The PDP8 That Never Was: Hollow State Logic
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The PDP8 That Never Was: Hollow State Logic

May 08, 2023

[Outer World Apps] noted that there was no PDP-8/V made by DEC — a variant that used vacuum tubes. So he's decided to make one using about 320 6J6A tubes. He's got a plan and a few boards completed — we can't wait to see it finished.

The logic is actually done by crystal rectifiers, but the tubes do inversion. To make an and/or/invert gate requires a single triode or half of a 6J6A. A D flip flop requires three tubes or two tubes for a latch. In addition to the "crystal diodes," the memory and I/O are a Raspberry Pi, and there are transistors to do level conversion between the tube logic and the Pi.

The final product will run at a blistering 28 kHz or less. Possibly a lot less. So far, there is the sequencer with 80 tubes and the memory address register with 48 tubes. Remaining are the PC register, the AC register, and the next one slated for completion, the ALU.

What a labor of love! The PDP-8 had a simple instruction set, so it makes sense to pick it for a tube implementation. We get the concession to use some transistors and the Pi, although we would have loved to see this with core memory and all tubes. We have seen a PDP-8 transistorized. Of course, as long as you’ve got the Pi, you really have everything.