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 at classiccmp.org> wrote:
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...