Man-machine interface, GPRS module, inverter and intelligent instrument. In this system, the signal transmission between PLC and frequency converter does not need D/a conversion, but through the RS485 interface communication connection between PLC and frequency converter to achieve digital communication, so the anti-interference performance of the system has been greatly improved. . The Ethernet module (cp243-1) supports the TCP/IP protocol and can be connected to industrial Ethernet via a switch