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How does PLC work?

The PLC work with the mode of cyclic scanning. In the PLC, the user programs are stored in order, and the CPU starts executing the program from the first instruction until the first character is returned after encountering the terminator, so that the cycle continues cyclically. The scanning process of the PLC work is divided into internal processing, communication operation, program input processing, program execution, and program output. The time required to scan the entire process is called the scan cycle.

When the PLC works in the stop state, only internal processing and communication operation services are performed. When the PLC works in the running state, it performs cyclic scan work from internal processing, communication operation, program input, program execution, and program output.

plc work

1. Input processing
Input processing is also called input sampling. When PLC working at this stage, the state of the terminals of all the input terminals is sequentially read, and the read information is stored in the corresponding image register in the memory. The input image register is refreshed here. Then enter the program execution phase. When the program is executed, the input image register is isolated from the outside world. Even if the input signal changes, the contents of the image register will not change. The information can only be read in the input processing stage of the next scan cycle.

2. Program execution
According to the principle of PLC ladder program scanning, following the steps of first left and then right first up and down, the program is executed step by step. A program jump instruction is encountered, and the jump address of the program is determined according to whether the jump condition is satisfied. When the user program involves the input/output state, the PLC reads the state of the corresponding input terminal picked up from the previous stage from the input image register, reads the corresponding image register from the output image register, performs logical operation according to the user program, and saves In the relevant device register. For each device, the contents of the device image registers are subject to change as the program is executed.

3. Output processing
PLC works after the program is executed, the output image register, that is, the state of the Y register in the device image register, is dumped to the output latch in the output processing stage, and the power amplifier circuit is driven through the isolation circuit to output the output terminal to the outside. Control signals that drive external loads.

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