Siemens SIMATIC S7-400H System Manual page 208

Using I/Os in S7–400H
13.5 Other options for connecting redundant I/Os
Hardware configuration and project engineering of the redundant I/O
Strategy recommended for use of redundant I/O:
1. Use the I/O as follows:
2. Wire the I/O in such a way that it can be addressed by both subsystems.
3. Configure the signal modules so that they have different logical addresses.
Redundant I/O in the user program
The sample program below shows the use of two redundant digital input modules:
● Module A in rack 0 with logical start address 8 and
● module B in rack 1 with logical start address 12.
One of the two modules is read in OB 1 by direct access. For the following it is generally
assumed that the module in question is A (value of variable MODA is TRUE). If no error
occurred, processing continues with the value read.
If an I/O area access error has occurred, module B is read by direct access ("second try" in
OB 1). If no error occurred, processing of module B continues with the value read. However,
if an error has also occurred here, both modules are currently defective, and operation
continues with a substitute value.
208
– in a one-sided configuration, one signal module in each subsystem
– in a switched configuration, one signal module each in two ET 200M distributed I/O
devices.
NOTICE
It is not advisable to configure the input and output modules with the same logical
addresses. Otherwise, in addition to the logical address, you will also need to query the
type (input or output) of the defective module in OB 122.
The user program also has to update the process image for redundant 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 first output old values on
the single-channel one-sided output modules of the reserve CPU when the system
changes to redundant mode.
System Manual, 03/2012, A5E00267695-11
S7-400H