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Circular scanning the implementation process of PLC

Circular scanning process execution of user programs, can be divided into input sampling, program execution, output refresh three stages, as shown in figure 1.
1 the input sampling stage
In the input sampling stage, PLCC to scan the input signal the state of all the input (ON/OFF state) read to the input image storage register, called the sampling of the input signal. And then transferred to the program execution phase, during the execution of a program, even if the input state changes, the input image register contents will not change. Change only the input state in the next work cycle of the input sampling stage was re read.
Fig 1 the process of the implementation of the PLC program
The 2 program execution stage
In the program implementation stage, PLC on the program sequentially scanning. If the program with the ladder diagram representation, a sequential scan of the right is always in and after the first, after the first left. Each scan to the input state requires a instruction or other elements of the state, respectively, by reading the input image register or the output image register, then the corresponding logical or arithmetic operation, operation results are then deposited in the special register. If the execution of the program output instructions, the corresponding operational results into the output image register.
The 3 output refresh stage
In all the instructions after the execution is completed, the output image register state is to the output state. In the output refresh stage which will be transferred to the output latch circuit, and then through the output terminal output signal to drive the user output device, the actual output that is PLC. PLC repeated execution of the above three stages, each repeat is a working cycle (or scan cycle). Length and the program on the length of working cycle.
Because of the time constant module filter input / output, output relay causes mechanical hysteresis and implementation procedures according to the working cycle, will make the input / output response lag phenomenon, for general industrial control equipment, the hysteresis phenomenon is allowed. But some signals of some equipment requirements to make quick response, therefore, some PLC with high speed response of input / output module, and some of the sequential program into a low-level program advanced program of quick response and the response speed of two kinds of general. Such as the FANUC-BESK PLC provisions 8ms senior program each scan, and the automatic partitioning segments low-level program, when the start of program execution, first to perform advanced sequential program, segments and then perform low-level procedures in 1, and then to perform advanced procedure, segments and then execute the lower 2 of the procedures, so that each executed a the segments of low-level program, you will need to re scan the execution of a high-level program, to ensure the speed signal in response to the senior program.

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