General Question about UNIBUS backplanes

william degnan billdegnan at
Tue May 24 07:50:38 CDT 2016

There have been a lot of parallel discussions on my PDP 11/40, I am
consolidating them all here:

I now have 5 backplanes installed.  Here they are, described in order.  I
walked through this will Paul Anderson last night (thanks Paul!) to verify
everything was in the right place and in the right order, in the right
power, etc.  The +5V is a little weak but I can tweak it, etc.

1.  CPU backplane.  This is the same backplane I have had running/working
for 10 years.  There is no M2737 or M787 installed, just a stock CPU card
set.  No changes have been made.

2.  core backplane #1 32k x 16 - 18 Memory backplane 500344C.    This
backplane has one set of core cards installed in the order they came from
another system.  These are electrically ok, but not tested/known to pass
memory tests.  The M8293 is set to 0000
9/11: M981 (1-2)
11: M8293 (3-6)
12: M7259 (1-2)
13: G114
14: H217C
15: G235

The 2nd half of the 1st core backplane is empty.

3.  The 2nd core backplane is empty.  There are M920's on either end to
connect them

4.  The next backplane is a DD11B.  This is empty other than grant cards in
each slot D.

An M9202 connects the DD11B to the next backplane

5.  The final (5th) backplane is a DD11C

In the first slot (closest to the front) GC in slot 4.

In the 2nd there is a M7261 RT11 drive controller.  This slot requires a
NPG G7273 installed if not in use, the M7761 requires this kind of slot.
It's not hooked up to anything (RL02).

In the 3rd slot is an M9312 with console prom and RL01/02 boot prom.  Not a

In the 4th slot is a M930 on top with a G7273 in 3/4.  The G7273 is
required because previously the NPG was removed from wirewrap per
requirements for installation of M7762 RL11.  This was done before I
learned I could have just used slot 2nd, which comes from the factory that
way.  Bummer but that's what I've got.

WHEN I POWER UP - I have full console access, everything "works" and now I
have to start diagnosing the core to find at least 16K that I can play
with. With the current set of core cards  I can toggle in data into
addresses, but it's not perfect (line 20000 sticks).  Once I am operating
with a working set of core I will then turn my attention to the RT11 card,
and find a working serial card so I can use a terminal and PDP11GUI, etc.

I have spare core cards to draw upon, I had pulled a good set for my 11/05
I could always use those if need be.

Thanks everyone for your help and suggestions.


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