Siemens SIMATIC PCS 7 System Manual page 286

Supplementary information
16.14 Other options for connecting redundant I/Os
The sample program is based on the fact that following an access error on module A and 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.
Note
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.
Figure 16-26 Flow chart for OB 1
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CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 09/2014, A5E32631667-AB