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When PLC controller is put into operation, its working process is generally divided into three stages, namely, input sampling, user program execution and output refresh. The completion of the above three stages is called a scanning cycle. In the whole operation process, the CPU of PLC controller repeatedly executes the above three stages at a certain scanning speed.

2. Input sampling stage:

In the input sampling stage, the PLC controller reads in all input states and data by scanning and stores them in the corresponding units in the I/O image area. After the input sampling is completed, execute the user program and refresh the output. In these two stages, even if the input state and data change, the state and data of the corresponding unit in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scanning period to ensure that the input can be read under any circumstances.

3. The user program execution stage:

In the user program execution stage, the PLC controller always scans the user program (ladder diagram) from top to bottom. When scanning each ladder diagram, always scan the control circuit composed of contacts on the left side of the ladder diagram first, and perform logical operation on the control circuit composed of contacts in the order of first left and then right, first up and then down, and then refresh the state of the corresponding bit of the logic coil in the system ram storage area according to the result of logical operation; Or refresh the state of the corresponding bit of the output coil in the I/O mapping area; Or determine whether to execute the special function instruction specified in the ladder diagram.

That is, in the process of user program execution, only the state and data of input points in I/O image area will not change, while the state and data of other output points and soft devices in I/O image area or system ram storage area may change, and the program execution results of the top ladder diagram will play a role in the bottom ladder diagram using these coils or data; On the contrary, in the ladder diagram arranged below, the status or data of the refreshed logic coil can only work on the programs arranged on it in the next scanning cycle.

4. Output refresh stage:

When the scanning user program is completed, the PLC controller enters the output refresh stage. During this period, the CPU refreshes all the output latch circuits according to the corresponding state and data in the I/O mirror area, and then drives the corresponding peripherals through the output circuits. This is the real output of PLC controller.

The same ladder diagram, different arrangement order, different execution results. In addition, the running result of scanning user program is different from the parallel running result of hard logic of relay control device. Of course, if the time occupied by the scanning cycle is negligible for the whole operation, there is no difference between the two.

Wiring diagram: