Dr. Scott M. Baker's Heathkit ET-3400's Memory I/O Expansion Board Is a Modern Retro Marvel

Dr. Scott M. Baker has built a memory upgrade kit with a difference: This MAX202-powered gadget is designed for the Heathkit ET-3400 microprocessor trainer, launched back in 1976, and gives it SSH connectivity and HDMI video output courtesy of an optional Raspberry Pi Zero W.

Released in 1976 for around $200, the Heathkit ET-3400 was designed to introduce users to the Motorola 6800 eight-bit microprocessor launched two years prior. Like many kits of the era, the system was undeniably basic — but an add-on dubbed the Memory I/O Expansion Accessory could turn it into a basic microcomputer, offering a built-in BASIC programming language, terminal connectivity, and the ability to save and load programs to and from cassette tape.

Today, the ET-3400 isn't too rare — unless you want the expanded version, that is. Baker's answer to the problem: Building a modern equivalent, swapping the hard-to-find or otherwise awkward original components for newer replacements without losing the heart of the original.

"The schematics for the ETA-3400 are well published on the web, and you can readily see exactly how Heathkit implemented the original accessory," Baker explains. "One option would have been to faithfully reproduce the original, but that would have been a little inconvenient in comparison to some of the more modern ICs that are available now."

"Rather than using eight static RAMS and a pair of ROMs, we could simply use one larger RAM and one larger ROM. Rather than implementing address translation in a ROM, it could be done in programmable logic. A modern MAX202 could easily replace the handful of transistors and discrete components used in the original. While slightly newer, these choices are still in the vintage theme of the original."

On its own, the custom-designed accessory does everything the original add-on offered — including providing the ability to program the system in BASIC or assembly. There's a bonus feature, though: A 40-pin header compatible with the Raspberry Pi family of single-board computers. "A [Raspberry] Pi Zero W can be added to the board and will be connected to the serial and cassette interfaces, Baker explains. "This allows you to hide a pi inside the case and have remote SSH access to your trainer (or hook up the HDMI and USB to drive a display and keyboard.)"

Full details on the project can be found on Baker's website, while the source code and design files area available on GitHub under the permissive Creative Commons Attribution-ShareAlike 3.0 license.