Avian’s Blog

Recently I've been working on a redesign of Galaksija for Cyberpipe's computer museum. People from the museum would like to add this 80's-era computer to their collection and I offered to help them build one.

Galaksija was originally meant as a do-it-yourself project, so complete schematics and ROM contents are available on the net. At first it didn't look like it would be a complicated task to build one with modern components. Soon it turned out that the original author (Voja Antonić) had used a lot of dirty tricks in the computer's circuitry to keep the number of integrated circuits to a minimum. In the 80's he of course had a perfectly good reason for this: integrated circuits were expensive since the government restricted imports of western technology. However this also meant that the circuit relied on properties of certain chips that weren't documented and therefore weren't guaranteed to remain unchanged with new revisions. The result: the original circuit only works with 74LS series logic chips which are a bit hard to come by these days (they were mostly replaced with 74HC/74HCT series) and only works with some Z80 processors (or so I heard from people that built the original Galaksija).

There were some attempts to build Galaksija with more modern components. For example galaksija.org had a schematic of a Galaksija built with a GAL chip instead of discreet logic circuits (the link to schematics disappeared some days ago with the redesign of the page). However they said that this version also had many of the problems of the original (it also doesn't work with all Z80 processors).

So I set out to design a Galaksija compatible computer with the following goals:

• The computer must be completely software compatible with the original Galaksija.
• It should use modern 74HC-series chips instead of 74LS.
• Rely only on documented features of integrated circuits. Integrated circuits are cheap today, so the first priority is to make the circuit work reliably. The number of components comes second. This way the circuit should work with any series of logic chips and would probably only require minor modifications in the future.
• Skip the radio-frequency modulator that enabled the computer to be connected to an ordinary television and use a composite video output instead. All television sets today have a composite video input so including a RF modulator no longer make sense.

So far I spent something like 120 hours on this project and the circuit schematics itself is now more or less finished (you can find the current working draft of the motherboard here). The only thing that remains to be done at this point is to make the layout for the printed circuit board. Unfortunately I have some other things to attend to right now, so this will have to wait. In the mean time I'll try to post here some of the more interesting details of the original Galaksija's design.