I am having issues with slave devices dropping offline about once every 10-15 minutes or so. Has anyone had this similar issue and what have they done to resolve? I can see the master tx 3 times, and with no response from slave after 3rd time, it times out and goes offline, then moves on to another slave device. This happens to only 1 or 2 slave devices in a series modbus rtu circuit with about 9 slaves in series, at different time intervals. I have checked the wiring, grounding, termination resistors, comm, programming of slaves, o'scoped the signal, all these look good. Any other help would be appreciated.

 

Have you set it up in a full duplex or half duplex mode? Can you sequence your messages to occur only when the previous message has completed (either completed succesfully or failed).

 

Have you isolated the problem to the Master or the slaves?

Have you tried another Master or different type of slave?

Automated Solutions offers Modbus master and slave communications drivers that can help to isolate the problem.

Master: http://automatedsolutions.com/products/modbusrtu.asp

Slave:
http://automatedsolutions.com/products/modbusslave.asp

Fully functioning 30 day trial versions with pre-built HMI applications are available for download.

 

I have seen this before.

Our products do this, and it is configurable within the newer systems. Basically we perform a self-test every 15 minutes which lasts about 10-15 seconds. During the self-test NOTHING else happens. No communication, no calculation of the process variable, nothing... it is normal operation.