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Answer ILLEGAL_FUNCTION for a request with a reserved function code - #3075
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janiversen merged 2 commits intoNov 1, 2026
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Function codes 128 to 255 are reserved for exception responses, so a request using one is an illegal function (exception 01). The server decoder took such a request for an exception response, then failed in datastore_update and answered SLAVE_FAILURE (04). Two tests decoded an exception response with the server decoder; they now use the client decoder, which is the side that receives them. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Function codes with 0x80 set is defined in the modbus protocol as Exception Response.
The Modbus protocol do not define a Exception request, so a server cannot receive a function code > 0x80 bit generates it when the client sends something illegal.
The server should however as you state respond with ILLEGAL function.
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PR will be merged, when you solve the conflict.
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 request whose function code is 0x81 to 0xFF was decoded by the server as an exception response.
datastore_updatethen raisedNotImplementedException, and the server answered SLAVE_FAILURE (04):81 00 00got81 04. The Modbus Application Protocol (V1.1b3, 4.1) reserves codes 128 to 255 for exception responses. A request with one of them is therefore an illegal function: exception 01, as for any other code the server doesn't implement.The change: on the server side,
DecodePDU.decoderaisesModbusIOExceptionfor these codes. Unknown function codes already take that path, so the request handler answers ILLEGAL_FUNCTION (81 01). The client side still decodes them as exception responses.Two existing tests changed:
test_pdu.py::test_error_methodsandtest_extras.py::test_rtu_decode_exceptiondecoded an exception response with the server decoder. They now use the client decoder, the side that receives exception responses.Tests:
81 00 00gets81 01.dev(b9cd6c6) and pass here. codespell, ruff, pylint, zuban and pytest (100 % coverage) pass.Found by a differential test against libmodbus.
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