Dr. Scott M. Baker Builds Cees Meijer's Jupiter Ace Clone — and Three New Add-On Modules

Dr. Scott M. Baker has taken a break from Heathkit projects to put together a replica Jupiter Ace microcomputer, based on a design by Cees Meijer — then designed three of his own expansion modules for good measure.

"It all started when I acquired a speech synthesizer for the Timex Sinclair and set off to build myself a Timex Sinclair clone," Baker explains. "Along the way I stumbled onto the Jupiter Ace, and being a FORTH-based computer that was a commercial failure, I just couldn't resist the diversion of building one of these unique computers! That's often how these journeys go — you start down one path and encounter something interesting along the way."

Jupiter Cantab launched the Jupiter Ace in 1982, aiming to offer competition to the Sinclair ZX81 (released as the Timex Sinclair 1000 in the US) with some big differences under its plastic hood — not least of which was its use of the FORTH language in place of the BASIC which was common to most eight-bit microcomputers of the era. Only a few thousand were sold before the company folded a year after launch, making those that remain collectors' items — and prime fodder for the cloning process.

Electronics engineer Cees Meijer has gone through the process of designing a clone of the Jupiter Ace, right down to the plastic case — "because," Meijer explains in his project's Hackaday.io page, "what's lacking in all these rebuilds is the actual housing, which to me is a huge part of the charm of this machine. And using a 3D printer it cannot be too hard to replicate this."

As if to prove his point, Baker has taken Meijer's design and replicated it — having the KiCad PCB design files manufactured, sourcing the required components, and making only minimal changes, though not without some difficulties with the video output which required the removal of a 555 timer from the circuit and the addition of a delay in the RAM write signal to add compatibility with 6166 static RAM (SRAM) parts.

Baker wasn't happy to stop the project there, though, and set about building his own add-on modules for the new Jupiter Ace clone. "Making new modules wasn't difficult, though there are a few tricks I came up with for the edge connectors," he explains. "The edge card connectors need to be 'open' on the two ends. I made them by taking a larger-than-necessary connector and using a hacksaw blade to carefully cut off the ends. For a 'pass-through' so you can piggyback one module behind another, I used a small stub board."

Baker's new modules are: a RAM board, which adds an extra 48kB of memory to the system; a speech synthesizer module, compatible with Martyn Davies "Big Mouth" board; and a Raspberry Pi Supervisor, powered by a Raspberry Pi Zero single-board computer. "The Pi Supervisor is a board that I first built for the RC2014," Baker explains.

"I recognized how useful it would be to just pause the CPU and read or write memory. You can PEEK and POKE your retro computer from the convenience of your desktop computer, to see what’s going on inside the retro computer. You can even push whole programs into memory, or save the current state of memory."

"On some computers," Baker continues, "such as the Jupiter Ace, I've been able to push keystrokes directly into the keyboard buffer and download screen captures. This lets me do software development from my Windows desktop across the room, yet still do it 'on' the retro computer. Rather than laboriously type a whole program on the Ace’s keyboard, I can just paste the whole thing into the keyboard buffer."

Baker's full write-up is available on his website; source code, schematics, and board design files for the add-on modules are available on GitHub under an unspecified license.