PDP-11/70 progress (and a cry for help)

Josh Dersch derschjo at gmail.com
Wed Feb 17 02:45:51 CST 2021

On Tue, Feb 16, 2021 at 5:08 PM Fritz Mueller via cctalk <
cctalk at classiccmp.org> wrote:

> > On Tue, Feb 16, 2021 at 2:36 AM Fritz Mueller via cctalk <
> cctalk at classiccmp.org> wrote:
> > So you could set up on t4 or t5 of that microinstruction with the KM11...
> > On Feb 16, 2021, at 11:08 AM, Josh Dersch <derschjo at gmail.com> wrote:
> > I can't, though -- all of this stuff works fine when running slowly :)
> Oh right -- I keep forgetting that part!  So that really does leave you
> with just the LA to catch things in the act I guess.
> > Right now I'm thinking it is most likely the AMX selecting the wrong
> input (I'm guessing that BMX is correctly selecting KOMX, to get the
> constant "2" for the add operation).
> I agree; seems a good place to look next.

I hooked the LA up to the two PROM bits that select the AMX input (RACC
UAMX00 H and RACC UAMX01 H), on the ROM & Address Control board.  These
come from the PROM at U101 and go through a 74S174 at E97 before heading
over to the DAP board.  And during FET.00 we have:

260  44  44  0

"0" here selects DR (Destination Register) input to the mux and is
incorrect; it should be 1 (PCB).  During a single-instruction-step run,
this value reads out OK on the analyzer.  I noted a few other discrepancies
in the capture (all of which match the ucode listing during a
single-instruction step) which makes me think that the high outputs of the
PROM are right on the bleeding-edge of acceptable TTL.  I checked out the
signal on the scope while running a BR .-1 instruction (which also doesn't
execute correctly but at least doesn't halt... I don't have a storage scope
to capture this during a single instruction execution) and it looks like
the voltage swing is from about 0.15V to 1.7V or so.

On the off chance it was the 74S174 at E97 pulling the signals down, I
socketed it and substituted a spare '174 in; no change.  I also noted that
the +5V at this chip was about 4.95V, I goosed it up to 5.10V to see if it
made a difference (it did not.)

So it seems likely it's the PROM.  Looks like I may have some typing to do,
though given that the ROM works well enough at slow speeds I might be able
to dump it with my Data I/O Model 29 and compare it against the listings in
the engineering drawings, to save some time...

I'll try to dump the PROM tomorrow and see what I get.

- Josh

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