Reference

Communication Protocol Description

The MIDI protocol shares many similarities with standard asynchronous serial interfaces, so you can use the UART pins of your Arduino to send and receive MIDI's event messages. The MIDI Shield can be mounted directly on top of an Arduino, connecting the MIDI-IN to the Arduino's hardware RX pin and the MIDI-OUT to TX.

Other Supplementary Information

The MIDI Shield provides an opto-isolated MIDI-IN port as well as a MIDI-OUT port. Potentiometers can be used to control volume, pitch, tone or anything else you'd like. Also connected on the shield are three momentary push buttons, a reset button, and green and red stat LEDs. The RUN/PROG switch allows you to program the Arduino over serial without having to remove the shield.