At 3:30 PM 6/12/97, Ken Nakata wrote:
>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?

No, you're right.  I just wasn't describing the whole picture.

RS-232 was nominally replaced by RS-449 (which gives a connector pinout
that nobody uses), and RS423 (which gives the same voltage levels and
imposes some pulse shaping to allow higher data rates, but says nothing
about pinout or functions), and RS422 (which likewise ignores pinout and
functionality and uses the same voltage levels and pulse shaping, but
requires two wire signals which should be detected by comparing them with
each other rather than to a seperate ground wire).

All of these standards were designed to interoperate at some level.  You
can (and Apple does) run the correct half of a RS422 signal into an RS232
receiver and pass data with no trouble (but only RS232 data rates).
Likewise you can (and Apple does) take an RS232 signal and run it into the
correct side of a RS422 receiver, ground the other side, and pass data with
no trouble (but only RS232 data rates).

When you hear of an RS232 interface running faster than 20KBaud most likely
what you have is RS232 pinout and RS423 electrical signals.  I may be a bit
out-of-date.  The latest version of RS232 (D?  E?) may have modified the
electrical characteristics to conform to RS423.

This has gotten a bit off-topic.  I assume there is some level of interest.

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h.b.hotz@jpl.nasa.gov, or hbhotz@oxy.edu