Troubleshooting serial communication - Arc protection - Auto synchronization - Bay control and measurement - Merging unit - Transformer protection - 2 winding - Feeder protection - Voltage regulation - Capacitor bank protection - Petersen Coil control - Grid automation - Busbar differential protection (high impedance) - Back-up protection - Motor protection - Busbar protection (voltage and frequency) - Transformer protection - 3 winding - Interconnection protection - Generator protection - Power management/Load shedding - Modbus Communication Manual - REX640 Protection and control - PCL4 - IEC - ANSI - 16.02.2023

The diagnostic capabilities can be used for investigating communication problems. If communication cannot be established to the relay, then proceed in this order.

1. Reset the serial driver and Modbus protocol diagnostic counters to make it easier to view the changes.
2. Check the serial driver diagnostic counters. If serial characters are not received, check the cable (Rx line) and the link setup parameters, also on the client side.
3. If serial characters are received, check if whole link frames are also received. Do this first on the driver side.
4. Go over to Modbus diagnostics and check if Modbus link frames are internally received. The serial driver detects every link frame on the line, but the Modbus protocol only reacts to Modbus link frames, which are addressed to its own protocol instance.
5. Check the receive and send delay settings in the relay. If link frames are not received properly there might be character timing problems.
6. Check for receive errors, checksum errors or several retransmissions. If these are found, the line may be noisy.
7. If Modbus link messages are received, check that the response messages are sent to the client.
8. Check the serial driver's transmitted character counter. If it is running, then the relay is transmitting. If the client receives nothing, then check the cable (Tx line).