions.",
+
+  "server_list": {
+    "srv": {
+      "comm": "tcp",
+      "host": "0.0.0.0",
+      "port": 5020,
+      "framer": "socket",
+      "device_id": 1
+    }
+  },
+
+  "device_list": {
+    "dev": {
+      "setup": {
+        "co size":    4096,
+        "di size":    4096,
+        "hr size":    4096,
+        "ir size":    1024,
+        "shared blocks": true,
+        "type exception": false,
+        "defaults": {
+          "value":  {"bits": 0, "uint16": 0, "uint32": 0, "float32": 0.0, "string": " "},
+          "action": {"bits": null, "uint16": null, "uint32": null, "float32": null, "string": null}
+        }
+      },
+      "invalid": [],
+      "write": [
+        [0, 0],
+        [25, 25],
+        [100, 101],
+        [200, 209],
+        [300, 301]
+      ],
+
+      "uint16": [
+        {"_quirk": "DB1 header marker. On an S7-1500 with MB_SERVER pointing at DB1, operators often reserve DB1.DBW0 for a fingerprint word so clients can verify they're talking to the right DB. 0xABCD = 43981.",
+         "addr": 0, "value": 43981},
+
+        {"_quirk": "Scratch HR range 200..209 -- mirrors the standard.json scratch range so the smoke test (S7_1500Profile.SmokeHoldingRegister=200) round-trips identically against either profile.",
+         "addr": 200, "value": 0},
+        {"addr": 201, "value": 0},
+        {"addr": 202, "value": 0},
+        {"addr": 203, "value": 0},
+        {"addr": 204, "value": 0},
+        {"addr": 205, "value": 0},
+        {"addr": 206, "value": 0},
+        {"addr": 207, "value": 0},
+        {"addr": 208, "value": 0},
+        {"addr": 209, "value": 0},
+
+        {"_quirk": "Float32 1.5f in ABCD word order (Siemens big-endian default, OPPOSITE of DL260 CDAB). IEEE-754 1.5 = 0x3FC00000. ABCD = high word first: HR[100]=0x3FC0=16320, HR[101]=0x0000=0.",
+         "addr": 100, "value": 16320},
+        {"_quirk": "Float32 1.5f ABCD low word.",
+         "addr": 101, "value": 0},
+
+        {"_quirk": "Int32 0x12345678 in ABCD word order. HR[300]=0x1234=4660, HR[301]=0x5678=22136. Demonstrates the contrast with DL260 CDAB Int32 encoding.",
+         "addr": 300, "value": 4660},
+        {"addr": 301, "value": 22136}
+      ],
+
+      "bits": [
+        {"_quirk": "Coil bank marker cell. S7 MB_SERVER doesn't fix coil addresses; this simulates a user-wired DB where coil 400 (=bit 0 of cell 25) represents a latched digital output. Cell 25 bit 0 = 1 proves the wire-format round-trip works for coils on S7 profile.",
+         "addr": 25, "value": 1},
+
+        {"_quirk": "Discrete-input bank marker cell. DI 500 (=bit 0 of cell 31) = 1. Like coils, discrete inputs on S7 MB_SERVER are per-site; we assert the end-to-end FC02 path only.",
+         "addr": 31, "value": 1}
+      ],
+
+      "uint32":  [],
+      "float32": [],
+      "string":  [],
+      "repeat":  []
+    }
+  }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Pymodbus/serve.ps1

@@ -21,7 +21,7 @@
 #>
 [CmdletBinding()]
 param(
-    [Parameter(Mandatory)] [ValidateSet('standard', 'dl205')] [string]$Profile,
+    [Parameter(Mandatory)] [ValidateSet('standard', 'dl205', 's7_1500', 'mitsubishi')] [string]$Profile,
     [int]$HttpPort = 8080
 )
 

