Easy Book
Manual, 05/2009, A5E02486774-01
31
How the S7-1200 works
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 scan. 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.
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STARTUP
RUN
A Clears the I memory area
① Writes Q memory to the physical outputs
B Initializes the outputs with either the last value
or the substitute value
② Copies the state of the physical inputs to I
memory
C Executes the startup OBs
③ Executes the progam cycle OBs
D Copies the state of the physical inputs to I
memory
④ Handles communication requests and
performs self-test diagnostics
E Stores any interrupt events into the queue to
be processed in RUN mode
⑤ Processes interrupts during any part of the
scan cycle
F Enables the writing of Q memory 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.
