PDP 11/24 - A Step Backwards

Brent Hilpert bhilpert at shaw.ca
Thu Mar 31 15:02:45 CDT 2022

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_Aug80.pdf

>> 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.

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 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.

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.

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