Siemens SIMATIC S5 Manual page 156

Using the Q area You can use I/O bytes QB 0 to QB 254 for the I/O modules in the expansion
unit and for distributed I/O.
Warning
!
There is a possibility of inputs or outputs receiving double assignments in
the Q area.
If you address the inputs of a DP slave in the Q area, it is important to ensure
that there is not an input module in the central programmable controller
under the same address but with a different page number.
The same applies to DP slaves with outputs.
If you use the entire Q area, do not insert an input or output module in the
host.
You can avoid the danger of inadvertent double assignments by reserving
input and output areas when setting the master parameters with
COM ET 200 Windows (see Section 8.2.3).
Restrictions The following restrictions apply to page addressing:
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a
IM 308-C – addressing, diagnostics and use of the FB IM308C
With P-page addressing, you cannot use station numbers 120 to 123. Only
the station numbers from 1 to 119 are available.
With Q-page addressing, you cannot use station numbers 108 to 123. Only
the station numbers from 1 to 107 are available.
If you use pages to address an output, you can no longer address the out-
put via the FB IM308C. The IM 308-C does not recognize the attempt to
access this output via the FB IM308C.
If you use an S5-135U programmable controller, you must always store
the page number in the bit memory address area for a programmable con-
troller restart.
Additional programming: (write page selection address and only then the
I/O byte as such)
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