Redundant I/O In The User Program - Siemens SIMATIC S7-400H Manual

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.
Notice
Variables BGA and PZF_BIT must also be valid outside OB1 and OB122. The
variable VERSUCH2, on the other hand, is used only in OB1.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Using I/O on the S7-400H
7-37