Siemens SIMATIC S7-400H Manual page 83

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-sided 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.
Note
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 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.
Redundant I/O in the user program
The following example program shows the use of two redundant digital input
modules:
module A in rack 0 with logical base address 8 and
module B in rack 1 with logical base address 12.
One of the two modules is read directly in OB1. It is assumed for the following,
without limiting general applicability, that it is module A (the value of variable BGA
is TRUE). If no error occurred, processing continues with the value read.
If an I/O access error has occurred, module B will be read by direct access
("second attempt" in OB1). If no error has occurred, processing continues with the
value read by module B. However, if an error has similarly occurred in this
instance, both modules are currently defective, and work continues with a
substitute value.
The example program is based on the fact that following an access error to module
A, module B is always processed first in OB1 after the former has been replaced.
Module A is not processed first again in OB1 until an access error to module B
occurs.
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Using I/O on the S7-400H
6-11