PDP-11/34a hung bus problem.

E. Groenenberg ed at groenenberg.net
Sun May 15 01:23:39 CDT 2016

On Sat, May 14, 2016 23:41, Noel Chiappa wrote:
>     > From Ed Groenenberg
>     > - insert both CPU cards, KY11-LB card & bootstrap card, 5 full grant
>     > cards, DL11-W  and bus terminator card.
>     > ...
>     > - cntrl + boot shows register dump at printer.
>     > ...
>     > All looks ok
> I'm surprised the bootstrap ran OK with no memory at all in the machine. I
> vaguely STR that I had a machine that would not work like that, but maybe
> I'm
> wrong. (DEC bootstaps tend to do things like set the NXM vector, in low
> memory, so they can size memory; and when it gets the NXM (since there is
> no
> memory) from trying to touch the NXM vector, and tries to push the old PS
> and
> PC to service _that_, and gets _another_ NXM, that 'double bus fault'
> often
> causes many -11 processors to do a cheap suit)

Could that be done on later models? (i.e. 11/24 and up?)
>     > power down machine, add memory (M7981, 128KW)
> What's an M7981? Did you mean an M7891 MS11-L?

Yes, a typo,  it is a M7891.

>     > - power up machine -> run light is on, does not get cleared by
>     > cntrl + halt.
> This is where a UA11 would really help. I had similar issues with an
> -11/04,
> and the UA11 was a huge help in figuring out what's going on. One glance
> and
> you can see if a bus line is wedged, or something.

Ah, that reminds me, I have an original DEC Unibus analyzer, which is
one of those small suitcase sized boxes and has a display for all
bus signals. Stupid that I didn't think about that.
>     > - power down & replace memory with grand card -> run light is off.
> Well, that's good sign - the memory card didn't fry anything, at least...
>     > - tried a 2nd memory card (M8722, 128KW)
> Ooops. The MS11-M needs +/-12V, which is _not_ standard in most
> machines/backplanes). The EUB in the 11/24 and 11/44 (which this card is
> intended for) does have it. The really bad part is that those same pins
> usually carry +/-15V in most MUD backplanes. So hopefully you didn't fry
> it.
> It does have standard UNIBUS as well as EUB, but there's a jumper, IIRC.

Ah, well it was not set for EBU mode, but ISTR that there is a jumper
which selects for +12 or +15 volt.

>     > So what could be the problem here? The bus works without the memory
> Two possibilities off the top of my head. i) The first memory card is bad
> (or
> configured incorrectly), or.. ii) The M7891 uses +/-15V as well as +5V? So
> maybe one of the other voltages is not so good? But you said the console
> worked, and I think that uses other voltages (at least, in EIA mode - not
> sure about 20mA, I never touch the stuff).

Yes, both +5 and +15 is used and measure ok on the backplane.

> 	Noel

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