RTU: Check CRC in exception ADU #118
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The first five response ADU bytes received are checked for a valid function code, assuming an error if it is unknown. The received ADU part is then treated as an exception frame. But even those must pass the CRC validation, otherwise the supposed "wrong" function code cannot even be trusted at all. This change adds the CRC check for the first five bytes, falling through to further processing when it fails. The overall CRC and function code is checked later in parse_response_adu(), so an invalid code will be detected again at that point. This is mainly relevant if the responses are received partially from the serial input buffer, which will cause many CRC and parsing failures. In those cases, the reported exception will likely be a CRCError or ValueError for the whole ADU, instead of an invalid function code based on only the first 5 bytes.
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The first five response ADU bytes received are checked for a valid
function code, assuming an error if it is unknown. The received ADU
part is then treated as an exception frame. But even those must pass
the CRC validation, otherwise the supposed "wrong" function code
cannot even be trusted at all.
This change adds the CRC check for the first five bytes, falling
through to further processing when it fails. The overall CRC and
function code is checked later in parse_response_adu(), so an invalid
code will be detected again at that point.
This is mainly relevant if the responses are received partially from
the serial input buffer, which will cause many CRC and parsing
failures. In those cases, the reported exception will likely be a
CRCError or ValueError for the whole ADU, instead of an invalid
function code based on only the first 5 bytes.
Note that it works best in conjunction with #117, failing early if the response function code does not match the request.