描述
PLC的cpu内有指示程序步存储地址的程序计数器,在程序运行过程中,执行一步该计数器自动加1,程序从起始步(步序号为零)起依次执行到最终步(通常为end指令),然后再返回起始步循环运算。plc每完成一次循环操作所需的时间称为一个扫描周期。不同型号的plc,循环扫描周期在1微秒到几十微秒之间。程序计数器这样的循环操作,这是dcs所没有的。这也是使plc的冗余不如dcs的原因。
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