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CI 840A is ment for redundant aplications so u need two of these and eighrter TU846 or TU847. These terminal units have two PROFIBUS ports each, so u need redundant signal.
To get redundant PROFIBUs lines from AC 800F You need a modulr named RLM01. In case of using single AC 800F it looks something like this:
In case of redundant AC 800F it will be more like this:
To learn more on RLM 01 modules check the ABB web site.