PDP-11/70 backplane wire list

Josh Dersch derschjo at gmail.com
Sat Jan 2 00:33:35 CST 2021

On Fri, Jan 1, 2021 at 10:06 PM Eric Smith <spacewar at gmail.com> wrote:

> On Fri, Jan 1, 2021 at 10:04 PM Josh Dersch via cctalk <
> cctalk at classiccmp.org> wrote:
>> Discovered a broken wirewrap wire on the 11/70 I'm slowly working on, it's
>> on the last slot (44), and runs from BD2 to ??.  White wire, part of a
>> white/black twisted pair.  I've been looking but haven't found a wire list
>> for the backplane
> There would be two different 11/70 backplane wirelists, depending on
> whether your CPU is a KB11-B or a KB11-C. Unfortunately I don't have either
> wirelist.

> If your 11/70 has an M8133 and an M8138, it's a KB11-B.
> If it has an MB8123 and MB8138-YA, it's a KB11-C.

I have a KB11-C here.

> You're undoubtedly aware that the A/B section of slot 44 is for the Unibus
> cable or terminator, so BD2 is signal BR4 L. It should probably go to the
> BR4 L pin of the slot 44 SPC, which would be pin DH2, which would be bussed
> through DH2 of the other SPC slots (40-43).

You know, that really should have occurred to me.  Convenient that the
break happened on such a convenient pin!

Thanks for the tip, I'll double-check everything tomorrow and see if I can
find my wire-wrap tool...

- Josh

> RH70 Massbus controllers normally use BR5 L, but I suspect that the BR4 L
> line would be wired to their slots as well. I don't have an RH70 print set,
> so I don't know the details. From the RH70s (or slot 40, if BR4 L doesn't
> go to the RH70s), I would expect it to go to the Trap & Msc Control board
> (TMCB) in slot 11, pin FP1.
> Best regards,
> Eric

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