UPS 9910 cables to 170

My brain cells that contain a crucial bit of information have gone on 
strike.
With an IBM 9910 (BEST) UPS connected to a model 170, the signal cables 
for UPS serial comm and CPM have a gray end and a black end.
On newer systems there was a pin on the end of the cables to prevent you 
installing them the wrong way around, but older ones didn't have this.  If 
you connected the cables the wrong way the 400 would shut down.

Question - does the gray end of the UPS signal/CPM cables go into the UPS 
(which I think is right), or does the gray end go into the 400 ?
If I was at the office I could look at my own system and see the answer, 
but I'm out at a customer site.

...Neil
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