Notes :

1. R* value depend of the optocoupler , see table
2. 6N138 and a PIC , R*=1K work fine ...
3. Best optos for MIDI are 6N13x series. PC900 is very good too but with a reversed pinout... 4N2x work fine too. Avoid TIL11x.
4. Logic gates are 74HC14 or 74HCT14.
5. PSU is clean regulated +5VDC , ground at 0V.
6. For an already made pcb of this thing , look at the MPB3 Patch Bay , i/o boards can be used independantly...