PLC scanning
The scan time indicates how fast the controller can react to changes in inputs. Scan times vary with computer model and program content, and length. If a controller has to react to an input signal that changes states twice during the scan time, it is possible that the PLC will never be able to detect this change. When the inputs is closed, the input module senses a voltage and an ON condition (1) is entered into the input table bit ‘0012’. During the program scan the processor sets instructins ‘0012’ and ‘506’ to ON(1).The processor turns light output ‘506’ ON during the next I/O scan.