The basic function of PLC is to control, it collects various signals of controlled objects. After PLC processing, the control is realized through the executive device. The input circuit is the controlled object (the machine, equipment and production process that needs to be controlled) for detection, collection, conversion and input. In addition, the button installed on the console, switches, etc. can also send control instructions to the PLC. The function of the output circuit is to accept the control signal output by PLC and execute the control task on the controlled object.