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