Modbus tcp communication fault finding

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Often the client software has some configuration option to set a timeout for the expected response, a bit of historical baggage from serial links. When using TCP there is no need for this (or the often associated config. value of "retries") as the TCP stack will either deliver a response (possibly after TCP retransmissions) or will indicate a failure of the stream.

If there is a comms. or message timeout, try increasing it a bit (or even disable it if an option). Similarly set any retries option to 0.

It depends where you do the actual measurements. I would prefer to do measurements on both ends and also compare it with network statistics. But if you don't see any obvious network issue it could very well be a software glitch.