lmxopcua/archreview/plans/05-protocol-drivers-plan.md

Design + Implementation Plan — 05 Protocol Drivers

Status (2026-07-08): BOTH Criticals DONE. STAB-1 (S7 reconnect, task 4) — fix/archreview-crit3-s7-reconnect 25c0c6f6; IS7Plc/IS7PlcFactory seam + lazy EnsureConnectedAsync; 4 unit guards + LIVE-verified against real snap7 (container bounce → recovers, no redeploy). STAB-2 (TwinCAT orphaned ADS subs, task 5) — fix/archreview-crit4-twincat-replay af318fb4; replayable-intent store + reconnect replay + probe-recycle; 4 unit guards + 174/174 (unit-only — no TwinCAT docker fixture). Remaining (non-Critical): STAB-4/5 (AbCip lock, FOCAS caches) + Part B shared-template consolidation. See STATUS.md.

Source report: archreview/05-protocol-drivers.md Review commit: 9cad9ed0 (== current HEAD, so no drift) Verification date: 2026-07-08 Scope: Modbus, S7, AbCip, AbLegacy, TwinCAT, FOCAS, OpcUaClient (+ Contracts, + Cli)

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Verification summary

Every finding in the report was opened at its cited file:line and checked against the current tree. 37 actionable findings; 37 CONFIRMED; 0 stale. (STAB-13 is a "positive" observation carrying no work.) The line references were accurate to within 1–2 lines throughout. The zero-drift result is expected: the review commit is the current HEAD.

Confirmed counts by section: Stability 12 actionable (STAB-1…12) + 1 positive (STAB-13); Performance 7 (PERF-1…7); Conventions 8 (CONV-1…8); Underdeveloped 9 (UNDER-1…9).

Top 3 by priority (align with OVERALL actions #3/#4 and report remediation order):

1. STAB-1 — S7 no reconnect path (Critical). Plc.OpenAsync only at S7Driver.cs:165; grep confirms it is the sole open site, no EnsureConnected.
2. STAB-2 — TwinCAT native subscriptions orphaned after reconnect (Critical). NativeSubscription (TwinCATDriver.cs:517-519) stores only disposable handles, not replayable intent; EnsureConnectedAsync swaps the client (:614-644) and nothing re-runs AddNotificationAsync; AdsTwinCATClient.Dispose clears _notifications (:426).
3. STAB-4 + STAB-5 — silent-corruption class. AbCip runs Read→GetStatus→Decode on shared libplctag handles with no per-runtime lock (AbCipDriver.cs:544-575; only GetRmwLock exists, bit-writes only), while AbLegacy guards the identical hazard with GetRuntimeLock (AbLegacyDriver.cs:786, used at :227/:331). FOCAS's fixed-tree caches are plain Dictionary (FocasDriver.cs:1184,1190,1192,1194) read/written across threads.

The two systemic threads the report calls out — theme #2 (fixes don't flow back to shared templates) and theme #6 (equipment-tag/TagConfig paths are second-class) — are both real and are given a dedicated consolidation section (Part B) so the fleet fixes land in one home rather than being re-patched per driver.

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Part A — Per-driver Criticals & Highs

A1. STAB-1 (Critical) — S7 has no reconnect path

Restatement: the S7 Plc is opened once in InitializeAsync; a transient PLC reboot/network blip permanently kills the instance until redeploy.

Verification: CONFIRMED. grep OpenAsync S7Driver.cs → single hit at line 165, inside InitializeAsync. No EnsureConnected/Reconnect symbol. Reads/writes call RequirePlc() under _gate and reuse the dead handle (S7Driver.cs:479-483, 995-1001). The probe loop detects the outage and transitions Stopped but never recycles. ReinitializeAsync (:212) just re-calls InitializeAsync (externally driven only). Zero S7 reconnect tests (tests/Drivers/.../S7.Tests has no reconnect file; only Modbus, OpcUaClient do).

Root cause: the Plc is a concrete S7.Net type new-ed inline (S7Driver.cs:156, field at :122), with no factory seam and no lazy-connect gate. The template calls for a lazy EnsureConnectedAsync (TwinCAT/AbCip have one; S7 never got it).

Design (mirror TwinCAT + OpcUaClient):

• Introduce a testable seam: IS7Plc (wrapping the members S7Driver uses: OpenAsync, Close, ReadBytesAsync/ReadMultiple, Write*, IsConnected) and IS7PlcFactory with Create(cpu, host, port, rack, slot). Default impl wraps S7.Net.Plc; the ctor takes an optional IS7PlcFactory? = null defaulting to the real one — exactly TwinCAT's ITwinCATClientFactory pattern (TwinCATDriver.cs:20,58,64).
• Add private async Task<IS7Plc> EnsureConnectedAsync(CancellationToken ct) guarded by the existing _gate: fast-path returns a live Plc; under the gate, if the Plc is null or a prior socket-level PlcException marked it dead, dispose and re-OpenAsync a fresh instance (with the same timeout-linked CTS as :163-164).
• Route ReadAsync/WriteAsync/PollOnceAsync through EnsureConnectedAsync instead of RequirePlc(). On a socket-level PlcException during a read/write, null the Plc (mark dead) so the next call reopens, and set Degraded (keep LastSuccessfulRead) rather than Faulted. Preserve the existing Faulted escalation for PUT/GET-denied (S7Driver.cs:921-1006) — that is a permanent config fault, not a transient drop.
• Classify socket-level vs protocol-level PlcException in a small IsS7ConnectionFatal(Exception) helper (S7.Net surfaces ErrorCode.ConnectionError / WriteData / ReadData / IO/socket exceptions). Do not reopen on a data-address/type error.

Alternatives considered: (a) let the probe loop own recycle-on-Stopped — rejected as sole mechanism because reads between probe ticks would keep failing on the dead handle; the lazy EnsureConnectedAsync repairs on the very next data call and the probe becomes a backstop. (b) Reconnect handler à la OpcUaClient's SessionReconnectHandler — overkill; S7.Net has no session-transfer semantics, a plain reopen is correct.

Implementation steps:

1. New file Driver.S7/IS7Plc.cs (+ S7PlcAdapter, IS7PlcFactory, S7PlcFactory). Change S7Driver.Plc field type to IS7Plc?.
2. Thread IS7PlcFactory through the ctor + S7DriverFactoryExtensions.CreateInstance.
3. Add EnsureConnectedAsync + IsS7ConnectionFatal; convert the two RequirePlc call clusters and PollOnceAsync (:1295-1309).
4. Optionally: on probe transition to Stopped, null the Plc so the next data call reconnects even if no read failed in between.

