Digital Equipment Corporation
pdp-11/20 processor

[JPEG image of 11/20]

    In late 1969, Digital Equipment Corporation introduced the first of the venerable pdp-11 series of machines that were to catapult DEC into a position of preeminance in the manufacture of minicomputers. That machine was the pdp-11/20.

    The 11/20 was DEC's first machine to feature a word length that was not a multiple of six bits (six bits being the length of the standard character in the 1960s); the designers chose to use a 16 bit word (two 8 bit octets), and a memory addressed in octets, not words as in previous machines.

    The 11/20 also introduced to the world the bus structure known as UNIBUS, an 18-bit adddress space in which the highest 8192 (8k) octets are reserved to handle the various I/O devices necessary for a computer to be useful. This structure was a radical departure from the prevailing model of the day that used seperate busses for I/O and memory. This allowed a single set of instructions to control the functions of I/O in addition to memory.

    This individual sports a test and checkout tag dated 1972/09/24 and was "acquired" from a friend who wanted it repaired and lost interest in it. It's still formally his, and if he wants it he gets it back.

    Like all the "early" pdp-11s, this one has a full-featured front panel with twenty-five toggle switches, a locking keyswitch, and 42 LED indicators. I believe the LEDs were a retrofit, made by the company where I worked years ago. The panel consists of the 16 data lights (one for each bit of the ALU word width), 18 address lights, 2 status lights, and 6 time-state indicators.

    Alone in the pdp-11 family, the 11/20 utilised time-state logic; all others sported microcode designs. The CPU is implemented in DEC M-series "Flip-Chip" modules, plugged into a wirewrap backplane (standard DEC construction practise in the 60s/70s) using TTL circuitry. The system uses switching-mode power supplies.

    The memory subsystem of the 11/20 is implemented as magnetic core; this example has 12 kw in three backplane frames of 4 kw each in the main unit.

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