- Reset the serial driver and Modbus protocol diagnostic counters to make it easier to view the changes.
- Check the serial driver's transmitted character counter.
If it is running, then the relay is transmitting. If the slave receives nothing, then check the cable (Tx line).
- If Modbus link messages are sent, check that the response messages are received by the slave.
Check the slave’s Rx diagnostic counters, if any. Also, double check the baud rate setting in master and slave devices.
- If no messages are being transmitted, check the protocol instance’s settings and make sure that the mapping file has been written to the relay with CMT.
- Check the serial driver's received character counters.
If serial characters are not received, check the cable (Rx line) and the link setup parameters, and also that the slave is transmitting.
- Go over to the Modbus Master diagnostics and check if Modbus link frames are internally received.
The serial driver detects every link frame on the line, but the Modbus Master protocol will discard erroneous messages. Check if any error counters are being incremented, if so; the communication line could be noisy.
- Check the receive and send delay settings in the relay.
If link frames are not received properly there might be character timing problems.