PLC USES software functions to replace a large number of intermediate relays, time relays, counters and other equipment in the relay control system, thus greatly reducing the design, installation and wiring of the control cabinet. The sequence control method can be used to design PLC ladder diagram program. This design method is very regular and easy to master. For complex control systems, if the correct design method is mastered, the time required to design the trapezoidal diagram will be much less than the time required to design the circuit diagram of the relay system