FRS' Sega Page

Sega Mega Drive joypad to Sega Mark III adapter

If you tried to use a Mega Drive joypad with your Sega Mark III, you probably noticed that the LEFT/RIGHT directions won't work, and only the A/START buttons work instead of the more useful B/C.

But this is very easy to fix with a super simple adapter, since only one pin, the pin7, needs to be rerouted.

Mega Drive to Mk III joypad
        adapter schematics

Not even a PCB is necessary. You just need cut off the pin7 of the female side, then solder the two connectors back-to-back like below. Then solder one side of a 10K resistor the pin7 of the DE-9 male connector, and the other side of this resistor to the pin5 of the same connector.  If you want to be luxurious, you can wrap the middle section of the adapter with some Heat-shrink tubing or insulation tape.

Photo of an example