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MPM-B1651F-2J74AA
PLC 的工作过程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一个扫描周期。
(1)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映象寄存器,即输入刷新,接着转入程序执行阶段。在程序执行期间,输入状态发生变化,输入映象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。
(2)PLC在程序执行阶段,按先左后右,先上后下的步序,执行程序指令。其过程如下:从输入映象寄存器和其它元件映象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映象寄存器中。
(3)在所有指令执行完毕后,将各物理继电器对应的输出映象寄存器的通/断状态在输出刷新阶段转存到输出寄存器,去控制各物理断电器的通/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会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。
TEL ACT 12 ACT 8 Develop Chemical Tank Working
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