General Question about UNIBUS backplanes

Mattis Lind mattislind at
Tue May 24 08:32:19 CDT 2016

2016-05-24 14:50 GMT+02:00 william degnan <billdegnan at>:

> 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
> terminator
> 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.
> Bill

But the M9312 has bus terminating resistors (unless you actually desoldered
those!). Then you have three terminators in your system! One in the M891 at
the CPU, one in the M9312 in slot 3 of last DD11, and then a M930 in slot 4
of DD11.

If you want the M9312 you should put it with W1-W5 installed in the slot 4
where you have the M930. You should not have both a M930 and M9312 (when
you have the M891).


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