PDP-11/20 in Iowa (x3)

Ethan Dicks ethan.dicks at gmail.com
Mon Mar 27 08:15:27 CDT 2017

On Mon, Mar 27, 2017 at 5:32 AM, Philipp Hachtmann via cctalk
<cctalk at classiccmp.org> wrote:
> On 03/27/2017 08:05 AM, Paul Birkel via cctalk wrote:
>> http://www.vcfed.org/forum/showthread.php?56897-Came-across-three-PDP-11-20-
>> units-today

Nice find, even if they are bit rough.

> Haha, those low-brain comments!
> User g4ugm: "have core memory... lower value"

Indeed... I'm still kind of annoyed that the 11/20 I rescued from the
dumpster at work in 1984 got mildly "souveniered" before I got it - at
least one core plane went to another employee who wanted a

Was there ever a quad MOS memory board for the 11/20?  It would have
to be 4K or less, I'd expect, given the era and size of RAM.  I know
by a few years later, with the 11/04, MOS was becoming common.
Obviously, despite the lack of supported examples in the configuration
guide, one could put some RAM in a DD11-CK or DD11-DK and hang it off
of the CPU chassis (as long as module order and grant priority isn't
violated) but I wonder if anyone _did_ that.  I would think the cost
of a 4K or larger MOS board and the relative cost of PDP-11s in
general in the mid-1970s, it would have been better to have just
retired the 11/20 and gone with a single-cabinet machine like an 11/04
or 11/34, so it probably would have been some college lab experiment
to use parts on hand rather than a commercial endeavor.  Obviously,
it's easy enough to make a modern quad-height Unibus memory board and
bypass debugging core problems, but that's back to the functional vs
"authentic" argument (I prefer authentic except where impractical or
unobtainable, like trying to find an RF11/RS11).

> The 11/20 without "/20" on the front panel is *THE* pdp11. The first one. I
> consider it more valuable than $200-300. Even rusty.

Right.  I'm sad mine says "/20" but only a little.  The real trick is
going to be buying a crate of Boxer fans and finding enough power to
run this thing.  When it was dumpstered, my old boss stripped the fans
and PSUs and tossed the rest out (which I saved).  So I have 3 BA11
boxes (including the intact front panel), all the backplanes, nearly
all the boards, but in the form of a very large and very heavy jigsaw
puzzle.  I also saved what C-sized prints I could find, and the rest
appear to be on Bitsavers.

I'd be interested in talking with whoever ends up with this haul (or
with anyone else who has an 11/20, really) to get some specific
pictures of the power wiring and fans so I can put mine back together.
Also, if anyone knows where to get a crate of 110V Boxer fans for
around $5/each, let me know (I see them on a regular basis for
$15-$20/unit, but I need at least 9).  Used at that price is fine, or
else I'll just have to pay full tick for new.


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