Tests:

• Unit (new S7DriverReconnectTests.cs): fake IS7PlcFactory whose IS7Plc throws a connection-fatal PlcException on read N, succeeds on read N+1; assert the driver creates a second IS7Plc, health goes Degraded→Healthy, and the tag recovers to Good. Assert a protocol error (illegal address) does not trigger a reopen.
• Integration (S7.IntegrationTests, skip-gated on Snap7ServerFixture): subscribe, bounce the snap7 container (lmxopcua-fix down s7 s7_1500 / up), assert values resume without redeploy. Note: no per-test container hook exists, so gate this as a manual/[Trait("Category","Reconnect")] recipe.

Effort: M. Risk: Medium — touches the hot read/write path; the IS7Plc seam is mechanical but broad. Mitigated by the fake-factory unit tests (which also close the TEST-1 gap).

A2. STAB-2 (Critical) — TwinCAT native subscriptions orphaned after reconnect

Restatement: native ADS notifications (the default mode) silently stop after a client swap — no Bad status, no error.

Verification: CONFIRMED. SubscribeAsync registers AddNotificationAsync once per tag (TwinCATDriver.cs:472) and stores the resulting handles in NativeSubscription(Handle, Registrations) (:495, :517-519) — no record of the symbol/type/interval/handler needed to replay. EnsureConnectedAsync disposes a stale client and creates a fresh one (:614-644); AdsTwinCATClient.Dispose clears _notifications (:426). Compounding: the fast path keys on IsConnected (:606), the local AMS-port state, not wire liveness; and a probe failure only transitions state, never forces a client recycle (ProbeLoopAsync :540-547).

Root cause: subscription state is stored as opaque handles rather than replayable intent, and there is no "client changed → replay" hook. Native ADS is push, so unlike the poll fallback it cannot self-heal by re-reading.

Design:

• Change NativeSubscription to store replayable registrations: record NativeRegistrationIntent(string DeviceHostAddress, string Reference, string SymbolName, TwinCATDataType Type, int? BitIndex, TimeSpan Interval, Action<string,object?> OnChange) plus the live ITwinCATNotificationHandle.
• Give DeviceState an epoch/generation counter incremented every time EnsureConnectedAsync installs a new client (:644). After installing a new client, replay every NativeRegistrationIntent whose DeviceHostAddress matches: call AddNotificationAsync again and swap the stored live handle. Do this inside EnsureConnectedAsync under ConnectGate so it cannot race a concurrent subscribe.
• Fix the liveness signal: the probe should call a real read (ProbeAsync already does a symbol read) and, on failure, dispose+null the device client so EnsureConnectedAsync rebuilds — i.e. make probe-detected outage force a recycle (ProbeLoopAsync catch block at :541-545 should null device.Client). Consider keying the fast path (:606) additionally on a "known-good since last probe" flag so a locally-connected-but-wire-dead port still triggers the rebuild-and-replay.

Alternatives: switch the default to the poll fallback (self-healing) — rejected; native push is the performance-correct default and the report explicitly wants replay, not abandonment.

Implementation steps:

1. Introduce a per-device ConcurrentDictionary<subId, List<NativeRegistrationIntent>> (or hang intents off DeviceState). Populate in SubscribeAsync.
2. Extract the per-tag registration into RegisterNotificationAsync(device, intent) used by both initial subscribe and replay.
3. In EnsureConnectedAsync, after device.Client = client (:644), iterate the device's intents and replay; log a re-registration count.
4. In ProbeLoopAsync, on probe failure null the device client (force rebuild).

Tests:

• Unit (new TwinCATReconnectReplayTests.cs) using the existing fake ITwinCATClientFactory/ITwinCATClient: subscribe a tag, simulate a client that reports IsConnected=false, force EnsureConnectedAsync to build a replacement, assert AddNotificationAsync is re-invoked with the same symbol/type/interval and that a subsequent fake push reaches OnDataChange. Assert the old handle is disposed.
• Unit: probe-failure recycles the client (assert a new client instance built).
• Note there is no TwinCAT docker fixture (13 integration tests need a real TC3 XAR); the fake-client unit tests are the only automated coverage — make them authoritative.

Effort: M. Risk: Medium — concurrency around ConnectGate/replay; must not double-register. The fake client makes it fully unit-testable (the report's key gap).

A3. STAB-3 (High) — Modbus transport desyncs after a response timeout

Restatement: a per-op timeout or TxId mismatch leaves the single-flight socket half-read; the next txn reads a stale response and the stream stays desynchronized.

Verification: CONFIRMED. IsSocketLevelFailure (ModbusTcpTransport.cs:257-261) matches only EndOfStreamException/IOException/SocketException/ ObjectDisposedException. The per-op deadline (:226-227 linked CTS CancelAfter) surfaces as OperationCanceledException; the TxId guard throws InvalidDataException (:234-235); the truncated-length guard throws InvalidDataException (:237) — none of which are socket-level, so the reconnect-and-retry catch (:155-165) is skipped and the socket is retained mid-desync.

Root cause: the transport conflates "protocol-layer error, keep the socket" with "framing/timeout error, the socket is unusable." Timeout and any framing violation leave unknown bytes in the receive buffer; the connection is fatally desynchronized even though the exception type is not IOException.

Design: treat per-op timeout and any framing violation as connection-fatal: tear the socket down before propagating, so the next SendAsync reconnects clean.

• In SendOnceAsync, wrap the read/write in a try that, on OperationCanceledException where the caller's ct is NOT cancelled (i.e. the timeout linked-CTS fired), and on InvalidDataException, calls TearDownAsync() then rethrows a normalized ModbusTransportDesyncException : IOException (subclassing IOException lets the existing :155 catch optionally reconnect-and-retry once). Distinguish caller cancellation (propagate as OCE, no teardown) from timeout (teardown).
• Simplest correct implementation: after TearDownAsync, rethrow as IOException so both the retry classifier and the desync-repair share one path.

Alternatives: add timeout/framing to IsSocketLevelFailure directly — works but must still tear down the socket first (the current retry path does TearDownAsync before reconnect, so adding the types and ensuring teardown covers it). Preferred to keep IsSocketLevelFailure about exception type and add an explicit teardown at the throw site for framing/timeout, since those need teardown even when auto-reconnect is off.

