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

Josh Dersch derschjo at gmail.com
Tue Feb 16 13:08:28 CST 2021

On Tue, Feb 16, 2021 at 2:36 AM Fritz Mueller via cctalk <
cctalk at classiccmp.org> wrote:

> > On Feb 16, 2021, at 1:13 AM, Josh Dersch <derschjo at gmail.com> wrote:
> > My money's on t5 going wrong -- an ALU input mux or operation being
> selected incorrectly, possibly.  This also somewhat explains why execution
> doesn't trap due to executing a HALT from an odd address -- it isn't
> actually executing from the wrong address, because the bus address is
> loaded from a still-good PCB in t1.
> Yes -- that seems to match the observations.  So you could set up on t4 or
> t5 of that microinstruction with the KM11 in single-clock-phase mode, and
> then have at the ALU with a logic probe to check...

I can't, though -- all of this stuff works fine when running slowly :).
It's possible that while running in single-clock mode I might be able to
see a waveform that's slightly off on the 'scope, if I can pinpoint the

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).

> There is a small ALU control ROM on the GRA (E74 in the upper left of
> sheet GRAA), which selects the ALU mode and operation based on lines coming
> over from the IRC.  I had a failure of this part on my 11/45, and ended up
> with incorrect ALU setup in some circumstances.  That part is a 256-bit
> DM8598 tri-state bipolar mask ROM, and the truth table is on sheet GRAK.
> Looking back at my notes, I think it was Glen who informed me (on this
> list, in 2016!) that the Signetics 82S123 PROM could be substituted here.
> Programmed one up, and it worked, in case you run into the same thing and
> it is useful info.

I think I may have ended up doing a similar substitution with a microcode
ROM on my 11/05.  Have the 11/70 microcode PROMs been dumped?  I had to
recreate the 11/05 PROM from the listings in the engineering drawings...

- Josh

>     cheers,
>       --FritzM.

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