Siemens SIMATIC S7-400H System Manual page 122

Using I/Os in S7-400H
10.3 Using single-channel, one-sided I/Os
Single-channel, one-sided I/O and the user program
When the system is in redundant 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 state, it is irrelevant whether the I/O is connected to
the master or to the standby CPU.
In single mode, access to one-sided I/O assigned to the partner subsystem is not possible.
Remember to take this into account in your program: Make sure that you only 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 the redundant system state, and
when the relevant subsystem is in single mode.
NOTICE
The user program also has to update the process image for single-channel, one-sided
output modules when the system is in single 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 initialize the single-channel
one-sided output modules of the standby CPU with the old values after the system change
to redundant mode.
Failure of the single-channel one-sided I/O
The fault-tolerant system with single-channel, one-sided I/O reacts to errors just like a
standard S7-400 system, in other words:
● 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, 09/2007, A5E00267695-03
S7-400H