Came across this discussion looking for what sounds like a very similar problem on one of my S7-1500F's.

Did a cold start on the CPU after some work on a remote panel ET200SP (unsure if this is related in any way). CPU was stuck in a "connecting" state as previously described.

The other modules in the rack appeared to be going through some sort of loop. The power/diag lights would flash then come on as if it were booting, all IO lights would come on, then after a few moments all of the IO/power/diag lights would turn off then start over. No alarms/diag text on the CPU.

As I was standing there googling, it managed to get in to run mode by itself after maybe 5 min and is running now. No idea what it was stuck on. 

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Split from Display show "connecting" and it is keep in "connecting".