Joseph Doherty
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9e2b5b330f
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Phase 3 PR 54 -- Siemens S7 Modbus TCP quirks research document. 485-line doc at docs/v2/s7.md mirroring the docs/v2/dl205.md template for the Siemens SIMATIC S7 family (S7-1200 / S7-1500 / S7-300 / S7-400 / ET 200SP / CP 343-1 / CP 443-1 / CP 343-1 Lean / MODBUSPN). Siemens S7 is fundamentally different from DL260: there is no fixed Modbus memory map baked into firmware -- every deployment runs MB_SERVER (S7-1200/1500/ET 200SP), MODBUSCP (S7-300/400 + CP), or MODBUSPN (S7-300/400 PN) library blocks wired up to user DBs via the MB_HOLD_REG / ADDR parameters. The driver's job is therefore to handle per-site CONFIG rather than per-family QUIRKS, and the doc makes that explicit. Key findings worth flagging for the PR 56+ implementation track: (1) S7 has no fixed memory map -- must accept per-site DriverConfig, cannot assume vendor-standard layout. (2) MB_SERVER requires NON-optimized DBs in TIA Portal; optimized DBs cause the library to return STATUS 0x8383 on every access -- the single most common S7 Modbus deployment bug in the field. (3) Word order is ABCD by default (big-endian bytes + big-endian words) across all Siemens S7 Modbus paths, which is the OPPOSITE of DL260 CDAB -- the Modbus driver's S7 profile default must be ByteOrder.BigEndian, not WordSwap. (4) MB_SERVER listens on ONE port per FB instance; multi-client support requires running MB_SERVER on 502 / 503 / 504 / ... simultaneously -- most clients assume port 502 multiplexes, which is wrong on S7. (5) CP 343-1 Lean is SERVER-ONLY and requires the separate 2XV9450-1MB00 MODBUS TCP CP library license; client mode calls return immediate error on Lean. (6) MB_SERVER does NOT filter Unit ID, accepts any value. Means the driver can't use Unit ID to detect 'direct vs gateway' topology. (7) FC23 Read-Write Multiple, FC22 Mask Write, FC20/21 File Records, FC43 Device Identification all return exception 01 Illegal Function on every S7 variant -- the driver MUST NOT attempt bulk-read optimisation via FC23 when talking to S7. (8) STOP-mode read/write behaviour is non-deterministic across firmware bands: reads may return cached data (library internal buffer), writes may succeed-silently or return exception 04 depending on CPU firmware version -- flagged as 'driver treats both as unavailable, do not distinguish'. Unconfirmed rumours flagged separately: 'V2.0+ reverses float byte order' claim (cited but not reproduced), STOP-mode caching location (folklore, no primary source). Per-model test differentiation section names the tests as S7_<model>_<behavior> matching the DL205 template convention (e.g. S7_1200_MB_SERVER_requires_non_optimized_DB, S7_343_1_Lean_rejects_client_mode, S7_FC23_returns_IllegalFunction). 31 cited references across the Siemens Industry Online Support entry-ID system (68011496 for MB_SERVER FAQ, etc.), TIA Portal library manuals, and three third-party driver vendor release notes (Kepware, Ignition, FactoryTalk). This is a pure documentation PR -- no code, no tests, no csproj changes. Per-quirk implementation lands in PRs 56+. Research conducted 2026-04-18 against latest publicly-available Siemens documentation; STOP-mode behaviour and MB_SERVER versioning specifically cross-checked against Siemens forum answers from 2024-2025.
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2026-04-18 22:50:51 -04:00 |
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