Update (March 11, 2009): This modification is also needed for Sonic 3 and the combination Sonic 3 + Knuckles. Thanks [ColdFace] for pointing out the issue.
The SEGA Mega Drive / Genesis cartridge for the game “Sonic & Knuckles” has slot on top that other carts can be plugged into. There are three special cases which unlock new games:
|Combination||Game variation||Extra ROM chip active||S2K Mod needed|
|S&K + Sonic the Hedgehog||Full version of Blue Sphere game||No||No|
|S&K + Sonic 2||“Knuckles in Sonic 2″||Yes||Yes|
|S&K + Sonic 3||“Sonic & Knuckles 3″||No||Yes|
|S&K + any other game||Single level of Blue Sphere game||No||No|
When combined with Sonic 2, the S&K cartridge uses an otherwise hidden ROM chip. Since this requires two extra control lines that the Retrode doesn’t use by default, we have to add two wires if we want to play “Knuckles in Sonic 2″.
Here’s how it works (click on images to magnify):