Hi !
I hacked my old dead Atari 1024stf with a handsaw and a dremel to get the midi in and out pcb with optocoupler, inverters and 556 timer.
(Off topic : do you know what the 556 do in midi out messages ? Midi clock ?)

It works great as a midi input optocoupler !

Here is the official midi schematic for inputs, outputs and through ports :
http://www.midi.org/techspecs/electrispec.php

The triangle amps followed by a small ring are inverters, like the 4049, used for buffering the signal.
http://en.wikipedia.org/wiki/Inverter_%28logic_gate%29

Hi, I just recieved for Xmas my Arduino Mega, and I wish to control circuit bent devices (toy keyboards, speak and spell, DIY filters...) via Midi with Arduino.

Any help would be appreciated !

Question : did anyone tried the midi library for Arduino ?

Thanks