Most PLC using batch input/output cycle work scan, running as user programs one by one in the order. A complete cycle can be divided into three stages:
(A) enter the refresh phase
Programs at the start of monitoring program the machine to scan input each signal on all input ports, and in turn into the corresponding input mapping registers.
(B) the processing stage
All input ports sampled after the logical operation begins, based on user input controls, starting from the first article, article-by-article implementation and the corresponding logical result, deposit corresponding to the intermediate element and output element mapping registers, when finally a program’s execution has been completed, becomes the output refresh processing.
(C) output refresh phase
Output the contents of element mapping registers, starting from the first output port, to the last end, read in turn corresponds to the output latch, which will drive the output devices to form actual output may be programmed.
In General, the PLC scanning cycle is about 10ms, in addition, programmable logic controller input/output and response lag (about 10ms input filter), relays mechanical delay of about 10ms, so a signal from input to output, the lag of approximately 20–30ms.
Effective width should be greater than 1 cycle of the input signal +10ms.