PDP 11/24 - A Step Backwards

[Rob Jarratt]

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Noel Chiappa via
> cctalk
> Sent: 03 April 2022 03:41
> To: cctalk at classiccmp.org
> Cc: jncx at mercury.lcs.mit.edu
> Subject: RE: PDP 11/24 - A Step Backwards
> 
>     > It was quite a struggle to separate those nylon connectors, is there
a
>     > trick to it?
> 
> You mean the Mate-n-lok's? Not really; just make sure the catch is
released.
> 
> What did you do about DCLO? (Oh, I think I see the answer, below.... looks
> like you're relying on the pullup on K3...)
> 
>     > When I powered on, the CPU LEDs did not light up.
> 
> Two of them ('0' and '1') are just bits in a special register, and thus
only do
> anything when the bootstrap code fondles them. When you get ODT
> running, you can amuse yourself turning them off and on manually! :-)
> 
>     > I did notice that the CLK LED flickered on briefly when I powered it
>     > off.
> 
> Interesting. Not sure exactly what we can deduce from that; but
interesting.
> 
>     > I put a scope probe on TP1 (p152 of the PDF), there was no activity,
>     > the pin remained high.
> 
> Yes; the signal there (MCLK H) is more or less the same one that drives
the
> 'CLK' LED (MCLK L); so no big surprise there. Still, that reduces the
problem
> space to a small part of K1.
> 
>     > The problem now is that I expect I will need to probe various pins
to
>     > find out what is going on. But I don't have a Unibus extender and I
am
>     > reluctant to remove the backplane. From what I can tell in the
>     > Technical Manual you can't install the CPU in other slots
> 
> Basically right; the backplane and CPU are designed to have it go in slot
1.
> It _might_ work in other MUD slots, with some loss of functionality (e.g.
> slot 2 doesn't have grant lines; MUD slots won't have the 'UNIBUS Map
board
> pesent' line - pin FE1, on K11, UB TO MA VIA UBMAP) but I wouldn't want to
> chance it, there might be a clash.
> 
>     > I am forced to tack solder probe wires to the chips, which works but
is
>     > time consuming. Any other ways?
> 
> Sorry, I don't have any experience to suggest any; too well supplied with
> extender cards, so I've never had to resort to alternatives!
> 
> 
>     > I *think* I have found something. There could be a fault in E52
(sheet
>     > K6, p157 of the PDF). While K6 BUS DCLO L is +5V, I am measuring K6
BUF
>     > DCLO H at an average 1.64V
> 
> Yeah, that's wrong. E52 is bad, and will have to be replaced.
> 
> (From the +5V on BUS DCLO, I guess you're relying on the pullup? DCLO on
> the UNIBUS, with the resistor network on the M9302, should be about 3.5V -
> but now I'm confused, even with the P/S connector unplugged, it should
still
> be 3.5V or so. Oh well, it's late, the brain is powering down... :-)

Sorry if I wasn't clear, I did plug the connector back in, so that DCLO and
LTC are connected, I just removed the ACLO pin.

> 
> The 'unused' gate in E52 is the one that the added wires from the ACLO ECO
> went to; I wonder if it was damaged by the -15V, somehow?

Oh, I forgot about that! That would seem highly likely.

> 
>     > logical 0 output should be 0.4V max
> 
> Which is what you should be seeing.
> 
>     > I also measured K6 BUF DCLO L to be always low, suggesting it thinks
>     > the K6 BUF DCLO H is a logical 1.
> 
> Yup; and that definitely explains why the clock isn't running - BUF DCLO L
is
> clearing E41 on K1.
> 
> Anyway, you'll have to replace E52 (which will be a bit of a pain, with
the 3
> ECO eires tacked to it). The DS8641 is an old chip, no longer in
production, so
> the usual suppliers may not have it, but there are some on eBait.


I didn't look for replacements last night. Is there a modern equivalent? My
initial searches show there isn't but I may have found a source of NOS.

Worried about replacing it with the ECO wires as you say, I always marvel at
how neatly those wires are done, I wish I knew how to do such a neat job.

Thanks

Rob

> 
> 	Noel