Implementation steps:

1. In SendOnceAsync, catch OperationCanceledException (timeout branch) and InvalidDataException; await TearDownAsync(); rethrow as IOException (ModbusTransportDesyncException).
2. Keep the outer SendAsync single-retry (:155-165) — it now covers the desync.
3. Ensure teardown even when _autoReconnect is false (the socket must not be reused desynced regardless).

Tests:

• Unit (ModbusTcpReconnectTests.cs extension): a fake stream that (a) stalls past the deadline, (b) returns a wrong TxId, (c) returns a truncated header. Assert each triggers TearDownAsync (socket disposed) and the following call reconnects. Assert a caller-cancellation (external ct) does NOT tear the socket down.
• Integration (Modbus.IntegrationTests + exception_injector): the existing rig already forces FC exceptions; add a timeout scenario (non-responding unit) and assert recovery on the next poll.

Effort: S. Risk: Low-Medium — must get the timeout-vs-cancellation distinction right or a legitimate shutdown could look like a desync (harmless, just an extra reconnect).

A4. STAB-4 (High) — AbCip concurrent ops on shared libplctag handles, no lock

Restatement: AbCip read/group/write run Read→GetStatus→Decode / Encode→Write→GetStatus on cached IAbCipTagRuntime with zero serialization while the server read path, poll loops, and the alarm-projection loop can hit the same handle.

Verification: CONFIRMED. ReadSingleAsync (AbCipDriver.cs:544-575), ReadGroupAsync (:614-637), WriteAsync (:708-726) take no runtime lock; the only locks are GetRmwLock (bit-in-DINT writes, :1243) and a creation lock. AbLegacy documents the exact hazard and guards it: GetRuntimeLock (AbLegacyDriver.cs:786), applied on both read (:227) and write (:331), comment "A libplctag Tag handle is not safe for concurrent Read/GetStatus/DecodeValue." AbCipAlarmProjection.cs:149/228 reads through the unlocked path. libplctag's GetStatus returns the last operation's status, so concurrent Read/GetStatus can cross-attribute results → torn values with Good status.

Root cause: the AbLegacy fix never propagated to AbCip (theme #2). AbCip caches a per-tag runtime but never serializes operations on it.

Design: port AbLegacy's per-runtime operation lock verbatim.

• Add GetRuntimeLock(string tagName) to AbCip's DeviceState (a ConcurrentDictionary<string,SemaphoreSlim> keyed by tag name, matching AbLegacyDriver.cs:786).
• Wrap the whole Read/GetStatus/Decode critical section in ReadSingleAsync and ReadGroupAsync, and the Encode/Write/GetStatus section in WriteAsync, in await opLock.WaitAsync(ct) / finally Release. The existing GetRmwLock (bit RMW) must nest inside or replace-by the runtime lock consistently — since RMW does read-modify-write on the same handle, take the runtime lock for the whole RMW too (this also fixes STAB-11's "direct parent write bypasses the lock" for AbCip: a direct parent write and an RMW now serialize on the same per-tag runtime lock).

Alternatives: a single per-device lock — rejected; too coarse, serializes independent tags. Per-runtime lock matches AbLegacy and preserves cross-tag parallelism.

Implementation steps:

1. Add _runtimeLocks + GetRuntimeLock to AbCip DeviceState.
2. Wrap the three op bodies. Ensure eviction-on-failure (EvictRuntime, :554) also disposes/removes the tag's lock or leaves it (a fresh runtime under the same name reuses the lock — fine).
3. Reconcile RMW: make WriteBitInDIntAsync acquire the per-runtime lock instead of (or in addition to) GetRmwLock; collapse to one lock domain per parent to also close STAB-11.

Tests:

• Unit (new AbCipRuntimeConcurrencyTests.cs, mirror AbLegacyRuntimeConcurrencyTests): a fake IAbCipTagRuntime that asserts no overlapping Read/GetStatus (e.g. a reentrancy counter that throws if >1); hammer concurrent ReadAsync+WriteAsync+alarm-read on one tag; assert serialization and no torn/cross-attributed status.
• Integration (abcip fixture on :44818): concurrent subscribe + write to overlapping tags, assert Good values stay coherent.

Effort: S. Risk: Low — additive locking, blast radius is AbCip only.

A5. STAB-5 (High) — FOCAS fixed-tree caches are plain Dictionaries mutated across threads

Restatement: LastFixedSnapshots/LastServoLoads/LastSpindleLoads/LastTimers are plain Dictionary, written by FixedTreeLoopAsync and read on ReadAsync threads.

Verification: CONFIRMED. Fields at FocasDriver.cs:1184 (Dictionary<string,double>), :1190 (Dictionary<FocasTimerKind,FocasTimer> = []), :1192 (Dictionary<string,double>), :1194 (Dictionary<int,int> = []). Writes at :752, :793, :836-839; reads at :912, :920, :945. Contrast the neighbouring _parsedAddressesByTagName = new ConcurrentDictionary (:40) — that field got the treatment, these did not. Concurrent write-during-Dictionary-resize is UB (throw or corruption).

Root cause: partial application of the concurrent-collection fix (theme #2).

Design: convert the four caches to ConcurrentDictionary<,>. TryGetValue and indexer-set are the only operations used, both supported. LastTimers and LastSpindleLoads collection-initializer [] becomes new().

• Alternative (immutable snapshot swap): build a fresh dictionary per loop pass and Volatile.Write the reference; readers Volatile.Read. Cleaner isolation but more churn; ConcurrentDictionary is the lower-risk minimal change and matches the in-tree precedent (:40).

Implementation steps: change the four field types (on DeviceState) + collection initializers; no call-site changes needed (indexer + TryGetValue are identical).

Tests:

• Unit (FocasDriver.Tests): a stress test spinning FixedTreeLoopAsync-style writes while readers call TryReadFixedTree; assert no exception over N iterations (would reliably throw on the plain Dictionary today).

Effort: S. Risk: Very low — type-only change, FOCAS-local.

A6. STAB-6 (High) — AbLegacy probe-loop shutdown race; S7 unsubscribe race

Verification: CONFIRMED both.

