What is the principle of the programmable controller?

PLC is to take sequential scan, continuous cycle, the way to work, that is, in the run-time of the PLC, PLC press control based on user requirements programmed and stored in the program in the user program memory, the instruction step number or address number for cyclicalscanning, if no jump instruction from the first instruction by one user program is executed in sequence, until the program ends and then return to the first instruction to start the next one scan, each time during the scanning process, but also complete rightinput signal sampling and output status refresh. A PLC scan cycle must be input sampling program execution and output to refresh three stages and some other auxiliary stage, in which the input sampling and output refresh in some occasions, also known as I / O refresh.