First post, by starhawk
I have a stack of weird reasons stuck together that makes me want to do this. Please don't ask unless you want a response so long it will have numbered chapters, AND you're willing to read something that's that long. No, I can't condense it, don't ask for that -- I will cackle, tell you that you were warned, and hand you a novel that you can expect to be quizzed on.
I have a datasheet for an Intel 82335SX chip that (theoretically) can take a 16MHz or 20MHz i386SX CPU and i387 FPU, and make it work in a system that expects to greet an 80286 when it gets up in the morning. I've attached it. Unfortunately, while I can get the chip from a questionable source in China (for the usual caveats, including a price that makes me wonder just how high the odds are that I'll be getting an experience similar to Sparkfun's early 2010 experience with Not Alibaba... especially considering that there are *four* *listings* *on* *all* *of* *eBay* and of the four, this is the odd one out at *half* the price of the others!) -- I can't find ANY schematics for how to use it, nor can I locate any modules/daughtercards/etc that are prebuilt that use it.
To be completely honest, the track record I have with putting circuitry together and having it actually *work* means that whether or not that chip is actually a chunk of bare copper flattened hard and stuck in a 132pin QFP probably doesn't actually matter much.
But I want to try.
Specifically what I want to make is a daughterboard that will adapt a 12MHz 286 motherboard to a 20MHz 386SX + a matching 387 coproc. I don't mind bodge-wiring the existing motherboard to heck and back if I have to, and I have a spare for it, but I basically want to "286-Overdrive" (as if such a thing ever existed by that name) the system in question. If it absolutely matters, the machine I'm upgrading (or trying to) is a Compaq Portable III, and I want to do it in a way that respects the era and heritage of that machine. This is *NOT* getting a RasPi inside; those people make me as close as I ever get to violently angry. A 20MHz or 25MHz 386 is about perfect for me -- my very first system of my own rocked a 25MHz AM386SX and that turns out to have been an incredible coincidence, because that's right about at the sweet spot for me where computing just makes sense. (Funny how that works.)
Laugh at me if you want, but can anyone find me that ref design... or *any* schematics that will let me build what I describe?