Digital Equipment Corporation PDP-8/I Processor

    Introduced by Digital Equipment Corporation in 1968, the PDP-8/I was the first TTL (Transistor- Transistor- Logic) implementation of the venerable PDP-8. According to the PDP-8 FAQ , 3698 of these machines were built from 1968 to 1971. The machine seems to be a direct "translation" of the straight-8 into TTL. Much of the original -8's character is preserved.

    This machine was produced in two versions -- a rack- mount design like my example here, and a very striking pedestal- mount design. From what I've been given to understand, not too many of the pedestal- mounted versions were produced and few, if any, actually shipped to customers.

    The processor is an odd mix of synchronous and asynchronous design and is an excellent example of how DEC's design philosophy was evolving.

    The machine's basic cycle time is centered around the memory subsystem which is asynchronous in nature. Memory timing is completely controlled by delay lines and flip- flops. Processor timing is controlled mainly by delay lines and pulses occurring at the end of time- states. In this configuration, the cpu "starts" the memory system, then waits for a completion pulse from the memory. One pulse occurs at the end of the read cycle and the memory then waits for a restart pulse from the processor to begin the write cycle. A final pulse from the memory at the end of the write signals the end of the process.

    As with all PDP-8s, this one has a 12-bit word size and a basic memory capacity of 4 kW. Total memory size was expandable on the PDP-8 line to hold 32 kW by use of memory expansion logic which worked along the bank- switching model. This particular example has 8 kW of 1.5 µSecond core.

    The machine is constructed from M- series Flip Chips ® on a machine- wrapped wire- wrap backplane. Unlike later /8s, the 8/I is not mounted in a standard- sized EIA enclosure, but rather has the backplane suspended on rails vertically as a 90 degree angle to the front panel.

    Sadly, this example sufferred a small electrical fire just behind the console at some point in its career (which may have ended it, actually), and will take quite a bit of effort to restore. Once I verify that the entire CPU is present, I may undertake the manufacture of a replacement indicator panel.

    Logic diagrams and maintenance documentation is available for the machine from Highgate . Thanks guys!!!

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