When LS PLC is put into operation, its operation process is generally divided into three stages, namely input sampling, user program fulfillment and output rewriting. The completion of these three phases is called a scan cycle. Throughout the entire running period, the PLC's CPU repeats the above three phases at a certain scanning speed.
(1) Input sampling stage In the input sampling stage, the PLC reads all input conditions and data sequentially by scanning method, and stores them in the corresponding unit in the I/O image area. After the input sampling is complete, it moves to the user program fulfillment and output rewriting phase. In both phases, even if the input status and data change, the condition and data of the corresponding cells in the I/O image area will not change. Therefore, if the input is a pulsed signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in any case.
(ii) User program fulfillment phase
During the user program fulfillment phase, LS PLC always scans the user program in a top-down order (ladder diagram). When scanning each ladder diagram, the control circuit composed of each contact on the left side of the ladder diagram is always scanned first, and the control circuit composed of contacts is logically operated according to the order of first left and then right, first up and then down, and then according to the results of logical operation, the situation of the corresponding position of the logic coil in the system RAM storage area is rewritten; Perhaps override the position of the output coil in the I/O image; It may be necessary to confirm whether the special functional instructions specified in the ladder diagram are to be fulfilled.
That is, in the process of user program performance, only the status and data of the input points in the I/O image area will not change, while the conditions and data of other output points and software devices in the I/O image area or the system RAM storage area may change, and the ladder diagram ranked above, the program performance results will play a role in the ladder diagram of those coils or data used in the below; Conversely, the condition or data of the overwritten logic coils in the ladder diagram below can only be used for the program above it until the next scan cycle.
In the process of program execution, if the immediate I/O command is used, the I/O point can be directly accessed. That is, if the I/O command is used, the value of the input process impression register will not be updated, and the program will directly take the value from the I/O module, and the output process impression register will be updated immediately, which is somewhat different from the immediate input.
(iii) Output rewriting phase
When the user program is scanned, the LS PLC enters the output rewriting stage. During this period, the CPU overrides all the output latch circuits according to the corresponding conditions and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. At this time, it is the real output of the PLC.
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