On Thu, 12 Jun 1997 10:29:02 -0700,
hotz@jpl.nasa.gov (Henry B. Hotz) wrote:
> 
> First the "official" answer:  The interface uses RS-232 voltage levels,
> therefore valid states are between 3-25 volts + or -.  Between -3V and +3V
> is undefined.

Um, don't Macs use RS-422 for their serial ports instead of RS-232?
RS-422 signals are differential but IIRC, only RD and TD signals are
actually differential and the rest are single-ended in Mac's serial
ports.  For the rest, the negative pins of diff. receivers are simply
grounded and only the positive pins are used.

Am I wrong?

Ken