Terry…
I saw messages about hooking up a battery to the LSI/11, and I happen to
have a DEC microcomputer processor handbook, circa 1978, that has some info
which may be of use to you.
Backplane pin AS1 is +12v Battery Power
Backplane pin AV1 is +5v Battery Power
Only certain devices use these battery connections.
AM1, BM1, AT1, BT1, AJ1, BJ1, AC2 & BC2 are all shown as
"SYSTEM SIGNAL GROUND – DC RETURN"
I just finished a course where we used the antique hardware for
lab exercises, so I've been using the handbook (#P03-1117005-00)
Hope this info is useful to you.