PDP-11/70 debugging advice

Josh Dersch derschjo at gmail.com
Sun Jan 31 21:55:07 CST 2021

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

- Josh

>     cheers,
>       --FritzM.

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