Dr. Scott M. Baker Builds a Downgrade for the Heathkit H8, Swapping the Intel 8080 CPU for an 8008

Engineer and vintage computing enthusiast Dr. Scott M. Baker is continuing to tinker with his Heathkit H8 eight-bit kit computer, this time creating a new processor board — but one which downgrades the stock Intel 8080, rather than upgrades it, by replacing it with Intel's original 8008.

"I’ve made boards for the 8080-based Heathkit H8 with newer CPUs, such as the [Intel] 8085, but I wanted to try my hand at incorporating something distinctly older," Baker explains of the project. "Can it be done? Can switching to an early CPU still yield a functional, useful computer? Yes! The [Intel] 8008 is widely regarded as the world’s first 8-bit programmable microprocessor. The 8008 was certainly before my time, before I ever sat down in front of a computer and started to program."

Released in 1972, the Intel 8008 was created by Intel based on a commission from the Computer Terminal Corporation. While its development was far from smooth, the chip proved a success — though not for CTC, which would ditch the part for discrete transistor-transistor logic (TTL) parts instead — and effectively launched Intel's stratospheric rise as one of the biggest microprocessor companies in the world.

The Intel 8008 was replaced two years after its launch by the 8080, a higher-performance part with features missing from the original — though lacking, unlike devices in the x86 range for which Intel would become known, binary compatibility with its predecessor. Despite that lack of compatibility between the two, it's the 8008 Baker chose to retrofit into the Heathkit H8 — which was launched in 1977, long after most new designs had switched to the 8080.

To get the processor switch to work, Baker had to address various difference between the chips: a 14-bit address space, accessing just 16kB of RAM; a clock speed down at 500kHz, rather than the 2MHz of the 8080; and just eight input and 24 output ports, to the 256 bidirectional ports available on the H8. Add in a lack of PUSH and POP instructions for stack management and troublesome interrupt support, and it's clear things wouldn't be easy.

Baker's resulting Heathkit board includes a clock circuit with three outputs, a reset circuit, a programmable logic device (PLD) for the bus, another for decoding input and output signals, buggers and latches to triplex the processor's bus, and on-board memory — used in place of the memory board fitted to the Heathkit H8 itself — with a memory mapper which expands the available address space from 16kB to 32kB RAM and 32kB ROM.

This is far from the first custom board Baker has developed for the Heathkit H8, though it's the only one which could be reasonably described as downgrading the device. Previously, the engineer has built a custom speech synthesis board, a 16MB RAM expansion card, a bridge to a Raspberry Pi single-board computer, and — matching the 8008 processor board for era, at least — a storage card built using bubble memory technology from the 1970s.

"It doesn’t run HDOS. It doesn’t run CP/M. It doesn’t work with the H8’s panel monitor. It (probably) won’t run any H8 assembly programs," Baker admits of the 8008 processor board.

"So what can you do with it once you’ve built it? I wrote a panel monitor and grafted it onto Jim Loos’s serial monitor, so we have a basic front panel monitor and a serial monitor. You can program in BASIC using Scelbi Basic (SCELBAL). You can play Star Trek, Hangman, and a few other games. You can write your own assembly programs and burn them to ROM or upload them over serial."

Baker's full write-up is available on his website, with schematics, gerbers, and source code available on GitHub under an unspecified open source license.