MSX and X68000 share the same controller port as many other computers and consoles, but they are not wired the same, so DO NOT just put a Mega Drive controller in without an adapter.
A joystick made for Atari 2600 / Commodore 64 / Amiga is fine, but they only have one working button, and many MSX games have support for two.
The Master System controller have _almost_ the same pinout as MSX – all you have to do is switch place on pin 9 to pin 7 on your SMS pad for 2 button support. You can modify an extension cable if you still want the controller to work with Master System.
Another option is to modify a Mega Drive Controller like this: https://www.msx.org/wiki/How_to_get_joypad_for_MSX
But the best option (as I see it) is to build/buy a joymega-adapter. This cheap adapter lets you use any Mega Drive controller without modifications. It even supports all the buttons on a 6 button-mega drive controller! Regular MSX games still only support 1-2 buttons though, but with Sofarun you can assign any key from the keyboard to a button on the controller.
Everything you need to know is on this Github-page: https://github.com/tebl/MSX-Mega-Adapter/tree/main
You can buy all components cheap from AliExpress and get your PCB from https://jlcpcb.com (or some other vendor, I’ve only used jlcpcb though). Then it’s just some easy soldering to put it together. Or you can just look if you can find one on etsy or ebay.
I made a custom 3D-printed shell for the JoyMega-adapter that you can download here:
