The best way to do this is to have a separate computer/laptop with the ability to MONITOR MODBUS by acting as a Master station.

The only cable required to test MODBUS is a three conductor, wired to one end with a standard 9 pin connector for the computer com port and the other end loose to tie into the customer interface terminals The 9 pin TERMINAL 2 is Rx, terminal 3 is Tx, and 5 is GND (or Common). So the Rx on one end goes to the TX on the other and vice versa, and the GND goes to GND.

To test an RS485 connection first establish that the RS232 side is working by disconnecting the RS232/485 converter and testing by the above method. Once the RS232 side is confirmed as working the only way to verify the RS485 connection with a PC is to USE another RS232/485 converter to convert the signal back into a RS232 signal that the PC can read. A converter on another PLC can be used for testing but the RS232 side of the converter must be disconnected first before USING the PC.

The best way to do this is to have a separate computer/laptop with the ability to monitor MODBUS by acting as a Master station.

The only cable required to test MODBUS is a three conductor, wired to one end with a standard 9 pin connector for the computer com port and the other end loose to tie into the customer interface terminals The 9 pin terminal 2 is Rx, terminal 3 is Tx, and 5 is GND (or Common). So the Rx on one end goes to the TX on the other and vice versa, and the GND goes to GND.

To test an RS485 connection first establish that the RS232 side is working by disconnecting the RS232/485 converter and testing by the above method. Once the RS232 side is confirmed as working the only way to verify the RS485 connection with a PC is to use another RS232/485 converter to convert the signal back into a RS232 signal that the PC can read. A converter on another PLC can be used for testing but the RS232 side of the converter must be disconnected first before using the PC.