PDP-11/10 repair started

Johnny Billquist bqt at update.uu.se
Mon Oct 5 10:44:42 CDT 2015

On 2015-10-05 17:24, Henk Gooijen wrote:
> -----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.
> Johnny
> =========
> 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
> mind!
> 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

Ok. I got the initial impression that you only had the CPU in. Thanks 
for the expanded info.

When you don't have any core memory, I wonder if you might need bus 
grants in those slots as well...? It's not as if they aren't a part of 
the Unibus... Memory sits on the Unibus, just like everything else, 
remember? Needs to check further if any special wiring are in place for 
those slots, though.


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