MODBUS RTU: should the master retransmit on timeout?

So the master *should* (must) retransmit last request if no answer is received from the slave.

Now, what happens if the request was correctly received and processed by the slave, but its answer wasn't received by the master? The master retries to transmit the request a second time, so the slave process the same request two times, because it can't detect the second request as a retransmission of the first.

If the request is a SET type, this retransmission could be dangerous (for example, I can move two times a motor or similar things).

Am I wrong?

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This is true of any comms scheme.

The ... value part of a set must be an absolute quantity and must not be a differential quantity, sfaik. It has to be a "move to ", not a "move degrees."

MODBUS RTU is a protocol for sharing registers over a wire, not commands per se.


Les Cargill
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Les Cargill

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