Digital Equipment Corporation PDP-12 Laboratory Computer

    Digital Equipment Corporation's PDP-12 is an interesting machine in that it embodies the architectures of two separate machines in one design. Introduced in 1969 and withdrawn in 1973, fewer than a thousand were ever built. This machine is serial number 179. An image of it in operation is also available .

    In the early 1960s, a computer design was created for laboratory instrumentation and control called the LINC (Laboratory INstrumentation Computer); this design was adopted by DEC who, over a few years manufactured fewer than a hundred examples. Subsequently, Digital created a machine called the LINC-8 which incorporated the original LINC design and the venerable PDP-8 ISP. The PDP-12 is the direct lineal descendant of the LINC-8. Unlike the LINC-8, which had a full PDP-8 and a partial LINC processor controlled by the -8, the PDP-12 utilises a single processor that "understands" both instruction sets and toggles betwixt them using a "mode bit" (from the Jargon File ).

    As a laboratory computer, the PDP-12 was required to interface gracefully with the "real world". This was accomplished via analogue inputs (passed through A/D converters) and various outputs like relay switches and analogue voltage supplies. These features made the machine an extremely useful adjucnt in the lab, capable of controlling and monitoring various experiments.

    The example here is known as a PDP-12/30, or "Advanced LINC". It has 8 kw of memory (12 bits wide), a real-time clock, and the dual instruction sets. Standard equipment also includes a pair of TU-55s (functioning as LINCtapes) and a VR-14 oscilloscope with screen dimensions of six inches by nine inches capable of displaying dot- matrix characters. A bit of the CRT screen is just visible directly above the programmers' console; the white binding posts below, and to the left of the console, are the six relay outputs. A Teletype ASR-33 was standard equipment with this system.

    The PDP-12 is constructed using DEC's M- series logic (TTL ICs on "Flip Chip ®" packaging) plugged into a machine- constructed wire- wrap backplane. Most of the options on the machine were already wired onto the backpanel and only required that the appropriate boards be inserted. Unlike most of the other minicomputers in my collection, this one is physically quite large at 35 inches wide by 36 inches deep, and 72 inches tall. The logic resides on a swing- out door at the rear of the system.

    The example here runs both OS/8 and LAP-6/DIAL. LAP-6/DIAL is a very interesting environment to work in, as it uses the PDP-12's analogue input devices (knobs) as cursor controls to manipulate the on- screen editor. The effect is remarkably like using a mouse on a X-window display.

    Outside of the PDP-10 line, this machine has one of the nicest programmers' consoles ever offerred by DEC. Both the LINC and PDP-8 modes are directly accessible via the front panel, which uses rocker switches and an indicator array of over one hundred (incandescent) lamps - a truly wonderful sight to behold whilst the machine is in operation!

    I have, with permission from Digital Equipment Corporation, made available here an HTML transcription of the PDP-12 Users' Manual . It's not quite complete yet, but it is usable.

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