PDP-11/40 modified to be a PDP-11/23

Ethan Dicks ethan.dicks at gmail.com
Wed Jun 22 19:35:54 CDT 2016

On Wed, Jun 22, 2016 at 8:28 PM, Michael Thompson
<michael.99.thompson at gmail.com> wrote:
> I looked at the backplane pictures that I took after the rescue. I assumed
> that the hex-wide 8-slot backplane in the front of the card cage was the
> original 11/40 processor backplane. On the back it says "LSI 11 BACKPLANE",
> so the operation is not so mysterious. I had never seen a hex-wide Q-bus
> backplane before this.

I have one... it came to me in an 11/34 with a lone IVB11 mounted in
it (Qbus IEEE-488) from a Physics Lab, and might have been marked on
the paper label DDV11CK.  It was all DEC, no Abel Qniverter, but I
can't recall what DEC module was on the Unibus to drive the Qbus (the
Qbus input paddle card was one of the ordinary ones used in a BA11N or

I never got the history, but presumably, some time in the late 1970s,
Ohio State ordered a PDP-11/34 with IEEE-488 and for whatever reason
(no IB11 available?  IB11 EOLed?), it came with an IBV11 and a bus
converter.  I'd like to dig that one out someday and try a Qbus SCSI
card on a Unibus machine.  It'll probably work if I can get a
bootstrap on the box.

> There are some pictures of the system and the Q-Bus to 11/40 front panel
> interface here: http://www.ricomputermuseum.org/Home/equipment/dec-pdp-1140


I've dug into the 11/70 front panel schematics, so I'm pretty sure
it's "easy" to get the Qbus to drive the LEDs, at least the address
and data LEDs, but I'm wondering about the switches and how many of
them work.


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