Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 69)

Memory concept
4.1 Memory areas and retentivity
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
4-7
Configurable process image with CPU317 (FW V2.3.0 or higher)
IN STEP 7, you can define a user-specific size of the I/O process images between 0 to 2048
for a CPU317, FW V2.3.0 or higher.
Note the information below:
Note
Currently, the dynamic setting of the process image only affects its update at the scan cycle
control point. That is, the process image of inputs is only updated up to the set PII size with
the corresponding values of the peripheral input modules existing within this address area,
or the values of the process image of outputs up to the set PIO size are written to the
peripheral output modules existing within this address area.
This set size of the process image is ignored with respect to STEP 7 commands used to
access the process image (for example U I100.0, L EW200, = Q20.0, T AD150, or
corresponding indirect addressing commands also). However, up to the maximum size of the
process image (that is, up to I/O byte 2047), these commands do not return any
synchronous access errors, but rather access the permanently available internal memory
area of the process image.
The same applies to the use of actual parameters of block calls from the I/O area (area of
the process image).
Particularly if these process image limits were changed, you should check to which extent
your user program accesses the process image in the area between the set and the
maximum process image size. If access to this area continues, the user program may not
detect changes at the inputs of the I/O module, or actually fails to write the data of outputs to
the output module, without the system generating an error message.
You should also note that certain CPs may only be addressed outside of the process image.