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/S7/S7_1500Profile.cs

@@ -0,0 +1,44 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.S7;
+
+/// <summary>
+///     Tag map for the Siemens SIMATIC S7-1500 device class with the <c>MB_SERVER</c> library
+///     block mapping HR[0..] to DB1.DBW0+. Mirrors <c>s7_1500.json</c> in <c>Pymodbus/</c>.
+/// </summary>
+/// <remarks>
+///     Unlike DL205, S7 has no fixed Modbus memory map — every site wires MB_SERVER to a
+///     different DB. The profile here models the *default* user layout documented in
+///     <c>docs/v2/s7.md</c> §per-model-matrix: DB1.DBW0 = fingerprint marker, a scratch HR
+///     range 200..209 for write-roundtrip tests, and ABCD-order Float32 / Int32 markers at
+///     HR[100..101] and HR[300..301] to prove the driver's S7 profile default is correct.
+/// </remarks>
+public static class S7_1500Profile
+{
+    /// <summary>
+    ///     Scratch HR the smoke test writes + reads. Address 200 mirrors the DL205 /
+    ///     standard scratch range so one smoke test pattern works across all device profiles.
+    /// </summary>
+    public const ushort SmokeHoldingRegister = 200;
+
+    /// <summary>Value the smoke test writes then reads back.</summary>
+    public const short SmokeHoldingValue = 4321;
+
+    public static ModbusDriverOptions BuildOptions(string host, int port) => new()
+    {
+        Host = host,
+        Port = port,
+        UnitId = 1,
+        Timeout = TimeSpan.FromSeconds(2),
+        Tags =
+        [
+            new ModbusTagDefinition(
+                Name: "Smoke_HReg200",
+                Region: ModbusRegion.HoldingRegisters,
+                Address: SmokeHoldingRegister,
+                DataType: ModbusDataType.Int16,
+                Writable: true),
+        ],
+        // Disable the background probe loop — integration tests drive reads explicitly and
+        // the probe would race with assertions.
+        Probe = new ModbusProbeOptions { Enabled = false },
+    };
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/S7/S7_1500SmokeTests.cs

@@ -0,0 +1,54 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.S7;
+
+/// <summary>
+///     End-to-end smoke against the S7-1500 <c>MB_SERVER</c> pymodbus profile (or a real
+///     S7-1500 + MB_SERVER deployment when <c>MODBUS_SIM_ENDPOINT</c> points at one). Drives
+///     the full <see cref="ModbusDriver"/> + real <see cref="ModbusTcpTransport"/> stack —
+///     no fake transport. Success proves the driver initializes against the S7 simulator,
+///     writes a known value, and reads it back with the correct status and value, which is
+///     the baseline every S7-specific test (PR 57+) builds on.
+/// </summary>
+/// <remarks>
+///     S7-specific quirk tests (MB_SERVER requires non-optimized DBs, ABCD word order
+///     default, port-per-connection, FC23 Illegal Function, STOP-mode behaviour, etc.) land
+///     as separate test classes in this directory as each quirk is validated in pymodbus.
+///     Keep this smoke test deliberately narrow — filtering by device class
+///     (<c>--filter DisplayName~S7</c>) should surface the quirk-specific failure mode when
+///     something goes wrong, not a blanket smoke failure that could mean anything.
+/// </remarks>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "S7")]
+public sealed class S7_1500SmokeTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task S7_1500_roundtrip_write_then_read_of_holding_register()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "s7_1500",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != s7_1500 — skipping (other profiles don't seed the S7 scratch range identically).");
+        }
+
+        var options = S7_1500Profile.BuildOptions(sim.Host, sim.Port);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "s7-smoke");
+        await driver.InitializeAsync(driverConfigJson: "{}", TestContext.Current.CancellationToken);
+
+        var writeResults = await driver.WriteAsync(
+            [new(FullReference: "Smoke_HReg200", Value: (short)S7_1500Profile.SmokeHoldingValue)],
+            TestContext.Current.CancellationToken);
+        writeResults.Count.ShouldBe(1);
+        writeResults[0].StatusCode.ShouldBe(0u, "write must succeed against the S7-1500 MB_SERVER profile");
+
+        var readResults = await driver.ReadAsync(
+            ["Smoke_HReg200"],
+            TestContext.Current.CancellationToken);
+        readResults.Count.ShouldBe(1);
+        readResults[0].StatusCode.ShouldBe(0u);
+        readResults[0].Value.ShouldBe((short)S7_1500Profile.SmokeHoldingValue);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/S7/S7_ByteOrderTests.cs

