Siemens SIMATIC S7-400H Manual page 76

Using I/O on the S7-400H
Single-channel, one-sided I/Os and user program
Information read in on one side – for example, from digital inputs – is transferred
automatically to the second subsystem via the synchronization link in the
Redundant system mode.
After the information has been transferred, both subsystems have the data from
the single-channel, one-sided I/O and evaluate them in the two identical user
programs that are present. It is therefore not decisive whether the I/O is connected
to the master CPU or to the standby CPU as far as processing of the information in
the Redundant system mode is concerned.
In Solo mode, the one-sided I/O assigned to the cooperating subsystem cannot be
accessed. This has to be taken into account in your programming as follows: you
have to assign functions to the single-channel, one-sided I/O that can only be
performed conditionally. In this way you make sure that certain functions for I/O
accesses are triggered only in the Redundant system mode and in Solo mode of
the subsystem concerned.
Note
The user program has to update the process image for single-channel, one-sided
output modules in Solo mode too (e.g. direct accesses). If using subprocess
imagers the user program must update the subprocess imager 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-sided output modules of the
standby CPU following transition to the Redundant system mode.
Failure of the single-channel, one-sided I/O
In the event of a malfunction the S7-400H with a single-channel, one-sided I/O
behaves like a standard S7-400 system, in other words:
When the I/O fails, the defective I/O is no longer available.
When a subsystem fails, the entire process I/O of that subsystem is not
available any more.
6-4
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04