What exception codes mean - and how to fix them fast
Tutorials › Modbus Error Codes
When a Modbus device cannot satisfy a request, it replies with an exception response: the original function code with the high bit set, followed by a one-byte exception code. Knowing these codes turns a frustrating "it doesn't work" into a quick fix.
The device does not support that function code. Use a different function (e.g. FC 04 instead of FC 03) and check the device docs.
The address does not exist on the device. The most common cause is 0-based vs 1-based addressing - if the manual says 40001, use address 0. Also try a smaller quantity.
The value or quantity in the request is out of range for the device. Check valid ranges and limits in the documentation.
An unrecoverable error occurred inside the device while processing the request. Retry, and check the device status/logs.
The request was accepted and is being processed (used with long operations). Not an error - poll again shortly.
The device is busy with another command. Wait and retry.
Two of the most common problems never produce an exception code at all, because nothing comes back:
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