I wonder, would it be possible to construct a fully fledging Sega 32/CD/Genesis all in one system? And create some kind of BIOS screen that'd allow you to select which system you'd like to boot upon power-up?
32X does work with a Sega CDX, so with (a lot of) case modifications you could do this. In fact, I plan to do just this, plus with an inbuilt screen, controller, and battery. :v:
Well, this fantastic article explains how you can make a DIY Neptune. If you were to get a CDX as Andlabs mentioned above (and probably a custom case), then it could be pulled off. I'm not too sure about that BIOS hacking part, though.
There's no need to do this, since MD games will run in a 32X (the extra hardware is simply ignored), and the SegaCD won't turn on if a ROM cartridge is inserted. (With an exception for that weird MD cartridge that could utilize a SegaCD after startup. I forget what it's called.)
Actually three: the official backup RAM cart, the one Nik mentioned, and the fanmade MegaCart. You will not be able to play 32X games made for a TV signal that isn't your own (that is, NTSC 32X games on a PAL 32X). Same concept for the Sega CD. For those you WILL need BIOS modifications. I know there is <a href="http://www.eidolons-inn.net/tiki-index.php?page=SegaCD+Multibios" target="_blank">a guide for the Sega CD part</a>.
I'd put the CDX/32X in a Saturn case. Plenty of room for it all there, with just about the perfect placement of the CD, a cart slot, and the controller ports.
Assuming though it'd be possible to make an entirely new smaller PCB combining all three circuit boards into one compact board?
Actually four, you forgot the Pier Solar cartridge can use the Sega CD to get the CD OST =P (not counting all the Sega 32xCD games).
After doing some research, we would have to either find schematics for all those systems and make a PCB based off the old hardware then remove all the chips and solder them on the new board. Or you have to find out how each chip works and replicate it using a processor which frankly even the NES isn't even replicated correctly now. You would need a team of engineers to do this in any kind of manner.
And I currently have found the Genesis Schematics: Sega Genesis Schematics: CPU and Custom Chips (from Charles MacDonald's site) http://emudocs.org/Genesis/mega1.png Sega Genesis Schematics: Sound and Video (from Charles MacDonald's site) http://emudocs.org/Genesis/mega2.png Sega Genesis Block Diagram (from Charles MacDonald's site) http://emudocs.org/Genesis/mega3.png
But I have no idea where I could possibly find schematic diagrams for the 32x and Sega CD, any ideas?