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FATVIC, my project where I reimplement the VIC 20's VIC chip in a cheap FPGA, is moving along nicely.
The goal is a proper drop-in replacement. It should provide every function the original chip has, plus a little extra actually. But the extras are always optional. I don't want to force features on anyone who didn't ask for them.
Two things I won't compromise on
I'm fairly picky about two points, and they drive the whole design.
The first is that nothing should stick out. No cables hanging outside the case, nothing that spoils the original look of the machine.
The second is that everything already there has to keep working as usual, and any modifications must be reversible. If someone wants to go back to stock, that has to be fully possible.
The VIC chip itself is the easy part. You just swap it. If it's in a socket, it's extremely easy.
The VGA output is the tricky bit
What complicates things is that I've chosen to offer VGA as an optional output for anyone who wants it. VGA needs a different connector, and that runs straight into the requirement of being able to restore everything.
Here's how I've solved it:
- Desolder the DIN connector, but leave the PCB completely untouched. No holes, no damage, no traces cut.
- I've designed a plastic adapter that hugs the edge of the motherboard. It uses one of the screws that already holds the motherboard down to stay put physically. It ends up being a longer screw than the original, but otherwise nothing new.
- Inside the adapter sits a small PCB with a mini DIN mounted upside down. It carries sync, the RGB signals, and stereo audio out. (The photo doesn't show the small PCB yet.)
- The whole thing connects to FATVIC with an FFC.
I haven't nailed down exactly how the interconnect should look. It could be a small PCB with FFC/FPC, a Rigid-FPC, or pure FPC. We'll see what lands best.
One small downside
There doesn't seem to be any standard for VGA over mini DIN, as far as I can tell. That means you'll have to make such a cable yourself if you want to use the output.
But on the whole I think this can turn out really well. You keep the original intact, get VGA out if you want it, and can undo it whenever you like.
More to come once the small PCB is in place and I've settled on the interconnect.