PDP-11/05 (was: PDP-11/05 microcode dump?)

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Jun 14 15:39:20 CDT 2021

    > From: Tom Uban

    > it has the early version M7261E Control Logic & Microprogram board and
    > the later version M7260 Data Paths board

Ah, I'm glad someone found all that stuff I wrote up there useful. As always,
I _think_ I got it all transcribed correctly, but do be on the lookout for

    > it seems like an older/newer combination, but maybe that was common. I
    > would not have guessed that the four possible combinations would all
    > work together, but maybe they do?

I honestly don't know. As far as I can tell, the DEC documentation doesn't
even _mention_ the two different board generations; perhaps a sign that they
are functionally interchangeable? (Although even the section on baud rate, in
both DEC-11-HKDBB-A-D and EK-KD11B-MM-001, 4.11, doesn't even mention the
early board. So maybe the manual just ignores the earlier version completely?)

I don't have an /05 up and running at the moment, or I'd check all 4 and see
if they all work.

    > Presently, the machine sometimes runs relatively well and other times
    > it does not.

What are the failure symptoms? (It's almost certainly going to take a 'scope
to fix it; I expect you have one?)

I'd start by monitoring the CPU clock, and make sure it's running when the
failure happens. (Note that the front console is handled by the microcode, so
if the microcode isn't running, the machine will be totally dead.
EK-KD11B-MM-001 has a good description of how that works.)

    > my initial messing with KM11 boards, reveals that I can step the
    > microcode with a KM11 in either the #1 or #2 position, but when two
    > KM11s are installed at the same time, they do not function properly
    > together. Is this expected or do I have an issue there too?

Not sure. EK-KD11B-MM-001 (available at:


and definitely something you need) says, at pg. 5-6 "KM11 switches have the
same function in slots KM-1 and KM-2", and on 5-7 "permits the user who has
only one KM11 to plug into either KM-1 or KM-2".

So that _sounds_ like you should be able to plug two in together. The first
indicates that the switches, the only input to the KD11-B from the KM11, are
wired in parallel, and the only other thing on the KM11 are the lights,
outputs. And why mention "user who has only one KM11", if having two is no
use because one can't use two at once?


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