The working principle of PLC when the PLC is put into operation, its working process is generally divided into three stages, i. e. input sampling, user program execution and output refresh. The completion of these three phases is called a scan cycle. During the whole running period, the PLC's CPU repeatedly performs the above three stages with a certain scanning speed. One input sampling phase, in the input sampling phase, the PLC reads all the input states and data by scanning in turn, and stores them in the corresponding unit in the I / O Image area. After the input sample is completed, go to the user program execution and output refresh phase. In both phases, even if the input state and data change, the state and data of the corresponding unit in the I / O Map area remain unchanged. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan cycle to ensure that the input can be read in any case. Two User Program Execution Stage, in User Program Execution Stage, PLC always scans user program ladder diagram in order from top to bottom. When scanning each ladder diagram, we always first scan the control circuit made up of each contact on the left side of the ladder diagram, and carry out logical operation on the control circuit made up of contact points in the order of left, right, first up, then down, then, according to the result of logical operation, the state of the corresponding bit of the logic coil in the system RAM storage area is refreshed, or the state of the corresponding bit of the output coil in the I / O Image area is refreshed; Or determine whether special functional instructions specified in the ladder diagram are to be executed. That is, only the state and data of the input point in the I / O Image region remain unchanged during the execution of the user program, the state and data of other output points and soft devices in the I / O Image area or the system RAM storage area may change, and the ladder diagram at the top, the result of the program will work on the ladder diagram below where these coils or data are used; on the contrary, the ladder diagram below, the status or data of the flushed logic coil can not affect the program on top of it until the next scan cycle. Three output refresh phase, when the end of the scanning user program, the PLC will enter the output refresh phase. During this period, the CPU refreshes all the output latches according to the corresponding state and data in the I / O Image area, and drives the corresponding peripheral via the output circuit. At this point, is the real output of PLC, the same number of ladder diagram, its sequence is different, the results of the implementation is different. In addition, the running result of the scanning user program is different from that of the hard logic parallel running of the relay control device. Of course, if the scan cycle time is negligible for the entire run, then there is no difference. Generally speaking, the scan cycle of PLC includes self-diagnosis, communication, that is, a scan cycle is equal to the sum of all time of self-diagnosis, communication, input sampling, user program execution, output refresh, etc. .