Siemens SIMATIC PCS 7 Function Manual page 25

Availability of a redundancy node despite faults
If a component in a redundancy node fails, the overall system continues to operate.
Total failure of a redundancy node
The following figure shows a complete system that has ceased to operate due to a failure of
the "Field bus (PROFIBUS DP)" redundancy node.
Fault-tolerant Process Control Systems (V8.0)
Function Manual, 03/2012, A5E02779471-02
Basics of Fault Tolerance
2.10 Redundancy nodes
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