This is mainly just a story of how I picked up my 486DX2s for testing. Nothing technically interesting here.
It is kinda long.
I'm trying to target a 486DX2 at 66mhz for this project. The other important specs are pretty common. VGA graphics and a SoundBlaster 2.0+ compatible sound card.
I hit up eBay for some 486 parts. Found a 486DX2 at 66mhz, on a motherboard with a couple of PCI slots. PCI-supporting 486 PCs weren't really that common, so I'm hoping this simplifies getting hardware supported by the thing. The main rare thing seems to be ISA VGA cards. Everyone is looking for them, and the ones online are expensive. I might just toss an old PCI GeForce card in there and not worry about it.
I was at WeirdStuff Warehouse looking for parts for the to-be-assembled 486, when I noticed a beat up old PC with no pricetag and no specs labeled on it. The only thing I could see on it were some serial ports, a VGA port, a pair of PS/2 ports, and - on the other side - a floppy drive an a 4x speed CD drive. I grew up with a 486 with a 2x speed CD drive, so that one marker put it in the right ballpark for being a slightly later 486.
One of the employees (or owner?) let me take it over to the workbench and crack it open. Couldn't tell much except that the bus speed was set at 33mhz with the jumpers. We plugged it into a monitor there and powered it on with no result. No signal. So I figured I'll need an ISA VGA card to stick in the thing, but otherwise it's probably pretty close to what I need.
One interesting sign was that the CPU had a heat sink on it. Just a heat sink, without a fan. Anything older than a 486/386 probably wouldn't bother with a heat sink. Anything newer (Pentium and up) would probably also have a fan.
I bought it with the knowledge that it'd probably be a bit of a fixer-upper. And it was. But when I got home and plugged it into my CRT, it popped up with a message that the CMOS settings were bad. Probably a dead battery on the board, but otherwise everything's good! It was exactly the right processor. An AMD 486DX2 at 66mhz. Perfect.
Well, that little dead battery turned out to be a lot more of a hassle than anticipated. The system wouldn't be able to recognize any hard drive without a working battery. The HD settings wouldn't be remembered between the time you set them and the time it looks for a boot device when you reboot.
To make matters worse, it turned out to be one of those Dallas 12887 Real Time Clock chips with the battery built in. Thanks to some handy internet guides, I learned how to modify these chips (with the help of a dremel and a soldering iron) to accept an external battery and bypass the internal one. By that night I had a functioning 486DX2! Note that the guides are mostly about the older 1287 (one '8', rather than two), but the modification is about the same for the 12887.
For anyone interested in the process, Peter H. Wendt's site has information on the mod. Terry Stewart's site also has information on it.
Other nice stuff found at WeirdStuff: a couple of Vibra 16 (Low end SB16) cards, and a few 3COM Etherlink III ISA network cards. The Etherlink III cards are great, and I'll go over that more in the "Networking DOS" section.