Siemens SIMATIC S7-400H Manual page 166

Using I/O on the S7-400H
Notice
When using redundant I/O, an extra value might have to be added to the
calculated monitoring times; refer to Section 6.3.2.
Hardware installation and configuration of the redundant I/O
If you wish to use a redundant I/O, we would recommend you the following
strategy:
1. Use the I/O in the following manner:
– With a one-way configuration, one I/O module in each subsystem
– With a switched configuration, two I/O modules in two distributed ET 200M
I/O devices.
2. Wire the I/O in such a way that it can be addressed by both subsystems.
3. Configure the I/O modules for different logical addresses.
Notice
We do not recommend configuration of the output modules you use to the same
logical addresses as the input modules; if it is done nevertheless, you have to
query the type (input or output) of the defective group in OB 122, in addition to the
logical address.
The user program must update the process image for redundant one-way output
modules in single mode too (e.g. direct accesses). If subprocess images are used,
the user program must update the subprocess images in OB 72 (redundancy
return) accordingly (SFC 27 "UPDAT_PO"). If it did not, old values would initially
be read out to the single-channel, one-way output modules of the standby CPU
following transition to redundant system mode.
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Automation System S7-400H Fault-tolerant Systems
A5E00068197-07