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490NRP25400 输入信号的采样和对输出状态的刷新
PLC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，按指令步序号（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。然后重新返回一条指令，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。
PLC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。
PLC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。
PLC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、晶体管或晶闸管）输出，驱动相应输出设备工作。
490NRP25400 输入信号的采样和对输出状态的刷新
PLC is the use of “sequential scanning, continuous cycle” way to work. That is, when the PLC is running, the CPU is prepared according to the user’s control requirements and exists in the user’s memory program, according to the instruction step number (or address number) for periodic cycle scanning, if there is no jump instruction, the user program is executed successively from the beginning of the instruction until the end of the program. Then a new command is returned to start the next round of scanning. In the process of each scan, it is necessary to complete the sampling of the input signal and refresh the output state.
A PLC scan cycle must be input sampling, program execution and output refresh three stages.
In the input sampling stage, PLC first reads the on-off status or input data of all the input terminals that temporarily exist in the input latch in order by scanning, and writes it into the corresponding input status register, that is, refresh the input. Then close the input port and enter the program execution phase.
PLC in the program execution stage: scan and execute each instruction according to the sequence of user program instructions stored, after the corresponding operation and processing, the result is written into the output status register, and all the contents of the output status register change with the execution of the program.
Output refresh stage: When all instructions are executed, the on-off state of the output status register is sent to the output latches in the output refresh stage, and output by a certain way (relay, transistor or thyristor) to drive the corresponding output device.
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