
https://profinetuniversity.com/wp-content/uploads/2018/04/Device-Redunancy-with-Field-Wiring.png
https://profinetuniversity.com/wp-content/uploads/2018/04/Device-Redundancy-Failover-1024x471.png
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On the left is a normal redundant system, with a primary connection to a device with active I/O connections and a backup connection to a device with inactive I/O connections. The system on the right shows a failure of the original “primary” device, and the new primary device exchanging valid IO data.