The Megadrive Expansion Port (http://www.gamesx.com/wiki/doku.php?id=schematics:megadriveexpport) was largely detailed based on tracing the circuits in a MD1 and a Teradrive. Since there are schematics on the wiki (http://www.gamesx.com/wiki/doku.php?id=schematics:console_related_schematics#sega) I added info from them to the expansion port pinout.
Still some details I'd like to know, about some of the signals, but I don't think it's terribly important.