Working principle of the PLC

PLC is adopted 'sequential scan, cycle'. In the operation of the PLC, namely the CPU according to the users according to the requirements of control is prepared in the program in memory, according to the instruction step number ( Number or address) As a periodic cycle scan, if no jump instruction, is from the first instruction to order a detailed user program, until the end of the program. And then back to the first instruction, the next round of the new scan. In the process of each scan, and complete to the input signal sampling and refresh the output status, etc. A PLC controller scan cycle necessary input sampling, refresh program execution and output stages. PLC in the input sampling stage: first of all in scanning manner in order will all temporary presence of input terminals in the input latch on and off of the state or the input data is read, and write the corresponding input in the status register, namely to refresh the input. Then close the input port, enter the stage of program execution. PLC in program execution stage: according to the user program instructions for scanning the order execute each instruction, after the corresponding operation and treatment, the result then written to the output status register, output status register all of the content and subject to change with the execution of the program. Output refresh stage: when all instruction execution is completed, the output of the status register on off state in the output refresh stage to the output latch, and through a certain way, Relay, transistor and thyristor) Output, drive the corresponding output devices work.
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