Siemens Simatic S7-200 System Manual page 122

Basic Concepts for Programming an S7-200 CPU
Process-Image Input and Output Registers
It is usually advantageous to use the process-image register rather than to directly access
inputs or outputs during the execution of your program. There are three reasons for using the
image registers:
The sampling of all inputs at the top of the scan synchronizes and freezes the values of
the inputs for the program execution phase of the scan cycle. The outputs are updated
from the image register after the execution of the program is complete. This provides a
stabilizing effect on the system.
Your program can access the image register much quicker than it can access I/O points,
allowing faster execution of the program.
I/O points are bit entities and must be accessed as bits, but you can access the image
register as bits, bytes, words, or double words. Thus, the image registers provide
additional flexibility.
An additional benefit is that the image registers are large enough to handle the maximum
number of input and output points. Since a real system consists of both inputs and outputs,
there is always some number of image register locations not used. You can use the unused
locations as extra internal memory bits. See Section 8.1.
Immediate I/O
Immediate I/O instructions allow direct access to the actual input or output point, even though
the image registers are normally used as either the source or the destination for I/O
accesses. The corresponding process-image input register location is not modified when you
use an immediate instruction to access an input point. The corresponding process-image
output register location is updated simultaneously when you use an immediate instruction to
access an output point.
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S7-200 Programmable Controller System Manual
C79000-G7076-C230-02