@@ -0,0 +1,132 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.S7;
+
+/// <summary>
+///     Verifies the Siemens S7 big-endian (<c>ABCD</c>) word-order default for Float32 and
+///     Int32 against the <c>s7_1500.json</c> pymodbus profile. S7's native CPU types are
+///     big-endian end-to-end, so <c>MB_SERVER</c> places the high word at the lower register
+///     address — <b>opposite</b> of DL260's CDAB. The driver's S7-family tag config must
+///     therefore default to <see cref="ModbusByteOrder.BigEndian"/>; selecting
+///     <see cref="ModbusByteOrder.WordSwap"/> against an S7 would decode garbage.
+/// </summary>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "S7")]
+public sealed class S7_ByteOrderTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task S7_Float32_ABCD_decodes_1_5f_from_HR100()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "s7_1500",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != s7_1500 — skipping (s7_1500 profile is the only one seeding HR[100..101] ABCD).");
+        }
+
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition("S7_Float_ABCD",
+                    ModbusRegion.HoldingRegisters, Address: 100,
+                    DataType: ModbusDataType.Float32, Writable: false,
+                    ByteOrder: ModbusByteOrder.BigEndian),
+                // Control: same address with WordSwap should decode garbage — proves the
+                // two code paths diverge on S7 wire bytes.
+                new ModbusTagDefinition("S7_Float_CDAB_control",
+                    ModbusRegion.HoldingRegisters, Address: 100,
+                    DataType: ModbusDataType.Float32, Writable: false,
+                    ByteOrder: ModbusByteOrder.WordSwap),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "s7-float-abcd");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(
+            ["S7_Float_ABCD", "S7_Float_CDAB_control"],
+            TestContext.Current.CancellationToken);
+
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(1.5f, "S7 MB_SERVER stores Float32 in ABCD word order; BigEndian decode returns 1.5f");
+
+        results[1].StatusCode.ShouldBe(0u);
+        results[1].Value.ShouldNotBe(1.5f, "applying CDAB swap to S7 ABCD bytes must produce a different value — confirms the flag is not a no-op and S7 profile default must be BigEndian");
+    }
+
+    [Fact]
+    public async Task S7_Int32_ABCD_decodes_0x12345678_from_HR300()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "s7_1500",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != s7_1500 — skipping.");
+        }
+
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition("S7_Int32_ABCD",
+                    ModbusRegion.HoldingRegisters, Address: 300,
+                    DataType: ModbusDataType.Int32, Writable: false,
+                    ByteOrder: ModbusByteOrder.BigEndian),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "s7-int-abcd");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["S7_Int32_ABCD"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(0x12345678,
+            "S7 Int32 stored as HR[300]=0x1234, HR[301]=0x5678 with ABCD order decodes to 0x12345678 — DL260 would store the reverse order");
+    }
+
+    [Fact]
+    public async Task S7_DB1_fingerprint_marker_at_HR0_reads_0xABCD()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "s7_1500",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != s7_1500 — skipping.");
+        }
+
+        // Real-world MB_SERVER deployments typically reserve DB1.DBW0 as a fingerprint so
+        // clients can verify they're pointing at the right DB (protects against typos in
+        // the MB_SERVER.MB_HOLD_REG.DB_number parameter). 0xABCD is the convention.
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition("S7_Fingerprint",
+                    ModbusRegion.HoldingRegisters, Address: 0,
+                    DataType: ModbusDataType.UInt16, Writable: false),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "s7-fingerprint");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["S7_Fingerprint"], TestContext.Current.CancellationToken);
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe((ushort)0xABCD);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.csproj

@@ -26,6 +26,8 @@
     <ItemGroup>
         <None Update="Pymodbus\**\*" CopyToOutputDirectory="PreserveNewest"/>
         <None Update="DL205\**\*" CopyToOutputDirectory="PreserveNewest"/>
+        <None Update="S7\**\*" CopyToOutputDirectory="PreserveNewest"/>
+        <None Update="Mitsubishi\**\*" CopyToOutputDirectory="PreserveNewest"/>
     </ItemGroup>
 
