LR 4220E
LR 4220E
LR 4220E
PLC采用 “ 顺序扫描、不断循环 ” 的工作方式, 这个过程可分为输入采样,程序执行、输出刷新三个阶段 ,整个过程扫 描并执行一次所需的时间称为扫描周期。
输入采样阶段:PLC在输入采样阶段,以扫描方式顺序读入所有输入端的通/断状态或输入数据,并将此状态存入输入状态寄存器,即输入刷新。接着转入程序执行阶段。在程序执行期间,即使输入状态发生变化,输入状态寄存器的内容也不会改变,只有在下一个扫描周期的输入处理阶段才能被读入 。程序执行阶段:PLC 在执行阶段,按先左后右,先上后下的步序,执行 程序指令。其过程如下:从输入状态寄存器和其它元件 状态寄存器中读出有关元件的通 / 断状态,并根据用户程 序进行逻辑运算,运算结果再存入有关的状态寄存器中。
输出刷新阶段:在所有指令执行完毕后,将各物理继电器对应的输出状态寄存器的通 / 断状态,在输出刷新阶段转存到输出寄存器,去 控制各物理继电器的通 / 断,这才是 PLC 的实际输出。
由 PLC 的工作过程可见,在 PLC 的程序执行阶段,即使输入发生了变化,输入状态寄存器的内容也不会立即改变,要 等到下一个周期输入处理阶段才能改变。暂存在输出状态寄存器中的输出信号,等到一个循环周期结束, CPU 集中将 这些输出信号全部输出给输出锁存器,这才成为实际的 CPU 输出。因此全部输入、输出状态的改变就需要一个扫描周 期,换言之,输入、输出的状态保持一个扫描周期。
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