PDP-11/70 debugging advice

Josh Dersch derschjo at gmail.com
Sun Jan 31 22:19:13 CST 2021

On Sun, Jan 31, 2021 at 7:55 PM Josh Dersch <derschjo at gmail.com> wrote:

> On Sun, Jan 31, 2021 at 7:04 PM Fritz Mueller via cctalk <
> cctalk at classiccmp.org> wrote:
>> > Yeah.  I want to get the fans installed and then go triple-check all
>> the power signals and get the voltages dialed in nicely.  But then things
>> come out on extenders :).
>> Yup -- I'm surprised how picky my '45 is about +5 undervolt; it really
>> seems happiest with trimmed up to about 5.1 at the backplane.
>> Looks like E106 on the RAC (M8123) might be a good place to start
>> (drawing RACA, lower left.)
> I was just looking in chapter 4 of the processor manual to learn more
> about how the processor clocks are generated on the M8139 (TIG) board; on
> page II-4-2 (p. 136 in the PDF on Bitsavers) section 4.1.3 it says:
> "The third source of timing [the other two being the crystal clock and a
> diagnostic R/C network] is the manually-operated, single-step MAINT STPR
> switch S4, located on the maintenance card.  This switch is only enabled
> when maintenance card switches S2 and S3 are both set to 1."
> Section 4.2.3 confirms this:
> "The maintenance card S2 and S1 switches are both set to 1 to allow single
> timing pulses to be generated by MAINT STPR switch S4.... Removing the S2
> or S1 input conditions the MS EN flip-flop to be cleared."
> Well, what's interesting here is that on my system, switch S4 (MAINT STPR)
> steps the processor with switches S1 and S2 set to *any* configuration.
> Tried it with the other KM11 I have, same behavior.  This being the case, I
> wonder if the logic that selects the clock source is faulty, and is always
> selecting the MAINT STPR input.  This would definitely explain the behavior
> I'm seeing.  I hope the fans arrive tomorrow so I can start debugging this
> :).

Ah, and page II-6-20 (p. 178) indicates that when DCLO is asserted, it

"UBCE ROM INIT H - forces the ROM to ZAP.00 (200), and stops and clears the
Timing Generator and the Cache timing."

I suppose that this is the more likely explanation for the behavior here
and that the maintenance card behavior is a side-effect of this.  Time to
actually probe things, rather than speculating...

- Josh

> - Josh
>>     cheers,
>>       --FritzM.

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