This adapter is compatible with all MiSTers. It’s connected to USER IO port (USB3 with official IO and DB9 with my MiSTers). Mini-USB cable for official IO is required. It’s for connect to real MIDI synthesizer like Roland MT-32 through DIN 5-pin socket.

It has only MIDI output. If you want a device with input too I recommend buy this one from Thanks to @Oskarz for testing because I have not Roland MT-32.