First, you need a midi interface for the Amiga, which the most common are Parallel Port Midi interfaces.
It's the serial port actually. MIDI uses a different connector and voltage to RS232, other than that the usual baud rate is a little weird (a lot of serial ports can't do 31.25K baud).
You could probably built a midi interface for the Amiga with IN & OUT using some connectors and a max232 chip. You could probably add THRU as well but you'd probably have to start following the spec a little more closely in terms of isolation.
