PDP 11/24 - A Step Backwards

Wayne S wayne.sudol at hotmail.com
Thu Mar 31 16:49:20 CDT 2022

A suggestion. Can you sub in the aclo and dclo voltages from a bench power supply and see what happens after disconnecting the true ac/dc lo from connecyor of course.

Sent from my iPhone

> On Mar 31, 2022, at 14:15, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:
> I made some interesting discoveries this evening. See below.
>> -----Original Message-----
>> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Brent Hilpert
> via
>> cctalk
>> Sent: 31 March 2022 21:03
>> To: General Discussion: On-Topic and Off-Topic Posts
>> <cctalk at classiccmp.org>
>> Subject: Re: PDP 11/24 - A Step Backwards
>> On 2022-Mar-31, at 12:36 AM, Noel Chiappa via cctalk wrote:
>>>> From: Tony Duell
>>>> A short in FET Q15 on the bias/interface board in the PSU could do it.
>>>> The gate of that FET is driven from an LM339 comparator the -ve
>>>> supply of which is -15V.
>>> Ah; I hadn't even looked at the P/S prints.
>>> (Like I said, I'm really weak on analog: for digital, I have the
>>> advantages that i) although I'm basically/mostly a software person,
>>> the MIT CS department is part of the EE department, and they made sure
>>> that all the CS people had a decent grounding in the fundamentals of
>>> digital hardware; and
>>> ii) in my early years, I was involved in a number of actual hardware
>>> projects, including a UNIBUS DMA network interface that tuned into an
>>> actual product. So I'm pretty good with a digital circuit diagram,
>>> like these CPU prints. But analog stuff is still a mostly-closed book
>>> to me! :-)
>>> Anyway, I'm happy to let you provide the analysis of the P/S... :-)
>>>> From: Rob Jarratt
>>>> [Perhaps] something else on the CPU caused Q15 to fail (if indeed it
>>>> did).
>>> I'd guess 'unlikely' (if Q15 has failed); UNIBUS ACLO is connected, on
>>> the CPU card, to only a single gate (on K2), and that 383 ohm pull-up
>>> (on K3), and the 1K pF cap there (the purpose of which I still don't
>>> understand, unless it's just a smoother). Although I suppose that if
>>> that cap failed, shorted, maybe that could have taken out Q15 somehow.
>> Note: It's Q14 that controls ACLO, not Q15, Q15 is involved in the +5
> startup.
>> Unless there are two versions of the schematic and I'm looking at a
> different
>> one than everyone else.
>>    pdfPg.30 of
>> http://www.bitsavers.org/pdf/dec/pdp11/1124/MP01018_1124schem_Aug8
>> 0.pdf
> Thanks for pointing that out, I had not noticed the other line going down to
> Q13 and Q14.
>>>> Perhaps I should ... and disconnect ACLO, DCLO and LTC, they are all
>>>> on the same connector
>>> Now why didn't I think of just un-plugging that whole connector!
>>> Duhhhh! My only concern would be leaving inputs floating...
>>> DCLO, no problem; it has that pull-up on K3. (Ditto for ACLO, if the
>>> buffering input gate isn't dead.) LTC, let's see... It's on K6, upper
>>> left corner. I'm too lazy to work out what leaving that input floating
>>> will do, and, if it has bad consequences, trace out all the places it
>>> goes (it should be connected up to cause an interrupt, somewhere), but
>>> there's no point; the KW11 has an 'interrupt enable' that has to be
>>> set by software before it can do anything; so at the moment it's safe
>>> to just ignore it for now, and stay with a focus on getting the main
>>> CPU clock running. (LTC is not on the UNIBUS, so there's no pull-up on
>>> the M9302 for it the way there is for ACLO & DCLO.)
>>> So unplug that connector, and see if E70 (on K2, lower right corner) is
> OK.
>>> (Remember, the pull-up will give it an Ok input with BUS ACLO
>>> disconnected.) If yes, great, go check the main CPU clock.
>> Removing DCLO and ACLO from the PS to the bus may allow the CPU/clock to
>> work. Or it may not.