     <ItemGroup>

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

@@ -0,0 +1,116 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class MelsecAddressTests
+{
+    // --- X / Y hex vs octal family trap ---
+
+    [Theory]
+    [InlineData("X0", (ushort)0)]
+    [InlineData("X9", (ushort)9)]
+    [InlineData("XA", (ushort)10)]      // hex
+    [InlineData("XF", (ushort)15)]
+    [InlineData("X10", (ushort)16)]     // hex 0x10 = decimal 16
+    [InlineData("X20", (ushort)32)]     // hex 0x20 = decimal 32 — the classic MELSEC-Q trap
+    [InlineData("X1FF", (ushort)511)]
+    [InlineData("x10", (ushort)16)]     // lowercase prefix
+    public void XInputToDiscrete_QLiQR_parses_hex(string x, ushort expected)
+        => MelsecAddress.XInputToDiscrete(x, MelsecFamily.Q_L_iQR).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("X0", (ushort)0)]
+    [InlineData("X7", (ushort)7)]
+    [InlineData("X10", (ushort)8)]      // octal 10 = decimal 8
+    [InlineData("X20", (ushort)16)]     // octal 20 = decimal 16 — SAME string, DIFFERENT value on FX
+    [InlineData("X777", (ushort)511)]
+    public void XInputToDiscrete_FiQF_parses_octal(string x, ushort expected)
+        => MelsecAddress.XInputToDiscrete(x, MelsecFamily.F_iQF).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("Y0", (ushort)0)]
+    [InlineData("Y1F", (ushort)31)]
+    public void YOutputToCoil_QLiQR_parses_hex(string y, ushort expected)
+        => MelsecAddress.YOutputToCoil(y, MelsecFamily.Q_L_iQR).ShouldBe(expected);
+
+    [Theory]
+    [InlineData("Y0", (ushort)0)]
+    [InlineData("Y17", (ushort)15)]
+    public void YOutputToCoil_FiQF_parses_octal(string y, ushort expected)
+        => MelsecAddress.YOutputToCoil(y, MelsecFamily.F_iQF).ShouldBe(expected);
+
+    [Fact]
+    public void Same_address_string_decodes_differently_between_families()
+    {
+        // This is the headline quirk: "X20" in GX Works means one thing on Q-series and
+        // another on FX-series. The driver's family selector is the only defence.
+        MelsecAddress.XInputToDiscrete("X20", MelsecFamily.Q_L_iQR).ShouldBe((ushort)32);
+        MelsecAddress.XInputToDiscrete("X20", MelsecFamily.F_iQF).ShouldBe((ushort)16);
+    }
+
+    [Theory]
+    [InlineData("X8")]     // 8 is non-octal
+    [InlineData("X12G")]   // G is non-hex
+    public void XInputToDiscrete_FiQF_rejects_non_octal(string bad)
+        => Should.Throw<ArgumentException>(() => MelsecAddress.XInputToDiscrete(bad, MelsecFamily.F_iQF));
+
+    [Theory]
+    [InlineData("X12G")]
+    public void XInputToDiscrete_QLiQR_rejects_non_hex(string bad)
+        => Should.Throw<ArgumentException>(() => MelsecAddress.XInputToDiscrete(bad, MelsecFamily.Q_L_iQR));
+
+    [Fact]
+    public void XInputToDiscrete_honors_bank_base_from_assignment_block()
+    {
+        // Real-world QJ71MT91 assignment blocks commonly place X at DI 8192+ when other
+        // ranges take the low Modbus addresses. Helper must add the base cleanly.
+        MelsecAddress.XInputToDiscrete("X10", MelsecFamily.Q_L_iQR, xBankBase: 8192).ShouldBe((ushort)(8192 + 16));
+    }
+
+    // --- M-relay (decimal, both families) ---
+
+    [Theory]
+    [InlineData("M0", (ushort)0)]
+    [InlineData("M10", (ushort)10)]     // M addresses are DECIMAL, not hex or octal
+    [InlineData("M511", (ushort)511)]
+    [InlineData("m99", (ushort)99)]     // lowercase
+    public void MRelayToCoil_parses_decimal(string m, ushort expected)
+        => MelsecAddress.MRelayToCoil(m).ShouldBe(expected);
+
+    [Fact]
+    public void MRelayToCoil_honors_bank_base()
+        => MelsecAddress.MRelayToCoil("M0", mBankBase: 512).ShouldBe((ushort)512);
+
+    [Fact]
+    public void MRelayToCoil_rejects_non_numeric()
+        => Should.Throw<ArgumentException>(() => MelsecAddress.MRelayToCoil("M1F"));
+
+    // --- D-register (decimal, both families) ---
+
+    [Theory]
+    [InlineData("D0", (ushort)0)]
+    [InlineData("D100", (ushort)100)]
+    [InlineData("d1023", (ushort)1023)]
+    public void DRegisterToHolding_parses_decimal(string d, ushort expected)
+        => MelsecAddress.DRegisterToHolding(d).ShouldBe(expected);
+
+    [Fact]
+    public void DRegisterToHolding_honors_bank_base()
+        => MelsecAddress.DRegisterToHolding("D10", dBankBase: 4096).ShouldBe((ushort)4106);
+
+    [Fact]
+    public void DRegisterToHolding_rejects_empty()
+        => Should.Throw<ArgumentException>(() => MelsecAddress.DRegisterToHolding("D"));
+
+    // --- overflow ---
+
+    [Fact]
+    public void XInputToDiscrete_overflow_throws()
+    {
+        // 0xFFFF + base 1 = 0x10000 — past ushort.
+        Should.Throw<OverflowException>(() =>
+            MelsecAddress.XInputToDiscrete("XFFFF", MelsecFamily.Q_L_iQR, xBankBase: 1));
+    }
+}

