Introduction to the working principle and working process of PLC

How PLC works

PLC works in a "sequential scanning, continuous loop" mode. That is, when the PLC is running, the CPU performs periodic loop scanning according to the instruction step number (or address number) according to the program compiled by the user according to the control requirements and stored in the user memory. If there is no jump instruction, the user program will be executed sequentially from the first instruction until the program ends. Then it returns to the first instruction and starts the next round of scanning. In each scanning process, the sampling of input signals and the refreshing of output status must be completed.

A PLC scan cycle must go through three stages: input sampling, program execution and output refresh.

In the input sampling stage of PLC: firstly, the on/off status or input data of all input terminals temporarily stored in the input latch are read in sequence by scanning mode, and written into the corresponding input status register, i.e., the input is refreshed. Then the input port is closed and the program execution stage is entered.

During the program execution phase, the PLC scans and executes each instruction in the order in which the user program instructions are stored. After the corresponding calculations and processing, the results are written into the output status register. All contents in the output status register change as the program is executed.

Output refresh phase: When all instructions are executed, the on/off status of the output status register is sent to the output latch in the output refresh phase, and is output in a certain way (relay, transistor or thyristor) to drive the corresponding output device to work.

How PLC works

PLC working process PLC adopts

The working mode is "sequential scanning and continuous looping". This process can be divided into three stages: input sampling, program execution, and output refresh. The time required to scan and execute the entire process once is called a scanning cycle.

(1) In the input sampling phase, the PLC sequentially reads the on/off status or input data of all input terminals in a scanning manner and stores this status in the input image register, i.e., input refresh, and then enters the program execution phase. During program execution, the input status changes, but the content of the input image register does not change. It can only be read in the input processing phase of the next scan cycle.

(2) During the program execution phase, the PLC executes program instructions in the order of left first, right second, top first, bottom first. The process is as follows: the on/off status of the relevant components is read from the input image register and other component image registers, and logical operations are performed according to the user program, and the operation results are stored in the relevant image registers.

(3) After all instructions are executed, the on/off status of the output image register corresponding to each physical relay is transferred to the output register during the output refresh phase to control the on/off of each physical circuit breaker. This is the actual output of the PLC.

It can be seen from the working process of PLC that during the program execution phase of PLC, even if the input changes, the content of the input status register will not change immediately, and will have to wait until the next cycle input processing phase to change. The output signal temporarily stored in the output status register will be output to the output latch when a cycle ends, and then it becomes the actual CPU output. Therefore, the change of all input and output states requires a scan cycle. In other words, the input and output states are maintained for one scan cycle.