PDP 11/24 - A Step Backwards

Rob Jarratt robert.jarratt at ntlworld.com
Mon Mar 28 16:45:47 CDT 2022

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Chris Zach via
> cctalk
> Sent: 28 March 2022 20:57
> To: cctalk at classiccmp.org
> Subject: Re: PDP 11/24 - A Step Backwards
> > I don't think the CPU is working at all. The reason being that there
> > is absolutely no LED activity. Including an LED that is supposed to
> > indicate a clock. Having hopefully eliminated all the power voltages
> > it left me wondering if there was a fault on the CPU or in the PSU.
> > Having had activity on those LEDs recently it seems most likely that
> > it will be the PSU, particularly as *something* in there blew up. The
> > only signal that I can identify that seems likely to have this kind of
> > effect is LTC, but I don't know enough about LTC to know if its
> > absence could cause the CPU board not to work at all, although I see
> > above that you think it unlikely. I suppose the fault could be
> > something I can't see on the CPU board, particularly as there do seem
> > to be some quite large spikes, otherwise I am not sure if there is
> > anything else from the PSU that could prevent the CPU getting going.
> I'm on a nice long train trip right now but I recently got my 11/24 running
> again. One thing that baffled me was it would not do anything on the serial
> port. No ODT, no nothing.
> Turns out you really need to make sure the slots are filled properly.
> The CPU top, then the memory map, then for the next 4 boards one has to
> be either a properly configured MS11-PL (the 128kw board) or the memory
> boards specific to that type of 11/24. Or you have to put G7273's in the CD
> slots.

I have been reluctant to put everything back in, in case the PSU fries something. And the ripple I noticed is certainly something that bothers me. Previously I had a burning smell from the memory board. I have since replaced all the electrolytics on the memory board, but I have not tried putting it back in the machine since. Just checking my notes, it seems I have had *intermittent* lack of activity on the CPU LEDs before, so it may be worth trying to put everything back in, although the ripple makes me hesitant to do so. For the record, right now I have only the M7133, M7134 and G7273 installed.

> Next you need proper devices or G7273's in the next two slots and a proper
> terminator in the left sockets of the last slots and a G7273 in the center slots.
> Only then will ODT work.
> Another oddity is that the 5.25 inch box has +5 and +12 I think and the
> 10.5 has +5 and +15. There are different memory boards that work in one
> and not the other, or both depending on jumper settings that have to be
> right. Unibus drives me bonkers sometimes with the number of different
> voltages requires (+5, +12, +15, +20, -15, etc....) It doesn't help that the +15
> and +12 are on the same pins.
> Plus it's possible someone screwed with some switches, make sure they are
> set properly (ie: default is a good start).
> If you're still stuck next week drop me a line and I'll fire up my 11/24 and see
> if I can replicate your failure.
> >
> >>
> >> The first will tell you that i) the CPU is basically functional,
> >> executing
> > micro-
> >> instructions, etc; ii) that the bus is basically functioning (for
> > master-slave
> >> cycles; DMA and interrupts will remain to be checked out); iii) that
> >> the console port is working. (Yes, on the KDF11-U, the console is on
> >> an
> > internal
> >> bus, and so in theory a machine could have the ODT 'front panel'
> >> working, _and_ still have a problem with the bus, but depending on
> >> the exact details of said problem, maybe not.)
> >>
> >> So, hook up a console, set the machine to 'halt', and power on. Is
> >> console ODT working? If so, congrats, you win, go to stage ii) below.
> >
> > I had a console attached. There is nothing on the console. When I
> > first got the machine I did get output on the console. But that was
> > before the PSU first failed on me, which is quite a few years ago now.
> >
> >>
> >> If not, you have a reduced area in which you have to investigate -
> >> and
> > you'll
> >> need a 'scope of some kind to make any progress. (If you don't have
> >> one, you're SOL. Get one.). In order i) is the CPU's internal clock
> >> (and thus, probably the microcode) running? (At this point you will
> >> need to consult
> > the
> >> "PDP-11/24 System Technical Manual", EK-11024-TM.) If so, is it
> >> trying to
> > talk
> >> to the console's registers? (See Section 4.6 of the TM, "Internal
> >> Address
> >> Decode".) If so, is the UART working properly? (4.7 of the TM,
> >> "Serial
> > Line
> >> Units".)
> >>
> >> If so, console ODT is running, you're now at stage ii): you can see
> >> if the
> > CPU
> >> will run. Deposit a 0777 ('BR .') in a likely location (I usually use
> >> 0100 or 01000); read it back to make sure the write succeeded. (If
> >> not,
> > likely
> >> either the UNIBUS or the main memory has a problem.) Start the
> >> machine; the 'Run' light should come on - if you're lucky!
> >>
> >> Depending on which bin you wound up in, further assistance s available.
> > :-)
> >>
> >> 	  Noel
> >

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