• AbLegacy: probe task fire-and-forgot _ = Task.Run(...) (AbLegacyDriver.cs:118); ShutdownAsync (:156-170) cancels then immediately disposes ProbeCts + runtimes without awaiting loop exit — a winding-down loop can raise OnHostStatusChanged and touch a disposed CTS. AbCip already fixed this: retained ProbeTask (:298) + phased shutdown with await probeTask.WaitAsync(10s) (:332-365).
• S7: UnsubscribeAsync (:1185-1193) cancels + disposes the CTS without draining PollTask; Task.Delay on the disposed source can throw ObjectDisposedException the loop only catches as OCE (:1233-1234). ShutdownAsync does drain PollTask (:239), so the bug is confined to the Unsubscribe path.

Root cause: cancel-then-dispose-without-await; PollGroupEngine.StopState (PollGroupEngine.cs:99-112) already solves this by awaiting the loop (bounded) before disposing the CTS — neither bespoke path adopted it.

Design:

• AbLegacy: retain the probe task in DeviceState.ProbeTask and phase shutdown like AbCip (await probeTask.WaitAsync(bounded) before ProbeCts.Dispose()).
• S7: in UnsubscribeAsync, Cancel(), then state.PollTask.Wait(bounded) (or await), then Cts.Dispose() — mirroring StopState. Better: once S7 migrates to PollGroupEngine (see B1) this deletes itself.

Tests: unit — subscribe/unsubscribe rapidly in a loop and assert no ObjectDisposedException escapes; shutdown while a probe tick is mid-transition.

Effort: S. Risk: Low.

A7. UNDER-1 (High) — FOCAS advertises writes it cannot perform

Verification: CONFIRMED. WireFocasClient.WriteAsync returns FocasStatusMapper.BadNotWritable for every address (WireFocasClient.cs:71-73), yet FocasDriverOptions.Writable defaults true (FocasDriverOptions.cs:168) and FocasEquipmentTagParser hard-codes Writable: true (FocasEquipmentTagParser.cs:35). The driver's write exception-mapping (FocasDriver.cs:353-381) is dead code against this backend. Net: equipment tags surface as Operate-writable OPC UA nodes whose writes always fail.

Root cause: the writable default was written speculatively (doc cites "write tests") but the only real backend is read-only.

Design (force writable-false at the seams until PMC writes exist):

• FocasEquipmentTagParser.cs:35 → Writable: false.
• FocasDriverOptions.Writable default → false (and/or force it false at discovery materialization). Keep the option field for the future PMC-write feature but do not advertise writable nodes today.
• Leave the write status-mapping in place (it becomes live if/when PMC writes ship).
• Alternative (larger): implement PMC writes in WireFocasClient via pmc_wrpmcrng — a separate feature, out of scope for a hardening pass.

Tests: unit — assert a FOCAS equipment/authored tag materializes without the CurrentWrite access bit; assert a write attempt returns BadNotWritable (guards the regression if someone flips the default back).

Effort: S. Risk: Very low (removes a false capability).

A8. UNDER-2 / UNDER-3 (High) — OpcUaClient: dead UnsMappingTable gate; broken alarm-filter encoding

UNDER-2 verification: CONFIRMED. ValidateNamespaceKind hard-fails Equipment-kind configs lacking UnsMappingTable (OpcUaClientDriver.cs:335-338) but grep shows the table is read nowhere except that gate and the options DTO (OpcUaClientDriverOptions.cs:169,188). DiscoverAsync renders refs purely from remote browse via StableReference (:808-809). Operators must author a table with zero runtime effect.

UNDER-3 verification: CONFIRMED. Persisted refs are stable nsu= strings (:808-809), but the alarm source filter is an ordinal HashSet of caller refs (:1337) matched against EventFields[SourceNode].ToString() — a session-relative ns=N;… rendering (:1538-1539). The two encodings never match, so any non-empty filter drops every event; only empty-filter subscriptions work.

Root cause: UNDER-2 is a validation gate for an unbuilt feature; UNDER-3 is an encoding mismatch between the stored nsu= form and the runtime ns=N form.

Design:

• UNDER-2: delete the gate (remove the Equipment-kind UnsMappingTable mandatory check) — the honest minimal change, since the feature does not exist. Keep the option DTO field (forward-compatible) but stop forcing operators to author dead config. Alternative (build the feature): have DiscoverAsync translate remote refs → UNS refs via the table — a real feature, larger, defer.
• UNDER-3: normalize both sides to the same encoding before compare. Convert the incoming EventFields[SourceNode] NodeId to the stable nsu= reference via the driver's NamespaceMap.ToStableReference (the same function used at :808-809) before the HashSet.Contains check; build the filter set from stable refs. Then a non-empty filter works.

Implementation steps: remove ValidateNamespaceKind's Equipment branch (or make it a no-op warning); in OnEventNotification, map the source NodeId through _namespaceMap before comparing (:1538-1539); build sourceFilter from stable refs (:1337).

Tests: unit — event with a known source arrives, non-empty stable-ref filter matches and the event is delivered (today it is dropped); Equipment config without a mapping table no longer throws at validation.

Effort: S (both). Risk: Low — OpcUaClient-local; UNDER-3 fix is a behavior change that enables previously-dead filtering (verify no consumer depended on the "filter silently drops all" bug).

A9. STAB-13 (Positive) — no action

Write-outcome surfacing is honoured by every protocol driver (per-item WriteResult with typed exception→status), and OpcUaClient's reconnect machine is exemplary (double-arm guard, re-arm on exhaustion, session swap under _probeLock, :1908-2058). CONFIRMED. Use OpcUaClient's machine as the reference for A1/A2. One caveat surfaced: Session.TransferSubscriptionsOnReconnect is left unset (mentioned only in the doc at :1948), so subscription transfer rests on the SDK default — worth explicitly setting it (folds into UNDER-9 cleanup).

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Part B — Shared-template consolidation (themes #2 and #6)

The report's cross-cutting message: every High is a fix present in one sibling and absent in another. These four items give the fleet a single home for the fixes so they stop being re-patched per driver. Do these after the per-driver criticals but treat them as the highest-leverage structural work.

