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fix(twincat): review — a connect failure no longer burns the sum-chunk warning; doc precision

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- ReadBucketAsync: the once-per-device "sum-read chunk failed" Warning fires only after a successful
  connect; a connect/backoff failure is the connect path's to report. TwinCATSumChunkWarningTests pins
  it (no warning across connect failures, exactly one across two later failing sum polls; red without
  the guard) plus a source guard that ReadGroupAsync builds its SumRead only via CreateSumRead.
- Docs: diagnostics live-gated via the driver harness (/hosts not exercised); TC2 costs 8 reads on the
  first tick and after each reconnect, then <= 4; read counts attributed to the unit tests.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe
This commit is contained in:
Joseph Doherty
2026-10-08 21:26:26 -04:00
parent 664ff75ee2
commit a92fb81f00
4 changed files with 118 additions and 9 deletions
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docs/drivers/TwinCAT.md
+10 -5
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@@ -224,12 +224,16 @@ command is unverified, hence opt-in.
### `/hosts` diagnostics (`IDriverDiagnostics`, opt-in)
Behind `diagnosticsEnabled` (default **off** — not yet live-gated). The driver implements the optional
Behind `diagnosticsEnabled` (default **off**). Live-gated read-only 2026-10-08 on Z35474 (TC3) and Z34834 (TC2) by
driving `TwinCATDriver` from a scratch harness with an in-process AMS router; the `/hosts` page end-to-end was
**not** exercised (a stock host has no AMS router). The driver implements the optional
`IDriverDiagnostics` bag (the S7 `NegotiatedPduSize` precedent, #640), deliberately not synthetic raw tags.
When enabled, after each healthy probe tick **and after the host-state transition** it reads up to four
`TwinCAT_SystemInfoVarList` symbols per device — `_AppInfo.AppName`, `_AppInfo.OnlineChangeCnt`,
`_TaskInfo[1].CycleTime`, `_TaskInfo[1].LastExecTime` (100 ns ticks) — so up to **4 extra read requests per
device per probe interval** (default 5 s). The ADS state is the one `ProbeAsync` already read; no second
`_TaskInfo[1].CycleTime`, `_TaskInfo[1].LastExecTime` (100 ns ticks) — so at most **4 extra read requests per
device per probe interval** (default 5 s) once settled. On **TwinCAT 2** (fallback below) the first tick, and the
first tick after every reconnect, costs **8** (the four TC3 names answer "not found", then the four TC2 names);
after that ≤ 4. The ADS state is the one `ProbeAsync` already read; no second
ReadState. One device: flat keys `TwinCAT.AdsState`, `.AppName`, `.OnlineChangeCnt`, `.CycleTimeMs`,
`.LastExecTimeMs`, `.HeadroomMs` (cycle − last exec; negative = the task overran; this is spare time, not
jitter). Several devices: one line per device, `TwinCAT.<DeviceName or host>` =
@@ -239,8 +243,9 @@ best-effort: a runtime that does not export one keeps the others, and a symbol a
found" is not re-asked until a new client is installed (reconnect). **TwinCAT 2** has no
`TwinCAT_SystemInfoVarList`; once a TC3 name answers "not found" the driver reads the TC2 globals instead —
`.SYSTEMINFO.PROJECTNAME` (AppName), `.SYSTEMINFO.ONLINECHANGECOUNT` (UINT), `.SYSTEMTASKINFOARR[1].CYCLETIME` /
`.LASTEXECTIME` (same 100 ns units). Live on TC2 2.11: 8 reads on the first tick, 4 after; a TC3 target never
reads the TC2 names.
`.LASTEXECTIME` (same 100 ns units). The read counts — 8 on the first tick and after each reconnect, 4 after, and
none of the TC2 names on a TC3 target — are pinned by `TwinCATDiagnosticsTests`; the live run proved the values
(all six keys on TC2 2.11), not the counts.
## Capability surface