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Modbus Freeze
Modbus communication failure

Hope some expert could give some advice on the problems which I’m facing.

I have multi VFD equipped with Modbus. every month there are at least 10 incident in the same place but different sector having PLC failure to communicate with my VFD. I’ve tried to decrease the polling rate and ground the signal for 2.5 month. I’ve thought it’s going to resolved the issue but I’m wrong. somehow today they test failed after the long wait.

Are you using RS485 at the physical layer?

If so, check that you have "Fail Safe Biasing" enabled.
For more information Google "AN-1057" OR "10 Ways to Bullet Proof your RS485".