B1. PollGroupEngine v2 — absorb S7's fork; wire onError; add backoff (CONV-1, STAB-9, STAB-8, PERF-3, STAB-6-S7)

Verification of the pieces:

• PollGroupEngine already has the onError sink (PollGroupEngine.cs:36,55-67,164-169), StopState draining (:99-112), and structural array compare (:203-209). It does not have failure backoff — PollLoopAsync uses a fixed Task.Delay(state.Interval) (:131).
• STAB-9 CONFIRMED: no consumer passes onError — the five constructions (ModbusDriver.cs:115, AbCipDriver.cs:133, AbLegacyDriver.cs:65, TwinCATDriver.cs:69, FocasDriver.cs:72) omit it, so the sink is dead fleet-wide.
• CONV-1 / PERF-3 / STAB-6-S7 CONFIRMED: S7 re-ships the loop (S7Driver.cs:1165-1307), which has backoff (ComputeBackoffDelay :1284-1293) and failure-health (HandlePollFailure :1258-1274) the engine lacks, but lacks the structural array diff (uses Equals(lastSeen?.Value, current.Value) :1304 → arrays fire every tick) and the drain-before-dispose (UnsubscribeAsync :1189-1190).

Design (single home for the poll loop):

1. Add optional backoff to PollGroupEngine. Port S7's ComputeBackoffDelay (capped exponential, ticks-based, saturating at a PollBackoffCap) into the engine. Constructor gains TimeSpan? backoffCap = null (null ⇒ current fixed-cadence behavior, preserving every current consumer). PollLoopAsync tracks consecutiveFailures, resets on success, and delays ComputeBackoffDelay(interval, failures).
2. Route failures to health. The engine already forwards to onError; make each of the five drivers pass an onError that degrades _health (keeping LastSuccessfulRead) + logs — i.e. lift S7's HandlePollFailure into a small shared Action<Exception> per driver. Consider making onError required (the report suggests it) once all five pass it.
3. Migrate S7 onto the engine and delete the fork. Replace S7Driver's _subscriptions dict + PollLoopAsync/PollOnceAsync/ComputeBackoffDelay/ HandlePollFailure (:1162-1310) with a PollGroupEngine instance (reader = ReadAsync, onChange = forward OnDataChange, onError = degrade health, backoffCap = 30 s). This deletes PERF-3 (structural compare now inherited) and STAB-6-S7 (StopState drain now inherited) for free.

Implementation steps:

1. Extend PollGroupEngine ctor + PollLoopAsync with backoff; add unit tests for ComputeBackoffDelay parity with S7's (move the existing S7 test asserts over).
2. Add an onError handler in Modbus/AbCip/AbLegacy/TwinCAT/FOCAS constructions that degrades health; assert via unit test that a throwing reader degrades health.
3. S7: construct the engine, delete the fork, keep the 100 ms floor and the PollFailureHealthThreshold/Degraded-not-Faulted nuance in the onError handler.

Tests:

• Engine unit: backoff schedule (0 failures = interval, N failures = capped exponential); onError invoked once per caught reader exception; structural array equality still suppresses no-change arrays; StopState drains before CTS dispose.
• S7 unit: array tag no longer fires every tick (regression for PERF-3); rapid subscribe/unsubscribe no ObjectDisposedException (STAB-6); sustained poll failure degrades health with backoff.

Effort: M (engine change is S; S7 migration + 5 onError wirings is M). Risk: Medium — S7 migration touches its subscription surface; the array-diff change alters publish cadence (intended). Gate behind the existing S7 subscribe tests + a new array-cadence test.

B2. Shared strict ReadEnum for equipment-tag parsers (CONV-2, UNDER-6)

Verification: CONFIRMED. An identical private generic ReadEnum<TEnum>(…, TEnum fallback) with silent fallback is copy-pasted six times — a typo'd dataType quietly becomes the default type and reads/writes the wrong width with Good status: ModbusEquipmentTagParser.cs:65-67, S7EquipmentTagParser.cs:59-61, AbCipEquipmentTagParser.cs:86-88, AbLegacyEquipmentTagParser.cs:60-62, TwinCATEquipmentTagParser.cs:47-49, FocasEquipmentTagParser.cs:43-45 (all byte-identical bodies). Contrast the factory's fail-loud ParseEnum (ModbusDriverFactoryExtensions.cs:191-201, lists valid names). Probe-parity is also broken for Modbus: ModbusDriverProbe binds the runtime ModbusDriverOptions (ModbusDriverProbe.cs:36) not the factory DTO, so timeoutMs is silently dropped.

Root cause: duplication-by-convention (theme #2); the three parse tiers (factory strict / probe permissive / parser silent) diverged.

Design:

• Hoist one strict helper into Core.Abstractions, living beside EquipmentTagRefResolver (theme #6's home): e.g. TagConfigJson.ReadEnum<TEnum>(JsonElement o, string name, TEnum @default) with the rule present-but-invalid ⇒ throw (parse failure), absent ⇒ default. The parser wraps the throw so the resolver's _parseRef returns null on a bad enum, and the driver surfaces BadNodeIdUnknown instead of silently reading the wrong width.
• Delete all six private copies; the parsers call the shared helper. Enums stay per-driver (the helper is generic over TEnum), so no coupling of driver vocab.
• Probe parity (Modbus): make ModbusDriverProbe deserialize the same factory DTO shape the factory uses (or share a TryParseConfig used by both), so timeoutMs binds identically. OpcUaClient already documents this rule (OpcUaClientDriverProbe.cs:22); Modbus should follow.

Writable/capability parity (UNDER-6, folds in here): Modbus/AbLegacy/FOCAS parsers hard-code Writable: true (ModbusEquipmentTagParser.cs:54-57, AbLegacyEquipmentTagParser.cs:52, FocasEquipmentTagParser.cs:35), so read-only equipment tags are unauthorable on three drivers (AbCip honours a writable key, AbCipEquipmentTagParser.cs:53-54). FOCAS additionally must force writable-false (A7). Add a shared ReadBool(o, "writable", default) helper and honour it in all six parsers; FOCAS overrides to false until PMC writes exist. FOCAS equipment tags also bypass the FocasCapabilityMatrix gate authored tags get (FocasDriver.cs:243-247 vs :115-119) — route equipment-tag resolution through the same matrix validate.

Implementation steps:

1. New Core.Abstractions/TagConfigJson.cs with strict ReadEnum + ReadBool.
2. Replace the six ReadEnum copies; wrap parse failure so _parseRef returns null.
3. Honour writable in the three hard-coded parsers; FOCAS → false + capability gate.
4. Fix ModbusDriverProbe to parse the factory DTO shape.

Tests:

• Shared conformance test (the report explicitly wants one): a parameterized suite run per driver asserting (a) unknown dataType ⇒ resolve failure ⇒ BadNodeIdUnknown (not silent default); (b) writable:false honoured; (c) absent enum ⇒ documented default. Put it in a shared test helper consumed by each driver's .Tests.
• Modbus probe: a config with timeoutMs round-trips the timeout.

