♫ SEEEEEEEEEGAAAAAAAAAAAAAAA!!! ♫
To complement my previous post, here is a schematic of the regular Sega Genesis controller. You could actually make one of these from scratch from non-specialty items; unlike the NES controller, which uses a proprietary 7-pin connector, Sega used the common-as-dirt DE-9 female D-sub connector, following in the footsteps of Atari both physically and electrically.
The circuit above could be built in fairly little time using almost exclusively items from RadioShack, if it’s well stocked. There’s probably still a DE-9 connector kicking around there. You’re not likely to find a 74HC157 in a local store, but it’s easy enough to make a 2-1 mux using 74HC00 quad NAND ICs. If you can’t track those down, it appears to be possible to make a non-inverting 2-1 mux in as few as 9 or so transistors (probably MOSFETs), but my personal recommendation, which is more time-intensive but overall less masochistic, is to have a working stock of a key few 74HC-series ICs available for when you get curious.
If I were to do it this way, I’d pull a couple of 74HC00 from my stash. A 2-1 mux—let’s call it MUX(M,N,S)—implements the expression MS OR N(NOT S); that is, “reflect M if S is high; reflect N if S is not high”. That’s equivalent to the expression (M NAND S) NAND (N NAND (S NAND 1)), which is four NAND gates. One 74HC00 = 4 NAND gates = 1 mux.
A 74HC157 packs four of these, but only two are genuinely in use. Most game software probably ignores the left and right signals while the select line is low, so it’s most likely okay to pass the left and right buttons directly out pins 3 and 4 of the port (as is already the case with up and down). As for the rest, pin 6 would be MUX(A button, B button, select) and pin 9 would be MUX(start button, C button, select).
Of course, as if it even bore mentioning, this controller would be a cinch to implement on an Arduino-like platform using just a DE-9 breakout cable. (Hint: Vcc on pin 5, ground on 8, an input on 6, and the rest are outputs.) Do this only on a temporary basis, though—you have better things to do with an Arduino. :-)