SMSM is an interface that takes MIDI data and controls the sound chip inside of the SEGA Master System in real time. The idea is to send MIDI data to the SEGA Master System using a MIDI sequencer or MIDI keyboard via the SMSM, which then sends the appropriate audio signal out of the console via its AV port or a direct output port.
So for example, if a MIDI note C is sent to the SMSM, then the SEGA sound chip will play a tone with pitch of a C. The PSG SN76489 sound chip is supported as well as the FM YM2413 sound chip (which is found in Japanese models of SEGA Mater System).
The aim of this post is to cover the simplest hardware setup that can be used to create an SMSM interface. This is achieved via a Arduino Serial setup. Other methods are possible too.