Effort: M. Risk: Medium — changing silent-default to hard-fail is a behavior change: a deployment with a currently-silently-defaulted typo will start returning Bad. That is the correct outcome, but call it out in the migration notes and verify live on the rig (author a typo'd tag, confirm Bad rather than wrong-width Good).

B3. ResolveHost via the resolver — fix per-host breaker isolation (CONV-4, theme #6)

Verification: CONFIRMED. Every multi-device driver's ResolveHost consults only _tagsByName then falls back to the first device: AbCipDriver.cs:485-487, AbLegacyDriver.cs:498-500, TwinCATDriver.cs:587-591, FocasDriver.cs:1082-1087 (Modbus :125-131 same shape, but per-tag UnitId isn't in its equipment parser). An equipment-tag reference (raw TagConfig JSON, resolved via _resolver, not in _tagsByName) misses and returns the first device's host — so the per-host Polly bulkhead/circuit-breaker key is wrong for equipment tags, and device B's outage can trip device A's breaker. Since equipment tags are the primary post-Galaxy authoring model, the IPerCallHostResolver is effectively inert for the flagship path.

Root cause: ResolveHost never learned about the equipment-tag path; the resolver resolves defs but exposes nothing host-specific (EquipmentTagRefResolver is generic over TDef and never inspects it — confirmed).

Design: route ResolveHost through EquipmentTagRefResolver.TryResolve, then derive the host from the parsed def. The per-driver TDef records already carry the host: AbCipTagDefinition.DeviceHostAddress, AbLegacyTagDefinition.DeviceHostAddress, TwinCATTagDefinition.DeviceHostAddress, FocasTagDefinition.DeviceHostAddress (all parsed by the equipment parsers). So:

• Each driver's ResolveHost(ref) becomes: if (_resolver.TryResolve(ref, out var def)) return def.DeviceHostAddress; return <first-device-or-sentinel>. This covers both authored (_byName) and equipment (_parseRef) paths through one call and reuses the parse cache.
• Optional shared sugar: add a string? ResolveHost(string ref, Func<TDef,string> host) overload to EquipmentTagRefResolver so the pattern is one call. Keeps the host extraction driver-specific (the resolver stays generic) while giving theme #6 a single home.

Implementation steps: rewrite the four ResolveHost bodies to call _resolver.TryResolve; add the resolver overload if desired. Modbus: note UnitId isn't in the equipment parser — either add unitId to ModbusEquipmentTagParser (UNDER-6) or document that Modbus per-tag host resolution is out of scope.

Tests: unit per driver — an equipment-tag ref for device B resolves to device B's host (today returns device A/first); assert the Polly breaker key differs per device.

Effort: S-M. Risk: Low — corrects a mis-key; verify the breaker-key wiring downstream consumes the corrected host.

B4. Shared reconnect/backoff primitive (STAB-8, supports A1/A2)

Verification: CONFIRMED. No reconnect backoff exists except the Modbus transport's geometric backoff (ModbusTcpTransport.cs:179-208) and S7's poll-loop backoff. Dead devices pay full reconnect cost per tick: AbCip full Tag create + Forward Open per tag per tick (AbCipDriver.cs:891-897 evict-recreate), FOCAS full two-socket reconnect per tick of each of three loops (FocasDriver.cs:1089-1120), TwinCAT connect-attempt per call (:602-651).

Design: a small shared ConnectionBackoff primitive in Core.Abstractions (capped exponential, the same ComputeBackoffDelay math B1 adds to the poll engine — factor it out so both use it). Lazy-reconnect drivers gate their EnsureConnected/evict-recreate behind a per-device throttle: record _lastConnectAttemptUtc + a growing backoff, and short-circuit a reconnect attempt that is inside the backoff window (return the current Bad/Degraded state instead of hammering).

Implementation steps:

1. Extract ComputeBackoffDelay into Core.Abstractions/ConnectionBackoff.cs (shared by PollGroupEngine v2 and the lazy-reconnect drivers).
2. Add a per-device backoff gate to TwinCAT EnsureConnectedAsync, FOCAS EnsureConnectedAsync, and AbCip's evict-recreate path (skip the recreate if within the backoff window; reset on success).

Tests: unit — a device that fails to connect is not retried more often than the backoff schedule; success resets the schedule.

Effort: M. Risk: Low-Medium — must not delay recovery when the device comes back (reset promptly on the first success).

---

Part C — Remaining Mediums & Lows (batched)

All CONFIRMED. Grouped for efficient sweeps.

C1. One-liner conventions sweep (CONV-3, CONV-5)

• CONV-3 (Medium): S7/TwinCAT/AbCip/FOCAS Register take no ILoggerFactory (S7DriverFactoryExtensions.cs:19, TwinCATDriverFactoryExtensions.cs:18, AbCipDriverFactoryExtensions.cs:21, FocasDriverFactoryExtensions.cs:36), so DriverFactoryBootstrap.Register (:100,102,106,107) can't pass one and all four log to NullLogger in production — their poll-failure/prohibition/live-risk warnings are invisible. AbLegacy/Galaxy/Modbus/OpcUaClient already have the overload (:101,103,104,105). Fix: add the ILoggerFactory? = null param + thread to the driver ctor in the four factories; update the four bootstrap call sites. Effort S, risk very low. (Also unblocks A1/A2/B1 logging.)
• CONV-5 (Medium): four health-idioms (Volatile vs volatile vs plain field); AbLegacy no health-refresh on successful write (AbLegacyDriver.cs:344-346 vs AbCip :725); AbLegacy silently clobbers duplicate tag names (:91) where AbCip fails fast (:249-256); TwinCAT declares Healthy at init with zero wire contact (:115) and reads host state without the probe lock (:524-525). Fix: standardize on Volatile.Read/Write (or volatile record ref) across the fleet; add write-success health refresh to AbLegacy; make AbLegacy fail-fast on duplicate names; make TwinCAT prove the connection before Healthy. Effort S-M, risk low.

C2. Per-protocol read batching (PERF-1, PERF-2) — the scalability lever

• PERF-1 (High): five of seven drivers do one tag per round-trip. Priority order per the report: TwinCAT ADS sum-read (TwinCATDriver.cs:203-248), S7 multi-var (ReadMultipleVarsAsync, unused — S7 loops per tag :443-484 under one _gate), FOCAS PMC range reads (WireFocasClient.cs:296-334 widens to one value width), AbLegacy file-element spans (N7:0..N7:2). Each is a separate protocol feature.

