I built two of Mac RS422 to MIDI interface devices using the description given in the c't magazine. This interface works very well (powered from serial port, 1 x MIDI in, 2 x MIDI out). The interface design seems to be similar to John's MIDI interface. Which version to build is a matter of available parts, I think. The c't article is written in German, but anyone able to built such things will only need the parts list and PCB layout. (The PCB I used came from the eMedia shop.)