7.2 Automation system
7.2.1 Failure of the master CPU
Functionality
The initial situation is that the S7-400H is in "Redundant" system mode. The execution of the
user program is synchronized on both CPUs of the H system and, for example, CPU0 is the
master CPU and CPU1 is the backup CPU. Event-driven synchronization ensures that the
backup CPU always continues processing without interruption if the master CPU fails.
Example: Failure of the master CPU
If CPU0 fails, for example, the following LEDs light up on CPU1:
● REDF = Redundancy loss
● IFM1F = Interface fault Interface Module 1
This indicates the first fiber-optic cable of the synchronization line.
● IFM2F = Interface fault Interface Module 2
This indicates the second fiber-optic cable of the synchronization line.
The H system switches to "Solo" system mode. CPU1 ensures uninterrupted execution of the
user program. CPU1 is now the master CPU. The H system is no longer in "Redundant" system
mode.
Example: Reintegration of the failed master CPU
When the failed CPU0 is reintegrated, it does not become the master CPU. The master CPU
automatically performs the link-up and update of the reintegrated CPU0. Both processes are
necessary in order to check and synchronize the data in the memory of the master CPU and
the backup CPU. CPU0 then goes to RUN mode. Now the system is once again in "Redundant"
mode.
7.2.2 Failure of a fiber-optic cable
Requirements for the example
● The S7-400H is in "Redundant" system mode in the starting scenario.
● The CPU in Rack 0 is the master CPU and the CPU in Rack 1 is the backup CPU.
● The mode selectors of both CPUs are set to RUN.
Failure, switchover and reintegration of high availability components
7.2 Automation system
High Availability Process Control Systems (V9.0)
202 Function Manual, 05/2017, A5E39221836-AA
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