MIDI
MIDI was a popular musical interface standard in the home computer era and especially the Atari ST was famous for its built-in MIDI interface. Separate MIDI interfaces existed for virtually every other computer of that time.
Since MiSTle/MiSTeryNano started as an Atari ST recreation, a MIDI interface was added to the hardware. Most other cores adopted to this, and MIDI is also supported by the NanoMig, the NanoMac and the C64Nano.
On some machines like the NanoMac and the C64Nano additional hardware is implemented in the core to be compatible with common MIDI hardware for that platform. In some cases, this need to be explicitly activated through the On-Screen-Display (OSD).
Many games of that time also support external MIDI hardware. Especially the Sierra Online games like Space Quest will output additional music to an external MIDI synth like the Roland MT32.
The primary use for the MIDI interfaces is the connection to vintage music hardware, which can be a little pricey nowadays. It's also possible to use recent devices like the M5 SAM 2695 MIDI. This little device comes with physical MIDI connectors as well as a direct UART connection. The MIDI connectors are compatible with the usual MiSTle shields while the UART connector can be used to directly connect to the appropriate pins on the FPGA in setups where not physical MIDI is present.
The M5 MIDI device can also be connected directly with the TN20k or any other FPGA via its UART connector. This works with any setup and doesn't require physical MIDI connectors. This is still fully compatible with all cores supporting MIDI. The wiring is quite simple:
This works e.g. with the MisteryShield20k-Lite:
Due to the omnipresent MIDI interface on the Atari ST, it was also sometimes used for other purposes. Games like e.g. MIDI Maze and Frogs can use the MIDI connection to network multiple machines for multi-player gaming. This also works with a combination of genuine retro hardware and MiSTle based units.