> Well I can tell you it didn't, disconnecting those connectors left the CPU
> still not doing anything. However, there is a puzzle. On the CPU I found
> that the track from the pull up resistor to E70 has been cut. This would
> suggest that E70 pin 2 is floating, which I think means that K2 BUF ACLO H
> is also floating (I haven't put a probe on it as yet). But as the cut is
> deliberate, there must be a reason. The CPU did work for a while when I
> first got the machine. K2 BUS ACLO L however has been patched to E52 pin 4,
> which is the output of a gate on sheet K6. Can't say I understand why.
> However, for whatever reason it would seem that perhaps the ACLO signal from
> the PSU has always been considered bad?
>> DCLO & ACLO behave as power-on-reset signals to the system. If they are
>> allowed to just float up as the power supply comes up you have no
>> guarantees as to the end result ('end' meaning the state of things after
> the
>> power supply has come up), without doing an analysis of the pertinent
> logic
>> under their control.
>> JFETs are being used as the ACLO/DCLO control devices for a reason. In
>> contrast to bipolars, the normal/no-gate-voltage state of a JFET is
> Source-
>> Drain conducting, thus the initial state at power-up of ACLO-L & DCLO-L
> will
>> be 0V/low-impedance-to-GND. The point is to maintain that state until the
>> power supply levels are good so the logic can be forced into a known
> state.
>> Those three comparators in the H777 are looking at a time-delay ramp
> Is that a typo? This is the H7140 not the H777.
>> generated by C14 and the constant-current circuit of Q11.
>> What is supposed to happen:
>>    - everything is initially 0V: V+5, ACLO, DCLO.
>>    - power is switched on. Internal voltage levels begin to rise.
>>    - after some delay, E4 trips first to start the +5 supply.
>>    - after some more delay, E5 trips, de-asserting DCLO (DCLO =
>> High,+V).
>>    - after some more delay, E6 trips, de-asserting ACLO (ACLO =
>> High,+V).
>> The delays are presumably of some order of mS.
>> -15V is the expected level from the E6 comparator output if AC is good. A
>> Gate-Drain short in Q14 would be allowing that out to the bus. JFETs can
> be
>> flaky, a failed JFET wouldn't be a big surprise.
>> So E6.6 = Q14.G = -15V is expected after power-up but an additional
> concern
>> would be that a G-D short allowed excessive current from the bus through
>> the E6 comparator output and damaged E6, or if left on too long burned out
>> pull-up resistors on the CPU or bus terminator. However the LM301 is
>> supposed to have current limiting so those things may not have been
>> damaged.
> I am a bit confused here. E6 is an opto-coupled isolator. Do you mean E9? It
> doesn't look to me like Q14 is connected to anything that outputs -15V, but
> Q15 is. A short in Q15 would surely be more plausible as the reason for -15V
> on ACLO?
>> The scope could be used to observe what is going on with the +5, DCLO,
>> ACLO sequencing at power up (with bus pull-ups, but without CPU).
>> Removing only the ACLO PS-to-bus connection would allow DCLO to still
>> exercise it's proper POR control. Once bus-ACLO is disconnected from the
> PS,
>> look for the clock LED after powering up with both bus-ACLO open (pulled
>> high by bus & CPU) and bus-ACLO connected to GND. Manually
>> connecting/disconnecting bus-ACLO to GND after power-up will trigger the
>> CPU power-fail shutdown and disable the clock.
> Given that ACLO is not actually connected to E70 and DCLO seems to be
> operating correctly, I would think that disconnecting the connector is not
> likely to help in this case.
> I suspect I am going to have to check for shorts on the backplane and probe
> the CPU module when it is powered.
> Thanks
> Rob

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