> 
> 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?

You are correct.

No one's fully sure what the low-difference behavior of the input
buffers is, though.

Take care,

Bill