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

@@ -0,0 +1,70 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientAlarmTests
+{
+    [Theory]
+    [InlineData((ushort)1, AlarmSeverity.Low)]
+    [InlineData((ushort)200, AlarmSeverity.Low)]
+    [InlineData((ushort)201, AlarmSeverity.Medium)]
+    [InlineData((ushort)500, AlarmSeverity.Medium)]
+    [InlineData((ushort)501, AlarmSeverity.High)]
+    [InlineData((ushort)800, AlarmSeverity.High)]
+    [InlineData((ushort)801, AlarmSeverity.Critical)]
+    [InlineData((ushort)1000, AlarmSeverity.Critical)]
+    public void MapSeverity_buckets_per_OPC_UA_Part_9_guidance(ushort opcSev, AlarmSeverity expected)
+    {
+        OpcUaClientDriver.MapSeverity(opcSev).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void MapSeverity_zero_maps_to_Low()
+    {
+        // 0 isn't in OPC UA's 1-1000 range but we handle it gracefully as Low.
+        OpcUaClientDriver.MapSeverity(0).ShouldBe(AlarmSeverity.Low);
+    }
+
+    [Fact]
+    public async Task SubscribeAlarmsAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-alarm-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.SubscribeAlarmsAsync([], TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task UnsubscribeAlarmsAsync_with_unknown_handle_is_noop()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-alarm-unknown");
+        // Parallels the subscribe handle path — session-drop races shouldn't crash the caller.
+        await drv.UnsubscribeAlarmsAsync(new FakeAlarmHandle(), TestContext.Current.CancellationToken);
+    }
+
+    [Fact]
+    public async Task AcknowledgeAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-ack-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.AcknowledgeAsync(
+                [new AlarmAcknowledgeRequest("ns=2;s=Src", "ns=2;s=Cond", "operator ack")],
+                TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task AcknowledgeAsync_with_empty_batch_is_noop_even_without_init()
+    {
+        // Empty batch short-circuits before touching the session, so it's safe pre-init. This
+        // keeps batch-ack callers from needing to guard the list size themselves.
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-ack-empty");
+        await drv.AcknowledgeAsync([], TestContext.Current.CancellationToken);
+    }
+
+    private sealed class FakeAlarmHandle : IAlarmSubscriptionHandle
+    {
+        public string DiagnosticId => "fake-alarm";
+    }
+}

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

@@ -0,0 +1,62 @@
+using Opc.Ua;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientAttributeMappingTests
+{
+    [Theory]
+    [InlineData((uint)DataTypes.Boolean, DriverDataType.Boolean)]
+    [InlineData((uint)DataTypes.Int16, DriverDataType.Int16)]
+    [InlineData((uint)DataTypes.UInt16, DriverDataType.UInt16)]
+    [InlineData((uint)DataTypes.Int32, DriverDataType.Int32)]
+    [InlineData((uint)DataTypes.UInt32, DriverDataType.UInt32)]
+    [InlineData((uint)DataTypes.Int64, DriverDataType.Int64)]
+    [InlineData((uint)DataTypes.UInt64, DriverDataType.UInt64)]
+    [InlineData((uint)DataTypes.Float, DriverDataType.Float32)]
+    [InlineData((uint)DataTypes.Double, DriverDataType.Float64)]
+    [InlineData((uint)DataTypes.String, DriverDataType.String)]
+    [InlineData((uint)DataTypes.DateTime, DriverDataType.DateTime)]
+    public void MapUpstreamDataType_recognizes_standard_builtin_types(uint typeId, DriverDataType expected)
+    {
+        var nodeId = new NodeId(typeId);
+        OpcUaClientDriver.MapUpstreamDataType(nodeId).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void MapUpstreamDataType_maps_SByte_and_Byte_to_Int16_since_DriverDataType_lacks_8bit()
+    {
+        // DriverDataType has no 8-bit type; conservative widen to Int16. Documented so a
+        // future Core.Abstractions PR that adds Int8/Byte can find this call site.
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId((uint)DataTypes.SByte)).ShouldBe(DriverDataType.Int16);
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId((uint)DataTypes.Byte)).ShouldBe(DriverDataType.Int16);
+    }
+
+    [Fact]
+    public void MapUpstreamDataType_falls_back_to_String_for_unknown_custom_types()
+    {
+        // Custom vendor extension object — NodeId in namespace 2 that isn't a standard type.
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId("CustomStruct", 2)).ShouldBe(DriverDataType.String);
+    }
+
+    [Fact]
+    public void MapUpstreamDataType_handles_UtcTime_as_DateTime()
+    {
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId((uint)DataTypes.UtcTime)).ShouldBe(DriverDataType.DateTime);
+    }
+
+    [Theory]
+    [InlineData((byte)0, SecurityClassification.ViewOnly)]           // no access flags set
+    [InlineData((byte)1, SecurityClassification.ViewOnly)]           // CurrentRead only
+    [InlineData((byte)2, SecurityClassification.Operate)]            // CurrentWrite only
+    [InlineData((byte)3, SecurityClassification.Operate)]            // CurrentRead + CurrentWrite
+    [InlineData((byte)0x0F, SecurityClassification.Operate)]          // read+write+historyRead+historyWrite
+    [InlineData((byte)0x04, SecurityClassification.ViewOnly)]         // HistoryRead only — no Write bit
+    public void MapAccessLevelToSecurityClass_respects_CurrentWrite_bit(byte accessLevel, SecurityClassification expected)
+    {
+        OpcUaClientDriver.MapAccessLevelToSecurityClass(accessLevel).ShouldBe(expected);
+    }
+}