• PERF-2 (High): AbCip's whole-UDT batched read is half-built — the planner collapses N members via declaration-order layout, off by default (safe-but-unused, AbCipDriverOptions.cs:72 default false), while true member offsets are already fetched via the CIP Template Object (AbCipDriver.cs:151-182, AbCipTemplateCache) but consulted only by discovery, never the planner (AbCipUdtReadPlanner.cs:69). Highest-ROI batching win (design already in-tree): feed AbCipUdtShape template offsets into the planner, retire the unsafe declaration-order mode. Effort M-L, risk Medium (correctness of offset mapping — test against the abcip fixture with a known UDT).

  Sequencing: do PERF-2 first (design exists), then TwinCAT sum-read + S7 multi-var. Each is independently shippable. Effort L overall.

C3. OpcUaClient performance + monolith (PERF-4, PERF-5, UNDER-9)

• PERF-4 (Medium): all traffic serializes on one SemaphoreSlim(1,1) _gate (12 WaitAsync sites, OpcUaClientDriver.cs:78,626,713,836,1186,1377,1451,1653,1825). OPC UA sessions multiplex natively; a multi-second HistoryRead/re-discovery blocks every read/write. The gate is only needed for session-swap consistency, which _probeLock + re-read-in-critical-section already provide. Fix: scope the gate to session-swap only; let reads/writes/history run concurrently on the session. Effort M, risk Medium — must preserve session-swap safety; test the reconnect race.
• PERF-5 (Medium): EnrichAndRegisterVariablesAsync sends pending.Count*4 ReadValueIds in one call (:1029-1046); a server enforcing MaxNodesPerRead returns BadTooManyOperations and the blanket catch (:1049-1057) silently registers every var as Int32/ViewOnly/non-historized with no log/health. Fix: chunk client-side (respect MaxNodesPerRead / a conservative cap); log + health-signal the fallback. Effort S, risk low.
• UNDER-9 (Low): 2154-line monolith; extract the reconnect state machine for isolated testability; set Session.TransferSubscriptionsOnReconnect explicitly (:1946-1949 — currently unset, relies on SDK default); BuildCertificateIdentity loads only PKCS#12 (:483-484) while the option doc promises PEM (OpcUaClientDriverOptions.cs:68-75) — either implement PEM load or fix the doc. Effort M (extraction) / S (the two nits), risk low.

C4. Remaining stability/correctness Mediums (STAB-10, STAB-11, STAB-12)

• STAB-10 (Medium): AbLegacy never evicts failed tag handles on the data path (AbLegacyDriver.cs:246-259,269-274) though AbCip does (AbCipDriver.cs:891-897) and AbLegacy's own probe loop does (:456-461). Fix: evict-recreate on non-zero status/transport exception in the data path (port AbCip's EvictRuntime). Pairs with A4/A6 as an AbLegacy/AbCip parity pass. Effort S, risk low.
• STAB-11 (Medium): RMW locks don't cover direct parent writes (AbCip/AbLegacy/ TwinCAT) → lost update when a direct parent write interleaves an in-flight RMW; AbCip RMW hard-codes DINT while AbCipTagPath accepts .N on any parent (AbCipTagPath.cs:17-19); TwinCAT RMW reads the parent as 4-byte UDInt (AdsTwinCATClient.cs:96-107), an unverified assumption that will fail DeviceInvalidSize on WORD/BYTE parents. Fix: make direct parent writes take the same per-parent/per-runtime lock (AbCip's is subsumed by A4's runtime lock); width-aware RMW for TwinCAT (read the parent as its actual declared width). Effort M, risk Medium (TwinCAT width needs real-hardware verification — flag under UNDER-7).
• STAB-12 (Medium): FOCAS sync Dispose() does a close-PDU exchange (SendPdu+ ReadPdu) on a NetworkStream with no ReadTimeout/token (FocasWireClient.cs:164-175) — a stalled CNC wedges ShutdownAsync. TwinCAT ConnectAsync calls the blocking SDK Connect ignoring token+timeout (AdsTwinCATClient.cs:73-80). Fix: bound the FOCAS close read with a ReadTimeout; run TwinCAT Connect on a worker with the timeout/token honoured (or set _client.Timeout and wrap in Task.Run + WaitAsync(ct)). Effort S, risk low.

C5. Perf Lows (PERF-6, PERF-7)

• PERF-6 (Low): TwinCAT re-parses TwinCATSymbolPath per call (TwinCATDriver.cs:220,284,468) — cache like S7's _parsedByName; Modbus fresh DriverHealth per read (:285); AbCip BuildArray<T> boxes per element. None urgent.
• PERF-7 (Low): deadband + WriteOnChangeOnly exist only in Modbus (ModbusDriver.cs:136-159); generalize into/beside PollGroupEngine (driver-agnostic publish suppression) so noisy analog signals on the other poll drivers don't publish every jitter. Natural PollGroupEngine-v2 add-on (B1). Effort M, risk low.

C6. Underdeveloped Mediums (UNDER-4, UNDER-5, UNDER-7, UNDER-8)

• UNDER-4 (Medium, S7): S7TagDto has no ArrayCount and BuildTag never sets it (S7DriverFactoryExtensions.cs:80-90,137-151) → driver-config array tags silently scalar (arrays only work as equipment tags, S7EquipmentTagParser.cs:73-87); array writes throw NotSupportedException (S7Driver.cs:1014-1017); UInt32 surfaces lossily as Int32 (:1149-1152). Fix: add ArrayCount to the DTO + BuildTag; implement array writes; map UInt32→UInt32/Int64. Effort M.
• UNDER-5 (Medium, AbCip): AcknowledgeAsync fire-and-forgets write results (AbCipAlarmProjection.cs:149) → a failed ack is invisible to operator/health/Part 9 caller; with EnableControllerBrowse on, one unreachable PLC faults the entire multi-device DiscoverAsync (uncaught await foreach, AbCipDriver.cs:970-1037); browsed tags default writable/Operate (CipSymbolObjectDecoder.cs:91). Fix: surface ack outcomes to health + the caller; per-device try/catch in discovery so one dead PLC doesn't fault all; default browsed tags read-only. Effort M.
• UNDER-7 (Medium): aggregate the honest "unverified against real hardware" blocks into a "needs bench time" checklist — TwinCAT array-read shape (AdsTwinCATClient.cs:109-116), Flat-mode SubSymbols population (:263-277), bit-RMW width (STAB-11); AbLegacy array-decode's five unproven layout assumptions (LibplctagLegacyTagRuntime.cs:64-88); FOCAS cnc_getfigure id + servo-load scaling (FocasWireClient.cs:391-395,943-947). Deliverable: a doc checklist, not code. Effort S (doc).
• UNDER-8 (Medium): test coverage is codec-heavy, failure-path-light. The concrete gaps this plan closes with new tests: S7 reconnect (A1), TwinCAT replay (A2), AbCip concurrency (A4), FOCAS cache concurrency (A5), Modbus desync (A3), shared parser-strictness conformance (B2). Also: Cli.Common.DriverCommandBase has no direct tests — add a small suite.

