The PLC works by means of a "sequential scanning". That is, when the PLC is running, the CPU prepares a program which exists in the user's memory according to the control requirement, and performs periodic cyclic scanning according to the instruction step sequence number (or the address number), such as no jump instruction, and the user program is executed one by one from the first instruction until the program ends.

The first command is then returned to begin the next round of new scans. During each scanning process, the sampling of the input signal and the refreshing of the output state are also completed.

A scan cycle of the PLC is required for three stages of input sampling, program execution, and output refresh. In the input sampling phase, the PLC first reads the on-off state or the input data of all the input terminal in the input latch in the order of scanning, and writes the input data into the corresponding input state registers, i.e., the refresh input. The input port is then closed and the program execution phase is entered.

In the program execution stage, the PLC scans and executes each instruction according to the sequence stored in the user program instruction, and after the corresponding operation and processing, the result is written to the output state register, and all the contents in the output state register are changed in accordance with the execution of the program.

At the output refresh phase: when all the instructions are finished, the on-off state of the output state register is sent to the output latch in the output refresh stage and output by a certain mode (relay, transistor or thyristor) to drive the corresponding output device to work.