How to understand the difference between PLC ladder diagram and relay circuit?

Anyone who has learned PLC programming knows that it is best to understand the traditional relay control principle before entering the door and draw an electrical diagram. In understanding, it is also possible to convert the normally open, normally closed, time control, etc. in the electrical diagram with the plc ladder diagram, but the PLC ladder diagram and the relay control diagram are still different. The difference is that the PLC is serial control, relay Control is parallel control.

The above figure shows the electrical schematic diagram of the motor forward and reverse and the ladder diagram programmed by Siemens PLC. The comparison between the two can be found. In the relay control, when the accuracy is high, the normal rotation start button is pressed. Turning the normally closed button on there does not guarantee that it will be disconnected and then closed, because the relay control circuit is powered in parallel. The ladder diagram is a progressive read program, which is serially powered, so there is still a difference in control between the two.

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