wrs@Apple.COM (Walter Smith) (11/01/88)
A MIDI interface for the Macintosh consists of two parts. 1. The RS422-to-current-loop converter. MIDI is a current-loop interface, while the Mac uses RS422, so a converter is necessary. Most little Passport-type boxes use a transistor, optoisolator, and a few resistors and capacitors to do it. 2. The external clock. MIDI runs at 31.25 Kbps (1 MHz divided by 32), a non-standard (some say bizarre) data rate. The Mac serial ports cannot be run at this rate without outside help, because their clock is not a multiple of 31.25 kHz. Thus, the interface must provide the Mac with an external clock, which has been "standardized" by market pressure at 1 MHz although most software will also use 500 KHz and 2 MHz. You can build one of these things yourself in an afternoon, once you find a Schmitt-trigger optoisolator. Of course, it won't look as good as an Apple interface :-). A usable circuit appears in MacTutor, the issue before Kirk Austin's MIDI driver code (not a coincidence, since he wrote the hardware article too). - Walt -- Walter Smith wrs@apple.com, apple!wrs Apple Computer, Inc. (408) 974-5892 My corporation disavows any knowledge of my activities on the network.