Siemens SIMATIC S7-400H System Manual page 152

Using I/Os in S7-400H
10.6 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 I/O 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 base address 8 and
● module B in rack 1 with logical base 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 access error has occurred, module B is read by direct access ("retry" 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.
The sample program is based on the fact that, following an access error on module A or its
replacement, module B is always processed first in OB 1. Module A is not processed first
again in OB 1 until an access error occurs on module B.
NOTICE
The MODA and IOAE_BIT variables must also be valid outside OB 1 and OB 122.
The ATTEMPT2 variable, however, is used only in OB 1.
152
– In a one-sided configuration, one I/O module per subsystem.
– In a switched configuration, one I/O module each of 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 initialize the single-channel
one-sided output modules of the standby CPU with the old values after the system change
to redundant mode.
System Manual, 09/2007, A5E00267695-03
S7-400H