PDP-11/10 repair started

Henk Gooijen henk.gooijen at hotmail.com
Mon Oct 5 10:24:37 CDT 2015

-----Oorspronkelijk bericht----- 
From: Johnny Billquist
Sent: Monday, October 05, 2015 4:53 PM
To: cctalk at classiccmp.org
Subject: Re: PDP-11/10 repair started

On 2015-10-05 13:50, tony duell wrote:
> The 11/10 generally uses an M930 terminator.

Right. However, the SACK signal exists, even on the 11/10.
And if that signal is floating, or non-working, you can still have
problems. So, termination is still important.
The additional feature of of the M9302 is just to catch if BG signals
are sent out when noone was actually requesting an interrupt.

So your comment about open grant chain locking is relevant, but do not
negate the claim that you need to make sure you have terminators in
place, and that signals are carried through the whole backplane.

Note that my original comment was not about BG/BR continuity as such,
but termination and signals running through the whole bus. BG/BR is just
one part of that.

>>> But you do need some kind of termination/pullups on the Unibus. At least
>>> fit the CPU-end terminator (M930 in the right slot near the CPU boards) 
>>> if
>>> you haven't done so already.
>> You most likely want to terminate the other end as well.
> It may not be a perfect electrical match, but if all you have is the CPU 
> backplane,
> or even a BA11-K full of backplanes, I am certain a terminator at the CPU 
> end only
> will get the machine doing something, even running programs correctly. 
> Totally
> ignoring the front panel is not caused by a missing far-end terminator on 
> such a
> small machine.

The 11/10 do not have a built-in terminator at the CPU end. You are
supposed to have two M930 on the backplane.


Sorry for the delayed answer, I don't have email available at work -:/

I have one M930 in slot 3 position A-B, because that is the termination
for the processor. I am pretty sure (not 100%) that I also have an M930
in slot 9 position A-B. To be complete, this is the current state.
slot 1 A-F : M7260
slot 2 A-F : M7261
slot 3 A-B : M930   C-D : G7273
slot 4 A-B : M9970  C-D : G7273
slot 5 C-D : G7273
slot 6 empty
slot 7 empty
slot 8 empty
slot 9 A-B : M930

Slot 6-7-8-9 is for core memory, respectively G235, H217D, G114, M8293.
The documentation says that a G727 should go in slot 3-4-5 position D.
I hate those "knuckle-busters". Plus, I am lazy to check NPR continuity
on the backplane, so I installed G7273s instead of G727s. Always good.

I totally forgot that the GT40 is based on the 11/05. Great tip in case
it turns out that I have a "different" CPU module! You never know ...

When I got the 11/10, all boards were in their correct location, so after
the goof up of wrong placement, somebody more knowledgeable placed the
boards in their correct slots. I guess he hoped to see a working system.
So I do not know which board was place where :-/  That makes guessing
what might be damaged a bit tricky, although in/outs that go to fingers
of the board are first candidate for inspection. A visual check did not
reveal anything abviously blown or burnt.

@Bill : keep us posted of your progress :-)
@Johnny : I do not think an NPR problem exists, unless it is there,
because of damaged hardware.
@Jon : After I checked the correct levels of all power suplly voltages,
I also tried the system with slot 6-7-8-9 filled with the core memory.
Behavior did not change. Again, I am pretty sure NPR is not the problem.
@Noell : thanks for mentioning the possible versions!  Good to keep in

As Tony says, the M930 terminator close to the CPU (slot 3) should be
enough, especially as all other slots that matter are empty. But I am
pretty sure that for 100% correctness I also have one M930 in slot 9.

Thanks for all support, I hope to continue on Saturday ...
- Henk

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