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Modbus RS232 Communication
The communication between the PLC and HMI panel on Modbus RS232 is lost when I press a push button which is a digital input in PLC


I have a PLC which is communicating on Modbus RS232 with a HMI panel. The communication is ok, I can control the PLC from HMI. The issue is when i press a push button which is a digital input in plc, the communication with the hmi is lost.

The digital input is not affecting in software the communication. Actually and old hmi is working there with the same program without issues, but i want to change it with a new one.

This is happening only for that button. What issue can be?

Thank you

The Tx line for the RS-232 is wired through the NC momentary push button switch. When you push the button, the switch opens and the Tx line opens, killing comm between the HMI and PLC.

Seriously, so what is the DI that the push button is connected to programmed to do, if not affect the communication?

Is there more than one switch stacked on the back of the PB? Connected to?

If you have narrowed it down to the input transition event causing the PLC comms channel to close or lock up, then possibly, the addressing of the modbus read is off or the type is off.

By idiotsecant on 29 November, 2018 - 4:40 pm

Have you considered the behavior of the DI circuit that is being closed? Does the button have an additional set of poles that, for example, open a relay that does not have flyback voltage protection? If you 'hold' the DI closed does the communication eventually start working again or does it stay disabled the entire time you hold? It seems that you feel strongly you've eliminated software as the problem which leaves only electrical gremlins. How is your panel grounded? Is your power supply negative brought to earth ground? If you put a different button on the DI temporarily does the problem still happen? If you remove the logic from that DI does it still happen?