ra81's are irking me

After swapping out 3 HDA's and 2 servo boards, I had one RA81 working - mostly. For some reason the system wouldn't see it on port A, but it would on port B. Oddness. Anyways, since the day I had it working (and got RT11 loaded on it), the system will no longer see it. I have moved around and changed some of the cabling between the drive cabinet and the cpu cabinet, but I don't see that I've done anything wrong. I put the cabling back the way it was (I could be mistaken) and still no joy. During the basic UDA50 diag it says there are no drives connected (same thing for >>>B DU0). I'm still plowing through the docs (again), but can anyone suggest anything "obvious" that isn't clear in the docs with regards to connecting an RA81 to a UDA 50 that a newbie like me could be missing? I'm up for any input :) Jay West