TU838 3BSE008572R1 ABB

There is a program counter in the cpu of the PLC that indicates the storage address of the program step.During the program running process,the counter is automatically incremented by 1 after one step is executed,and the program is sequentially executed from the initial step(step number is zero)to the final step(usually the end instruction).),and then return to the starting step to cycle operations.The time required for the PLC to complete one cycle operation is called a scan cycle.Different types of plc,the cyclic scanning period is between 1 microsecond and several tens of microseconds.Loop operations such as the program counter,which dcs do not have.This is also what makes plc less redundant than dcs.

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。