Modern mice for old computers (part zero)

Anyone who collects and maintains “retro” computers will experience a recurring problem … and I don’t mean the one about it developing into a bit of an addiction which spouses tend to Have Views About.

I’m talking about when a peripheral dies for which there’s no easy replacement. A good example of this is the mouse.

Computer mice get a lot of abuse. They’re constantly being groped by manky, sweaty hands. They’re the first recipient of violent frustration when something isn’t going to plan. They get filled with the detritus that collects on the desk; dust, dead skin, biscuit crumbs, cat hair. And the cable suffers no-end of knot-tying terror before it begins to develop intermittent faults.

I collect and use a lot of Acorn computers – the BBC Micro, Archimedes, RiscPC and so on. As the majority of the computers I see have come from an educational background, then the abuse suffered jumps by an order of magnitude. They didn’t have just one abusive owner: they had an entire primary school of them!

Mice still get abuse these days, and the build quality is no better … but a replacement mouse for your modern PC might only cost you 99p so it doesn’t matter so much. However, those mice don’t communicate the same way, so we can’t use them as drop-in replacements.

There are a number of solutions to this problem. Broadly speaking, they fit into one of these three categories:

Solution one: Recondition your mouse

It’s certainly possible. Sometimes all a mouse might need is to be disassembled and have thirty years of fluff pulled out from the sensors. Older mice tend to be quite simple, so it’s certainly worth a shot. But as your mouse gets older, there’s less and less chance of that being sufficient. Cleaning out fluff is trivial, but soldering-up a replacement 10-way cable is not.

Solution two: Modify a modern mouse

Curiously, the majority of modern mice are powered by the same (or very similar) “encoder” ICs. These encoders take the quadrature signal that comes from the optical encoders in the mouse, and turn them into USB or PS/2 data. For older computers, we don’t want that encoding – we want the quadrature signals!

There are a number of guides around that show you how to modify a mouse to bypass these ICs. For the RISC-OS centric, A particularly comprehensive one appeared in Archive Magazine issue 23.11, written by Paul Porcelijn. If you’re not a subscriber, then you might be able to purchase a back-issue. Or ask Google for tutorials others may have written. There are a number of threads on the subject on StarDot, like this one.

Solution three: a converter that sits between the modern mouse and retro computer

I’m not a fan of solution two. This isn’t because there’s anything wrong with the idea (it’s actually quite elegant) but because I’m a banana-fingered buffoon with an appalling success record when modifying hardware.

I like the idea of an intermediate circuit that will convert data from a modern mouse and then generate the signals that the retro computer expects. It means I don’t have to pull apart any brand-new mice to attempt modification, and means I could include my RiscPC in my KVM-switcher setup. It also means I can freely replace the mouse any time I choose. I also get to build a sort-of in-circuit monitor that shows me how these things communicate. You never know: I might learn something!

So in the next part, I’m going to concentrate on this solution. It’s a collection of notes on how to build a circuit that will take the data from a modern mouse, and turn it into something suitable for older computers. I hope it proves useful to you!

Continue to part one!


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