Lutz Lisseck have built a bidirectional UART to Midi interface by using AVR Atmega8 microcontroller.

Interface circuit may work either as RS232 to Midi interface (Us MAX232) or USB to Midi (Use FT232). Data is sent as it is 1 to 1 so to make serial interface as Midi you need to install additional driver on PC which allows to send midi data via it. Author recommends using Roland Serial MIDI Driver for Windows XP / Windows 2000.