my corner of cyberspace =Dan Roganti

PDP-8 clone – using Transistors —

As a result of running the Build Your Own Transistor Logic Circuit workshop for VCF East 7.0, I’m stumbled upon another project that will definitely consume more time which I’m usually running out of [uh oh – ending a sentence with a preposition again]. But this one project is mostly out of desire than necessity. I never had a Transistor computer before. In the first photo below is an advertisement of the original PDP-8 minicomputer – this could actually fit on the table.

So after exploring what possible transistor computers I could attempt to build, I finally decided to build a clone of the PDP-8 computer from DEC. I can’t really say it will be a replica as it won’t look the same as the original. But rather the hardware design will make it software compatible with the original. I picked a common experimenter circuit card, a 22/44pin plug-in card, that I’ve used since the 70s.

I’m using traditional metal can transistors, 2N2219 in a TO-39 can. There’s no other engineering excuse for this other than “it looks cool” I rather stare at metal than a bunch of boring plastic To-92 packages any day of the week :) In the second photo, I made a prototype circuit with four NAND gates wired as a Flip Flop. In the third photo, it shows one of the first cards which hold an array of 42 transistors wired as NAND and NOR gates – AND/OR gates just take an extra transistor. Depending on the function of the circuit to make, it will contain a mix of these logic gates to wire together. These will be wired together to make the various modules, such as the Accumulator for the PDP-8 computer.

I started building a card rack which holds the backplane [pics coming soon]. It can hold 16 cards in one rack. I’m studying the PDP-8 hardware manuals still to figure out how many cards it will take to build this. But one thing is for sure, I’m packing as many transistors on one card as possible, 42 in all. So it will be fewer cards than the original where the largest card might have had about a dozen transistors. The major difference is due to the component packaging with the many resistors and diodes – which was still much larger in the early 1960’s. In the component layout arrangement, I position the passive components vertically to save space – much like the early radio electronic equipment – since they had to fit in tight quarters, such as on a ship, plane, etc.


Categorised as: Electronics | Hobbies | PDP-8 clone | Vintage Computers

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