Solution:

You must pay your attention to the following statements (RTS and Loopback), otherwise, you will get into trouble during communication. 1. RS-422 interface: It is necessary to use RTS signal to control the driver (Tx+, Tx-) open or close. That means you have to enable RTS before sending any message. 2. RS-485 interface: Hardware: You need to connect (Tx-, Rx-) and (Tx+, Rx+) first. Tx- and Rx- ==> Data- Tx+ and Rx+ ==> Data+ Software: You should use RTS to control the data flow. Besides, when you send the command to other devices, you will receive this command first, and receive the response from other devices later, because the receiver of ComPad-85 is always on and you connect Tx and Rx together. So you must delete the copy of your command before receiving the response in your program.