1. Observe the syntax in ladder diagram language. 2. Set the intermediate unit. In the ladder diagram, if multiple coils are controlled by a series parallel circuit of a contact, in order to simplify the circuit, an auxiliary relay controlled by this circuit can be set in the ladder diagram, which is similar to the intermediate relay in the relay. 3. Separate the intertwined circuit design ladder diagram, take the coil as the unit, respectively consider the control of each coil in the relay circuit diagram by those contacts and circuits, and then draw the corresponding equivalent ladder diagram circuit. 4 processing of input signals provided by normally closed contacts when designing input circuits, normally open contacts should be used as much as possible. If only normally closed contacts can be used, the normally open / normally closed type of the corresponding contacts in the ladder diagram should be opposite to the relay circuit. 5. Processing of instantaneous contact of time relay for time relay with instantaneous contact, the auxiliary relay can be paralleled at both ends of the coil of the corresponding timer in the ladder diagram, and the contact of the latter is equivalent to the instantaneous contact of time relay. 6. The time of power-off delay the processing of electrical appliances uses the timer of power-on delay to realize the function of power-off delay. 7. For the design of external interlocking circuit, in addition to the software interlocking composed of normally closed contacts connected in series with the coil of the corresponding output relay in the ladder diagram, the hardware interlocking circuit should also be set outside the PLC. Processing of overload signal of thermal relay 1) for automatic reset thermal relay, the overload signal provided by its contact must be provided to PLC through input circuit, and the overload protection is realized by ladder diagram; 2) The normally closed contact of the manual reset thermal relay can be connected in series with the coil of the AC contactor controlling the motor in the output circuit of the PLC. 9. Minimize the input and output signals of PLC. 10. Pay attention to whether the driving capacity of PLC output module can meet the requirements of external loads. Generally, PLC can only drive loads with rated voltage below AC220V. If the coil voltage is 380V, the coil should be replaced with 220V, or an intermediate relay should be set outside the original AC contactor of the PLC system.