PLC utilizes a standard scan method when evaluating user program.
Input signal: PLC reads the ON/OFF status of each input and stores the status into memory before evaluating the user program. Once the external input status is stored into internal memory, any change at the external inputs will not be updated until next scan cycle starts.
Program: PLC executes instructions in user program from top to down and left to right then stores the evaluated data into internal memory. Some of this memory is latched.
Output: When END command is reached the program evaluation is complete. The output memory is transferred to the external physical outputs.
Scan time The duration of the full scan cycle (read, evaluate, write) is called “scan time.” With more I/O or longer program, scan time becomes longer.