Processing Steps Within A Cycle - Siemens SIMATIC ET 200S Manual

Process image of the inputs and outputs
If the address areas "inputs" (I) and "outputs" (O) are addressed in the user
program, the signal states on the digital electronic modules are not checked, but
instead a memory area in the system memory of the CPU is accessed. This
memory area is referred to as the process image.
The process image is divided into two parts: the process image of the inputs and
the process image of the outputs.
Advantages of the process image
The advantage of accessing the process image over accessing the electronic
modules directly is that a consistent image of the process signals is available to the
CPU for the duration of cyclic program processing. If a signal state at an input
module changes during program processing, the signal state is retained in the
process image until the process image is updated in the next cycle. Furthermore,
accessing the process image requires much less time than accessing the
electronic modules directly because the process image is located in the system
memory of the CPU.
Updating the process image
The process image is updated by the operating system cyclically. The figure below
shows the processing steps within a cycle.
User program
Figure 8-5
ET 200S IM 151-7 CPU Interface Module
A5E00058783-04
Startup
PIQ
PII
CCP (OpSy)

Processing steps within a cycle

Functions of the IM 151-7 CPU
Startup program
Writing the process image of the outputs
to the electronic modules
Reading the inputs from the electronic modules and
updating the data in the process image of the
inputs
Processing the user program (OB 1 and
all blocks called in the program)
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