Serial Number S-686
The 11/20, along with its identical OEM version, the 11/15, was the first pdp-11. Introduced in 1970. It was, I believe, the only 11 not to make use of microcode.
Note on the front it just says 'pdp11' - no suffix. Just like the very earliest Macintoshes said 'Macintosh' as opposed to 'Mac 128k' or whatever came next... this marks it as a very early unit; later units said 'pdp11/20' to differentiate them from other pdp-11s. When this machine was sold, there *were* no other pdp-11 models to differentiate it from!
The CPU. Like the pdp-8/L, the backplanes are at the *top* of the chassis, with the cards hanging down. There must have been a reason... answers on a postcard please!
The first three system units are taken up with the multiple boards of the KA11 CPU. The empty unit is a standard DD11 Unibus peripheral backplane.
The observant amongst you will have noticed a minor detail... no memory! I got this system from Swan, a UK DEC broker, back in the mid-90s - it was in beautiful condition, but had no memory. Still haven't found any... note in the above photo how the fans intrude upon the sixth slot of the Unibus... yes, in this early design the sixth slot is only used for the power connectors. It is *impossible* to mount any hex-width Unibus modules in this chassis!
Looking down from the top... the power connectors for additional system units are visible, still taped-up from the factory... I'm presuming this thing at one point had an external memory box.
Despite the lack of memory it works perfectly (!) - I can power it up, step through addresses, and examine & deposit data into the registers. Anyone know a good little pdp-11 'blinkenlights' program that can run in just the registers? Email me!
http://www.village.org/pdp11/faq.pages/pricing.pdp11-20-aug-1971.html The 1971 pdp-11/20 pricelist makes interesting reading...
http://webpages.charter.net/thecomputercollection/pdp11/ Jay Jaeger has a couple of these...
http://telnet.hu/hamster/pdp-11/1120.html in Hungary has one, plus a rare 11/15...
http://www.ps8computing.co.uk/PDP11/11_20.htm Kevin Murrell has one, with an excellent webpage...
http://www.parse.com/~museum/pdp11/index.html Robert Krten has the extraordinarily rare ruggedised pdp-11/R20!
http://www.bolo.ch/?list=2&obj=484 These Swiss folks have the only green one I've ever seen! (and other cool stuff, including an IBM System/3).