(1)输入取样阶段
PLC在输入取样阶段，首先按顺序将所有输入端子的通断状态或输入数据读入，并将其存入(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。在程序执行阶段，即使输入状态有变化，输入状态寄存器的内容也不会改变。变化了的输入信号状态只能在下一个扫描周期的取样阶段被读入。
(2)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，所需的执行条件可从输入状态寄存器、内部继电器(寄存器)和当前输出状态寄存器中读入，经过相应的运算和处理后，其结果再次写入输出状态寄存器中。所以，输出状态寄存器中所有的内容将随着程序的执行而改变。
(3)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端并通过一定方式(继电器、晶体管或晶闸管等)输出，驱动相应输出设备工作，这就是PLC的实际输出。
经过这三个阶段，完成一个扫描周期。扫描周期的长短与用户程序的指令条数及执行各条指令所需的时间有关，一般不超过100ms。
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