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The PDP-8/I Minicomputer

The PDP-8/I was the first TTL version of the PDP-8 series. The schematics and engineering drawings I have date back to 1967 -- just when integrated circuits were coming into "popular" use. The machine consists of several hundred Flip-Chips -- small, general-purpose printed circuit boards, with several ICs each, connected to a large backplane. The backplane was wire-wrapped point-to-point by machine. A big manufacturing location for PDP-8/I backplanes was right down the street from me, at DEC's Herzberg Road plant in Kanata, Ontario.

This PDP-8/I came from Mississippi, and is now (2003 12 18) restored (read more). I just played chess on it (2003 12 18)!

The Front Panel

Front Panel of the PDP-8/I S M L XL

The PDP-8/I front panel is very impressive -- it displays just about everything that's happening inside of the machine -- from the current address being executed, the data in memory, the contents of the accumulator, and multiplier-quotient register to the decoded instruction being executed and the current state.

The Backplane

Backplane of the PDP-8/I S M L XL

This is the backplane for the PDP-8/I. It consists of 8 rows of 40 columns; a large number of which are filled with the Flip-Chip cards.

The Cards

The Cards in a PDP-8/I S M L XL

This is the reverse side of the backplane, with around two hundred cards installed. The purple cards are known as M-series ("M" for Magenta) cards, and contain general-purpose logic, such as 6 flip-flops per card, or a bunch of NAND gates. They are constructed from 74-series TTL chips for the most part. The green cards are known as G-series cards, and generally contain analog functions, such as the amplifiers for the core memory. White cards are interface cards, and generally contain little logic, mainly just connectors to go from one bus to another.

The Functional Groupings of the Cards in a PDP-8/I S M L XL

This is an annotated diagram, showing the functional groupings of the various cards. The real beauty of the PDP-8/I is that you can not only look at the schematic and get a good idea of what's going on, but you can physically find and touch various components. For example, the six cards at the bottom left, labelled "Major Registers", are where the flip-flops are that store the accumulator, the program counter, memory buffer, and memory address. There are two bits per card (2 x 6 = 12 bits).

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