Quick review
1.1 Basic information about how the CPU works
Getting started with S7-1200
8 Getting Started, 11/2009, A5E02486791-01
1.1 Basic information about how the CPU works
1.1.1 Tasks performed every scan cycle
The scan cycle provides consistent logic through the execution of the user program for a
given cycle and prevents the flickering of physical output points that might change state
multiple times in the process image output area. Each scan cycle includes writing the
outputs, reading the inputs, executing the user program instructions, and performing system
maintenance or background processing.
Under default conditions, all digital and analog I/O points use an internal memory area called
the "process image" to update the I/O synchronously with the scan cycle. The process image
contains a snapshot of the physical inputs ("I memory") and outputs ("Q memory") on the
CPU, signal board, and signal modules.
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A The input area of the process image (I memory) is cleared.
B The outputs are initialized with the last value.
C Any start-up logic (contained in special code blocks) is executed.
D The state of the physical inputs is copied to I memory.
E Any interrupt events are queued for processing during RUN mode.
STARTUP
mode
F The writing of the output area of the process image (Q memory) to the
physical outputs is enabled.
① Q memory is written to the physical outputs.
② The state of the physical inputs is copied to I memory.
③ The logic of your user program is executed.
④ The self-test diagnostics are performed.
RUN mode
⑤ Interrupts and communications are processed during any part of the
scan cycle.
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