4500 operates in a "sequential scanning, continuous cycling" manner. When the PLC is running, the CPU compiles a program that coexists in the user's memory according to the user's control requirements, and performs periodic cyclic scans according to the instruction step number (or address number). If there are no jump instructions, the user program is executed sequentially from the first instruction until the program is completed, and then returns to the first instruction to start the next new scan. During each scan process, We also need to complete tasks such as sampling the input signal and refreshing the output state. A scanning cycle of 4500 must go through three stages: input sampling, program execution, and output refresh. In the input sampling stage of 4500, the on/off status or input data of all input terminals that are temporarily stored in the input latch are first read in sequence through scanning, and written to the corresponding input status registers to refresh the input, then close the input port and enter the program execution stage. In the program execution phase, 4500 scans and executes each instruction in the order in which the user program instructions are stored. After corresponding calculations and processing, the results are written to the output status register, and all contents in the output status register change with the execution of the program. Output refresh stage: When all instructions are executed, the on/off status of the output status register is sent to the output latch during the output refresh stage, and is output through a certain method (relay, transistor, or thyristor) to drive the corresponding output device to work.