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Answer ILLEGAL_VALUE for a Write Single Coil value other than 0x0000/0xFF00 - #3074
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janiversen merged 1 commit intoOct 31, 2026
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…0xFF00 The specification (6.5) allows only 0xFF00 (ON) and 0x0000 (OFF); any other value is illegal, must not affect the coil, and gets exception 03. The server took any non-zero value as ON and echoed 0xFF00. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Your change is correct and according to the protocol documentation !
However many devices hace a more relaxed implementación, that says 0x00 is OFF and all other values are ON.
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Feel cree to open issues or make more PR, if you find other differences that also are not in the standard. |
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Refactored and added to PR #3068 (test from this PR is kept original to verify, it is implemented correctly). |
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The conflict was in test/server/test_server_asyncio.py: pymodbus-dev#3074 and this branch each added a server test at the same place. Both are kept. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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A Write Single Coil request (0x05) with a value other than 0xFF00 or 0x0000 was carried out. Any non-zero value turned the coil ON, and the response echoed 0xFF00, a value the client didn't send. The Modbus Application Protocol (V1.1b3, 6.5) says "All other values are illegal and will not affect the output", and its state diagram answers exception 03.
The change:
WriteSingleCoilRequestkeeps the decoded value, anddatastore_updateanswersILLEGAL_VALUEfor any other value before it touches the datastore. That is the same place #3066 checks counts. A request built through the API (no decoded value) is unchanged.Tests:
05 0000 0001now gets85 03. Before, it got05 0000 ff00and the coil was turned ON.dev(b9cd6c6) and pass here. codespell, ruff, pylint, zuban and pytest (100 % coverage) pass.Found by a differential test against libmodbus, which answers exception 03 to the same request.
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