First, the input sampling stage

During the input sampling phase, the programmable logic controller reads all input conditions and data in a scanning manner and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it is transferred to the user program fulfillment and output rewriting phase. In these two phases, the status and data of the corresponding unit in the I/O image area will not change even if the input status and data change. Therefore, if the input is a pulse 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.

Second, the user program fulfillment stage

During the user program fulfillment phase, the programmable logic controller always scans the user program (ladder) sequentially in a top-down order. When scanning each ladder diagram, first scan the control lines formed by the contacts on the left side of the ladder diagram, and perform logic operations on the control lines formed by the contacts in the order of left first, second right, first up and then down, and then according to As a result of the logic operation, rewriting the status of the corresponding bit of the logic coil in the system RAM storage area; or rewriting the status of the corresponding position of the output coil in the I/O image area; or confirming whether to fulfill the special specified by the ladder diagram Utility instructions.

In the process of user program fulfillment, the rewinder does not change the status and data of the input point in the I/O image area, while other output points and soft devices are in the I/O image area or system RAM storage area. The status and data are subject to change, and the ladder diagrams listed above, the program performance results will work on the ladder diagrams that use the coils or data below; instead, the ladder diagrams below, The status or data of the rewritten logic coil can only be applied to the program above it until the next scan cycle.

I/O points can be accessed directly in the process of program execution if I use immediate I/O instructions. That is, if the I/O instruction is used, the value of the input process impression register will not be updated, 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.

Third, the output rewriting stage

After the rewinder scans the user program, the programmable logic controller enters the output rewrite phase. During this period, the CPU rewrites all the output latch circuits according to the corresponding conditions and data in the I/O map area, and then drives the corresponding peripherals through the output circuit. At this point, it is the true output of the programmable logic controller.

Article from Jiangmen Nantian Machinery Manufacturing Co., Ltd.：http://www.jmntjx.com/