The causes of the fault are as follows: (1) Check whether the device serial number/COM port is correct, whether two application programs are opened, or whether the port is occupied by other programs. (2) Check whether the RS485 communication line is connected to the controller normally, to see whether there is A virtual connection or reverse connection, such as A, B reverse connection, 485 communication indicator RX green light will be steady on, the A, B line switch wiring can be; ③ Replace the 485 converter, if the communication is feasible, the previous 485 converter has problems ④ Replace the 485 controller, if the communication is feasible, the previous 485 controller has problems ⑤ line interference, can not communicate normally, short distance debugging, if the communication is feasible, the line has problems. ⑥ The controller to the computer master control center exceeds the effective length (500M), choose 485HUB or repeater to extend the communication distance. 7 Too many controller devices with the bus, disconnect some devices in the bus, if communication is feasible, please replace the 485 converter that can drive more device communication. Generally, the converter bought on the market, a bus can drive 40 controllers, if necessary, you can customize the 128/256 converter with the manufacturer.