Siemens SIMATIC S7-400H System Manual page 172

Using I/Os in S7–400H
13.2 Using single-channel, one-sided I/Os
Single-channel one-sided I/O and user program
In redundant system mode, the data read from one-sided components (such as digital
inputs) is transferred automatically to the second subsystem.
When the transfer is completed, the data read from the single-channel one-sided I/O is
available on both subsystems and can be evaluated in their identical user programs. For
data processing in the redundant system mode, it is therefore irrelevant whether the I/O is
connected to the master or to the standby CPU.
In standalone mode, access to one-sided I/O assigned to the partner subsystem is not
possible. This must be considered as follows when programming: Assign functions to the
single-channel one-sided I/O that can only be executed conditionally. This ensures that
specific I/O access functions are only called in redundant system mode and when the
relevant subsystem is in standalone mode.
NOTICE
The user program also has to update the process image for single-channel, one-sided
output modules when the system is in standalone mode (direct access, for example). If you
use process image partitions, the user program must update them (SFC 27 "UPDAT_PO")
in OB 72 (recovery of redundancy). The system would otherwise first output old values on
the single-channel one-sided output modules of the standby CPU when the system
changes to redundant mode.
Failure of the single-channel one-sided I/O
The fault-tolerant system with single-channel one-sided I/O responds to errors just like a
standard S7-400 system.
● The I/O is no longer available after it fails.
● If the subsystem to which the I/O is connected fails, the entire process I/O of this
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subsystem is no longer available.
System Manual, 03/2012, A5E00267695-11
S7-400H