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

@@ -0,0 +1,59 @@
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientCertAuthTests
+{
+    [Fact]
+    public void BuildCertificateIdentity_rejects_missing_path()
+    {
+        var opts = new OpcUaClientDriverOptions { AuthType = OpcUaAuthType.Certificate };
+        Should.Throw<InvalidOperationException>(() => OpcUaClientDriver.BuildCertificateIdentity(opts))
+            .Message.ShouldContain("UserCertificatePath");
+    }
+
+    [Fact]
+    public void BuildCertificateIdentity_rejects_nonexistent_file()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            AuthType = OpcUaAuthType.Certificate,
+            UserCertificatePath = Path.Combine(Path.GetTempPath(), $"does-not-exist-{Guid.NewGuid():N}.pfx"),
+        };
+        Should.Throw<FileNotFoundException>(() => OpcUaClientDriver.BuildCertificateIdentity(opts));
+    }
+
+    [Fact]
+    public void BuildCertificateIdentity_loads_a_valid_PFX_with_private_key()
+    {
+        // Generate a self-signed cert on the fly so the test doesn't ship a static PFX.
+        // The driver doesn't care about the issuer — just needs a cert with a private key.
+        using var rsa = RSA.Create(2048);
+        var req = new CertificateRequest("CN=OpcUaClientCertAuthTests", rsa,
+            HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
+        var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow.AddMinutes(-5), DateTimeOffset.UtcNow.AddHours(1));
+
+        var tmpPath = Path.Combine(Path.GetTempPath(), $"opcua-cert-test-{Guid.NewGuid():N}.pfx");
+        File.WriteAllBytes(tmpPath, cert.Export(X509ContentType.Pfx, "testpw"));
+        try
+        {
+            var opts = new OpcUaClientDriverOptions
+            {
+                AuthType = OpcUaAuthType.Certificate,
+                UserCertificatePath = tmpPath,
+                UserCertificatePassword = "testpw",
+            };
+            var identity = OpcUaClientDriver.BuildCertificateIdentity(opts);
+            identity.ShouldNotBeNull();
+            identity.TokenType.ShouldBe(Opc.Ua.UserTokenType.Certificate);
+        }
+        finally
+        {
+            try { File.Delete(tmpPath); } catch { /* best-effort */ }
+        }
+    }
+}

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 +1,91 @@
+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.SecurityPolicy.ShouldBe(OpcUaSecurityPolicy.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/OpcUaClientFailoverTests.cs

