User Manual

Easy Book
Manual, 03/2014, A5E02486774-AF
51
PLC concepts made easy
4
4.1
Tasks performed every scan cycle
Each scan cycle includes writing the outputs, reading the inputs, executing the user program
instructions, and performing system maintenance or background processing.
The cycle is referred to as a scan cycle or sca
n. Under
default conditions, all digital and analog I/O points are
updated synchronously with the scan cycle using an
internal memory area called the process image. The
process image contains a snapshot of the physical inputs
and outputs on the CPU, signal
board, and signal
modules.
● The CPU reads the physical inputs just prior to the execution of the user program and
stores the input values in the process image input area. This ensures that these values
remain consistent throughout the execution of the user instructions.
● The CPU executes the logic of the user instructions and updates the output values in the
process image output area instead of writing to the actual physical outputs.
● After executing the user program, the CPU writes the resulting outputs from the process
image output area to the physical outputs.
This process provides consistent logic through the execution of the user instructions for a
given cycle and prevents the flickering of physical output points that might change state
multiple times in the process image output area.