The PLC uses a ladder language that closely resembles the relay circuit diagram. If using PLC transformation relay control system, according to the relay circuit diagram to design ladder is a shortcut. This is because the original relay control system has been proved to be able to complete the control functions required by the system after a long period of use and testing. The relay circuit diagram has many similarities with the ladder diagram, so that the relay circuit diagram can be “translated” into Ladder, that is, with the PLC’s external hardware wiring diagram and ladder have many similarities, the function of the relay system. This design method generally do not need to change the control panel, maintaining the original external characteristics of the system, the operator does not change the long-term formation of operating habits.

First, the basic method

Relay circuit diagram is a pure hardware circuit. When changing it to PLC control, you need to use the external wiring diagram and ladder diagram of the PLC to equivalent relay circuit diagram. The PLC can be thought of as a control box whose external wiring diagram describes the external wiring of the control box, the ladder diagram is the internal “wiring diagram” of the control box, the input and output bits in the ladder diagram are the control box and External interface, the interface relay, so that you can use the analysis of the relay circuit diagram to analyze the PLC control system. When analyzing a ladder diagram, you can think of the contact of the input bit as the contact of the corresponding external input device and the coil of the output bit as the coil of the corresponding external load. External load coil in addition to the ladder diagram of the control, but also by the right external contact control.

The relay circuit diagram into a functionally identical PLC external wiring diagram and ladder steps are as follows:

1, to understand and be familiar with the working principle of controlled equipment, process and mechanical movements, according to the relay circuit diagram analysis and master the working principle of the control system.

2, determine the PLC’s input signal and output load

Relay circuit diagram of the AC contactor and solenoid valve and other implementing agencies if the PLC output bits to control their coils in the PLC output. Button, operation switch and travel switch, proximity switch, etc. Provide digital input signal of PLC relay The function of intermediate relay and time relay in the circuit diagram is completed by internal memory bits and timers of PLC, which are connected with PLC input bits, output bits Nothing to do

3, select the PLC model

Select the CPU module, power supply module, and digital input and output modules according to the required functions and sizes of the system. Configure the hardware to determine the mounting positions of the input and output modules in the rack and their starting addresses.

4, determine the PLC each digital input signal and output load corresponding to the input bit and output bit address, draw the PLC’s external wiring diagram. The addresses of the inputs and outputs in the ladder diagram depend on the start address of their module and the terminal number in the module.

5, Determine the middle of the relay circuit diagram, the relay in the ladder corresponding to the memory and timer, the address of the counter.

6, according to the correspondence between the draw ladder.

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