@@ -0,0 +1,81 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientFailoverTests
+{
+    [Fact]
+    public void ResolveEndpointCandidates_prefers_EndpointUrls_when_provided()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrl = "opc.tcp://fallback:4840",
+            EndpointUrls = ["opc.tcp://primary:4840", "opc.tcp://backup:4841"],
+        };
+        var list = OpcUaClientDriver.ResolveEndpointCandidates(opts);
+        list.Count.ShouldBe(2);
+        list[0].ShouldBe("opc.tcp://primary:4840");
+        list[1].ShouldBe("opc.tcp://backup:4841");
+    }
+
+    [Fact]
+    public void ResolveEndpointCandidates_falls_back_to_single_EndpointUrl_when_list_empty()
+    {
+        var opts = new OpcUaClientDriverOptions { EndpointUrl = "opc.tcp://only:4840" };
+        var list = OpcUaClientDriver.ResolveEndpointCandidates(opts);
+        list.Count.ShouldBe(1);
+        list[0].ShouldBe("opc.tcp://only:4840");
+    }
+
+    [Fact]
+    public void ResolveEndpointCandidates_empty_list_treated_as_fallback_to_EndpointUrl()
+    {
+        // Explicit empty list should still fall back to the single-URL shortcut rather than
+        // producing a zero-candidate sweep that would immediately throw with no URLs tried.
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrl = "opc.tcp://single:4840",
+            EndpointUrls = [],
+        };
+        OpcUaClientDriver.ResolveEndpointCandidates(opts).Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public void HostName_uses_first_candidate_before_connect()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrls = ["opc.tcp://primary:4840", "opc.tcp://backup:4841"],
+        };
+        using var drv = new OpcUaClientDriver(opts, "opcua-host");
+        drv.HostName.ShouldBe("opc.tcp://primary:4840",
+            "pre-connect the dashboard should show the first candidate URL so operators can link back");
+    }
+
+    [Fact]
+    public async Task Initialize_against_all_unreachable_endpoints_throws_AggregateException_listing_each()
+    {
+        // Port 1 + port 2 + port 3 on loopback are all guaranteed closed (TCP RST immediate).
+        // Failover sweep should attempt all three and throw AggregateException naming each URL
+        // so operators see exactly which candidates were tried.
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrls = ["opc.tcp://127.0.0.1:1", "opc.tcp://127.0.0.1:2", "opc.tcp://127.0.0.1:3"],
+            PerEndpointConnectTimeout = TimeSpan.FromMilliseconds(500),
+            Timeout = TimeSpan.FromMilliseconds(500),
+            AutoAcceptCertificates = true,
+        };
+        using var drv = new OpcUaClientDriver(opts, "opcua-failover");
+
+        var ex = await Should.ThrowAsync<AggregateException>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+
+        ex.Message.ShouldContain("127.0.0.1:1");
+        ex.Message.ShouldContain("127.0.0.1:2");
+        ex.Message.ShouldContain("127.0.0.1:3");
+        drv.GetHealth().State.ShouldBe(DriverState.Faulted);
+    }
+}

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

@@ -0,0 +1,91 @@
+using Opc.Ua;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientHistoryTests
+{
+    [Theory]
+    [InlineData(HistoryAggregateType.Average)]
+    [InlineData(HistoryAggregateType.Minimum)]
+    [InlineData(HistoryAggregateType.Maximum)]
+    [InlineData(HistoryAggregateType.Total)]
+    [InlineData(HistoryAggregateType.Count)]
+    public void MapAggregateToNodeId_returns_standard_Part13_aggregate_for_every_enum(HistoryAggregateType agg)
+    {
+        var nodeId = OpcUaClientDriver.MapAggregateToNodeId(agg);
+        NodeId.IsNull(nodeId).ShouldBeFalse();
+        // Every mapping should resolve to an AggregateFunction_* NodeId (namespace 0, numeric id).
+        nodeId.NamespaceIndex.ShouldBe((ushort)0);
+    }
+
+    [Fact]
+    public void MapAggregateToNodeId_rejects_invalid_enum_value()
+    {
+        // Defense-in-depth: a future HistoryAggregateType addition mustn't silently fall through.
+        Should.Throw<ArgumentOutOfRangeException>(() =>
+            OpcUaClientDriver.MapAggregateToNodeId((HistoryAggregateType)99));
+    }
+
+    [Fact]
+    public async Task ReadRawAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-hist-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.ReadRawAsync("ns=2;s=Counter",
+                DateTime.UtcNow.AddMinutes(-5), DateTime.UtcNow, 1000,
+                TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task ReadRawAsync_with_malformed_NodeId_returns_empty_result_not_throw()
+    {
+        // Same defensive pattern as ReadAsync / WriteAsync — malformed NodeId short-circuits
+        // to an empty result rather than crashing a batch history call. Needs init via the
+        // throw path first, then we pass "" to trigger the parse-fail branch inside
+        // ExecuteHistoryReadAsync. The init itself fails against 127.0.0.1:1 so we stop there.
+        // Not runnable without init — keep as placeholder for when the in-process fixture
+        // PR lands.
+        await Task.CompletedTask;
+    }
+
+    [Fact]
+    public async Task ReadProcessedAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-hist-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.ReadProcessedAsync("ns=2;s=Counter",
+                DateTime.UtcNow.AddMinutes(-5), DateTime.UtcNow,
+                TimeSpan.FromSeconds(10), HistoryAggregateType.Average,
+                TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task ReadAtTimeAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-hist-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.ReadAtTimeAsync("ns=2;s=Counter",
+                [DateTime.UtcNow.AddMinutes(-5), DateTime.UtcNow],
+                TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task ReadEventsAsync_throws_NotSupportedException_as_documented()
+    {
+        // The IHistoryProvider default implementation throws; the OPC UA Client driver
+        // deliberately inherits that default (see PR 76 commit body) because the OPC UA
+        // client call path needs an EventFilter SelectClauses spec the interface doesn't carry.
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-events-default");
+        await Should.ThrowAsync<NotSupportedException>(async () =>
+            await ((IHistoryProvider)drv).ReadEventsAsync(
+                sourceName: null,
+                startUtc: DateTime.UtcNow.AddMinutes(-5),
+                endUtc: DateTime.UtcNow,
+                maxEvents: 100,
+                cancellationToken: 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/OpcUaClientReconnectTests.cs

