PLC execution process is divided into three stages?

PLC execution process is divided into three stages, namely the input sampling stage, program execution phase, the output refresh stage, PLC scanning process: ( 1) The input sampling stage. Way at this stage, PLC to scan read all the input terminal of the input signal, and the state of input into the corresponding register the input image. At this point, the input image register was brushed off. In program execution stage and output refresh stage, the input image memory isolation with the outside world, its content remains the same, until the next scan input scan stage of the cycle is refreshed to read in the input signal. Visible, PLC in the execution procedure and process the data, not directly use the scene at the time of the input signal, and using the sampling data from the input to the image area. In general, the input signal is greater than the width of a scan cycle, otherwise may cause the loss of signal. ( 2) Program execution stage. In the process of execution of user programs, PLC controller in accordance with the principle of ladder diagram program scans, generally speaking, the PLC controller from left to right, from top to bottom of the steps one by one program execution. But met jump instruction program, to jump the decision process according to jump condition meets the address. The execution of a program, when instructions relating to both input and output state, PLC can 'read' from the input image register corresponding to the input terminal state, 'read' from the output image register corresponding components ( 'Soft relay') The current state. Then the corresponding operations and operations as a result, deposited in the output image register again. For the output image register, each element ( 'Soft relay') The state will change as the program execution process. ( 3) The output refresh stage. Program execution phase of the operation result is placed into the output image area, and not to the output port. In the output refresh stage, the PLC will output the output variable in the image area into the output latch, and then produced by the latch through the output module to control the output of this cycle. If the internal output relay status to '1', then the output relay contact closure, after the output terminal drive external load. All the state of the output device to keep a scan cycle.
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