PDP-11/70 debugging advice

Josh Dersch derschjo at gmail.com
Fri Feb 5 12:02:05 CST 2021

On Fri, Feb 5, 2021 at 12:29 AM Fritz Mueller via cctalk <
cctalk at classiccmp.org> wrote:

> > On Feb 4, 2021, at 9:45 PM, Josh Dersch <derschjo at gmail.com> wrote:
> > The 11/70 is now somewhat operable.  I can examine and deposit memory,
> at least for a short while -- after the system warms up for 30 seconds or
> so the Load Address switch stops functioning.
> Nice!!
> One handy test on '45 is to put the data display knob to bus address; if
> the microcode is still running in the control panel idle loop (halt switch
> down or fault) the data lights will follow along with the toggles.  This is
> a super quick way to tell if the microcode is still running or if it is
> wedged.
> (The '45 and '70 microcode seem quite similar, so your '70 might have the
> same feature?  I haven't looked at the flow diagrams yet to know for sure.)

Thanks for the tip, I'll check that out.  Before I went to bed I did a bit
more poking.  Upon closer examination, the Load Address switch is still
being recognized:  the Data lights get reset to 0 after it's toggled.  So
the microcode is taking a jump to ADR.00 when the switch is toggled.  It
looks like what isn't happening is the PCA <- BR in T5.  So I have a couple
of places to look -- it's possible that the T5 clock signal is going wrong
after warming up (note that ADR.00 is the only step in the console flow
that does anything in T5, except maybe RDP.00 -- I'm not sure if that's a
"5" or a 3").  (See flow 14, p. 18 of
http://bitsavers.org/pdf/dec/pdp11/1170/MP0KB11-C0_1170engDrw_Nov75.pdf) Or
something's going wrong with the PCA <- BR operation.

I put the KM11 back in, and while single-stepping or running under the RC
clock (which is currently set to a really low clock rate, like 7Mhz, for
some reason) Load Address works fine.  Seems like some component has slowed
down a bit and stops operating correctly at full speed.

Something to poke at this weekend...

- Josh

>    --FritzM.

More information about the cctalk mailing list