4500是采用“顺序扫描，不断循环”的方式进行工作的。即在PLC运行时，CPU根据用户按控制要求编制好并存于用户存储器中的程序，按指令步号(或地址号)作周期性循环扫描，如无跳转指令,则从条指令开始逐条顺序执行用户程序， 直至程序结束,然后重新返回条指令, 开始下一轮新的扫描， 在每次扫描过程中,还要完成对输入信号的采样和对输出状态的刷新等工作。4500的一 个扫描周期必经输入采样、程序执行和输出刷新三个阶段。4500在输入 采样阶段:首先以扫描方式按顺序将所有暂存在输入锁存器中的输入端子的通断状态或输入数据读入，并将其写入各对应的输入状态寄存器中，即刷新输入，随即关闭输入端口，进入程序执行阶段。4500在程序执行阶段:按用户程序指令存放的先后顺序扫描执行每条指令，经相应的运算和处理后，其结果再写入输出状态寄存器中，输出状态寄存器中所有的内容随着程序的执行而改变。输出刷新阶段:当所有指令执行完毕,输出状态寄存器的通断状态在输出刷新阶段送至输出锁存器中，并通过一定的方式(继电器、晶体管或晶间管)输出,驱动相应输出设备工作。 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.
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