The process and characteristics of PLC execute a program

PLC execution process is divided into three stages, namely the input sampling stage, program execution phase, the output refresh stage, as shown in figure 1. The input sampling stage in the input sampling stage, PLC to scan works in order to all samples at the input input status, and deposited in the input image, register the input image register be refreshed. Then entered the stage of process, the program or other phase, even if the input status changes, the input image will not change the contents of a register, the state of the input change only the next scan cycle can be sampled to input processing stage. Program execution stage during program execution stage, PLC to scan execution procedure in order. If a program with a ladder diagram, is always the first, after the first left after the right order. When meet the program jump instruction, according to whether meet to decide whether the program jump jump condition. When involved in the instruction to input, output, state, and component image registers the PLC from the input image read, according to the user program operation, the result of the operation in the component image register again. For component image register, its content will change along with the process of program execution. The output refresh stage when all after the completion of the program execution, into the output processing phase. In this stage, the PLC will output image register status (related to the output The output relay status) Transferred to the output latch and output through a certain way, drive the external load. Therefore, PLC controller in a scan cycle, the input of the state of the sample only in the input sampling stage. After the PLC program execution stage of input will be blocked, until the next input sampling scan cycle to resampling input condition. This way is called sampling, namely in a scan cycle, state of input samples for some time. If the output results in the user program assignment for many times, the last time is valid. In a scan cycle, state of the output in the output refresh stage only, from the output of the output image register to refresh the output interface. In other stage output state have been saved in the output image register. This way is called concentration output. For small PLC, the less I/O points, the user program is shorter, generally USES the output sampling, centralization works, although to a certain extent, reduce the response speed of the system, but make the PLC controller work most of the time from the external input/output devices, fundamentally improve the anti-interference ability of the system, enhance the reliability of the system. As for large and medium-sized PLC controller, its I/O points more, strong control function, the user program is longer, in order to improve the system response speed, can be used in a regular sampling, periodic output way, or interrupt input, output, and use a variety of ways such as intelligent I/O interface. From the above analysis shows that when the input end of the PLC input signal changes to the PLC outputs react to changes in the input, the need for a period of time, this phenomenon is referred to as the PLC input/output response lag. For the general industrial control, all this lag is allowed. It should be noted that this kind of response lag is not only caused by PLC scanning way, more important is the PLC input interface of the filter to the input lag, and the action time of the driving device in the output interface the output delay, but also related to the program design. The time lag is the design of PLC application system should pay attention to grasp a parameter.
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