I used the schematics from: http://www.midi.org/techspecs/electrispec.php
Instead of the Sharp PC-900 or HP 6N138 optocoupler I use a 6n137 as mentioned above. And I changed the resistor values a bit, instead of 280 Ohm I use 220 Ohm.
I generate a midi signal with my Arduino, a tone sweep example I found somewhere. That signal is normally high and pulls down.http://www.fairchildsemi.com/ds/6N%2F6N137.pdf
On the 6n137 the midi input goes to pin 2. I measured the output on pin 6 (Vo) of the 6n137. If necessary I could use a transistor to invert the signal, should be an easy fix.
I normally draw my schematics, but i just installed new firmware for my Wacom tablet and now it doesn't le me draw no more. I'm sorry if my explanation isn't clear. Not a native English speaker, and I absolutely have no technical electronics lingo.
I never worked with MIDI. Normally a MIDI signal is high and pulls down? That's the thing I can't figure out.