Siemens SIMATIC S7-400H System Manual page 209

Fault-tolerant systems

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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.

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 13-14 Flow chart for OB 1

System Manual, 03/2012, A5E00267695-11

13.5 Other options for connecting redundant I/Os

Using I/Os in S7–400H