C7. Lows — hygiene (CONV-6, CONV-7, CONV-8)

• CONV-6 (Low/Medium): Driver.OpcUaClient.Contracts drags the full OPC UA SDK because NamespaceMap lives there (OpcUaClient.Contracts.csproj:9 OPCFoundation.NetStandard.Opc.Ua.Client); every options-DTO consumer transitively loads the SDK. This is the previously-deferred OpcUaClient.Contracts-002 finding — move NamespaceMap into the driver project (or a new .Namespace project), leaving Contracts POCO-only like the other six. Effort M (new project boundary), risk low.
• CONV-7 (Low/Medium): CLI divergences — ParseValue/ParseBool copy-pasted six times (*/Commands/WriteCommand.cs); option Validate() absent in AbLegacy (AbLegacyCommandBase.cs — --timeout-ms 0 reaches the driver); Timeout init-setter throws in AbCip (AbCipCommandBase.cs:43) but no-ops in five others; only TwinCAT exposes browse though AbCip supports controller browse (AbCipCommandBase.cs:63 hardcodes EnableControllerBrowse=false); OpcUaClient has no CLI. Fix: hoist value parsing + a standard Validate() into Cli.Common; add an OpcUaClient CLI or document the gap. Effort M, risk low.
• CONV-8 (Low): dead/no-op knobs + stale docs — DisableFC23 no-op (ModbusDriverOptions.cs:83-89); AbCip ConnectionSize plumbed-never-applied (:97-103); AbLegacy class doc says read/write "ship in PRs 2 and 3" (AbLegacyDriver.cs:9-11); OpcUaClient header says "PR 66 ships the scaffold: IDriver only" (:12-14). No options class has Validate(). Fix: delete/wire the dead knobs; refresh the stale headers; optionally add per-driver options Validate() (S7's init guards :314-385 are the best-of-breed template). Effort S, risk very low.

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Sequencing & effort roll-up

Aligns with the report's "Recommended remediation order" and OVERALL actions #3/#4.

#	Item	Findings	Effort	Risk	Notes
1	S7 reconnect (IS7Plc seam + EnsureConnected) + reconnect test	STAB-1, UNDER-8	M	Med	Critical; template = TwinCAT/OpcUaClient
2	TwinCAT native-sub replay on client swap + probe recycle	STAB-2	M	Med	Critical; fake-client unit tests are the only automated coverage
3	AbCip per-runtime op lock + FOCAS ConcurrentDictionary caches	STAB-4, STAB-5	S each	Low	Silent-corruption class; port AbLegacy's GetRuntimeLock
4	Modbus timeout/framing ⇒ connection teardown	STAB-3	S	Low-Med	Distinguish timeout vs caller-cancel
5	Shutdown/unsub race fixes	STAB-6	S	Low	Deleted for S7 by item 8
6	FOCAS writable-false; OpcUaClient dead gate + alarm-filter encoding	UNDER-1, UNDER-2, UNDER-3	S each	Low	Kill false capabilities
7	Factory ILoggerFactory on 4 drivers	CONV-3	S	V.Low	Unblocks item 1/2/8 logging
8	PollGroupEngine v2: backoff + onError-health + migrate S7 off fork	CONV-1, STAB-8, STAB-9, PERF-3, STAB-6-S7	M	Med	Single home for poll fixes
9	Shared strict ReadEnum/ReadBool + probe parity + writable parity	CONV-2, UNDER-6	M	Med	Behavior change: typo ⇒ Bad; live-verify
10	ResolveHost via resolver (per-host breaker isolation)	CONV-4	S-M	Low	Flagship equipment-tag path
11	Shared reconnect/backoff primitive for lazy drivers	STAB-8	M	Low-Med	Extract ConnectionBackoff; don't delay recovery
12	Per-protocol batching: AbCip planner⇄template-cache, TwinCAT sum-read, S7 multi-var	PERF-1, PERF-2	L	Med	Biggest scalability lever; AbCip design in-tree
13	OpcUaClient gate-scoping + chunk discovery + monolith split	PERF-4, PERF-5, UNDER-9	M	Med	Preserve session-swap safety
14	Remaining mediums: evict-on-fail, RMW parent lock/width, blocking teardown	STAB-10, STAB-11, STAB-12	M	Med	TwinCAT width needs bench time
15	Underdeveloped: S7 arrays/UInt32, AbCip acks/discovery, bench checklist	UNDER-4, UNDER-5, UNDER-7, UNDER-8	M	Low-Med
16	Lows: Contracts SDK split, CLI hoist, dead knobs/docs, perf lows	CONV-6/7/8, PERF-6/7	M	Low

Where shared primitives live: Core.Abstractions — PollGroupEngine (v2 with backoff, item 8), ConnectionBackoff (item 11, shared with the engine), TagConfigJson.ReadEnum/ReadBool (item 9, beside EquipmentTagRefResolver), and the ResolveHost-via-resolver pattern (item 10). This is the concrete answer to themes #2 and #6: fleet fixes get one home instead of six copies.

Live-verify recipes (docker rig on 10.100.0.35): Modbus :5020 + exception_injector for STAB-3 (item 4) and write-failure paths; S7 :1102 (Snap7ServerFixture, bounce via lmxopcua-fix down/up s7 s7_1500) for STAB-1 (item 1); AbCip :44818 for STAB-4 concurrency + PERF-2 UDT (items 3, 12). TwinCAT has no docker fixture (needs a real TC3 XAR) — the fake-ITwinCATClient unit tests are authoritative for STAB-2/item 2.