@@ -0,0 +1,36 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Scaffold tests for <see cref="SessionReconnectHandler"/> wiring. Wire-level
+///     disconnect-reconnect-resume coverage against a live upstream server lands with the
+///     in-process fixture — too much machinery for a unit-test-only lane.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientReconnectTests
+{
+    [Fact]
+    public void Default_ReconnectPeriod_matches_driver_specs_5_seconds()
+    {
+        new OpcUaClientDriverOptions().ReconnectPeriod.ShouldBe(TimeSpan.FromSeconds(5));
+    }
+
+    [Fact]
+    public void Options_ReconnectPeriod_is_configurable_for_aggressive_or_relaxed_retry()
+    {
+        var opts = new OpcUaClientDriverOptions { ReconnectPeriod = TimeSpan.FromMilliseconds(500) };
+        opts.ReconnectPeriod.ShouldBe(TimeSpan.FromMilliseconds(500));
+    }
+
+    [Fact]
+    public void Driver_starts_with_no_reconnect_handler_active_pre_init()
+    {
+        // The reconnect handler is lazy — spun up only when a bad keep-alive fires. Pre-init
+        // there's no session to reconnect, so the field must be null (indirectly verified by
+        // the lifecycle-shape test suite catching any accidental construction).
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-reconnect");
+        drv.GetHealth().State.ShouldBe(Core.Abstractions.DriverState.Unknown);
+    }
+}

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

@@ -0,0 +1,54 @@
+using Opc.Ua;
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientSecurityPolicyTests
+{
+    [Theory]
+    [InlineData(OpcUaSecurityPolicy.None)]
+    [InlineData(OpcUaSecurityPolicy.Basic128Rsa15)]
+    [InlineData(OpcUaSecurityPolicy.Basic256)]
+    [InlineData(OpcUaSecurityPolicy.Basic256Sha256)]
+    [InlineData(OpcUaSecurityPolicy.Aes128_Sha256_RsaOaep)]
+    [InlineData(OpcUaSecurityPolicy.Aes256_Sha256_RsaPss)]
+    public void MapSecurityPolicy_returns_known_non_empty_uri_for_every_enum_value(OpcUaSecurityPolicy policy)
+    {
+        var uri = OpcUaClientDriver.MapSecurityPolicy(policy);
+        uri.ShouldNotBeNullOrEmpty();
+        // Each URI should end in the enum name (for the non-None policies) so a driver
+        // operator reading logs can correlate the URI back to the config value.
+        if (policy != OpcUaSecurityPolicy.None)
+            uri.ShouldContain(policy.ToString());
+    }
+
+    [Fact]
+    public void MapSecurityPolicy_None_matches_SDK_None_URI()
+    {
+        OpcUaClientDriver.MapSecurityPolicy(OpcUaSecurityPolicy.None)
+            .ShouldBe(SecurityPolicies.None);
+    }
+
+    [Fact]
+    public void MapSecurityPolicy_Basic256Sha256_matches_SDK_URI()
+    {
+        OpcUaClientDriver.MapSecurityPolicy(OpcUaSecurityPolicy.Basic256Sha256)
+            .ShouldBe(SecurityPolicies.Basic256Sha256);
+    }
+
+    [Fact]
+    public void MapSecurityPolicy_Aes256_Sha256_RsaPss_matches_SDK_URI()
+    {
+        OpcUaClientDriver.MapSecurityPolicy(OpcUaSecurityPolicy.Aes256_Sha256_RsaPss)
+            .ShouldBe(SecurityPolicies.Aes256_Sha256_RsaPss);
+    }
+
+    [Fact]
+    public void Every_enum_value_has_a_mapping()
+    {
+        foreach (OpcUaSecurityPolicy p in Enum.GetValues<OpcUaSecurityPolicy>())
+            Should.NotThrow(() => OpcUaClientDriver.MapSecurityPolicy(p));
+    }
+}

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>