>
>>  Strictly speaking, the RS422 voltage levels should be more in line
>> with typical RS485 signals (differential on top of a positive common
>> mode voltage), but the Mac doesn't seem to adhere to this convention.
>> Most models seem to use asymmetrical signals about 0v, like RS232.
>
>  That's not consistent with what you just described.  Can you explain
>more clearly how the Mac does not "adhere"?  I suspect you mean it will
>"tolerate" RS-232 rather than "use" RS-232 levels.
>

        I wouldn't bet much money on it, but I was always of the impression
that  RS422 and RS485 operated with a positive common mode voltage. The
schematics that I have (for the early Macs anyway) show AM26LS30 type
RS422/RS423 drivers powered from +/- 5V supplies, leading to my comment.
        Actually I haven't thought about this a great deal. I was too cheap
to buy an (overpriced) Mac cable about 10 years ago, made one and it
worked. When I went to a 28.8 modem ~5yrs ago, I remember having to fiddle
a bit with the handshaking, but wrote down what I did, and this is what
I've passed on.
        My initial comment in my post was meant as a dig at that large
component of the computer world that fails to realize that there are other
units out there besides the ones powered by Intel. The Macintosh is a major
player in that other world just as Mercedes is in North America to the
General Motors users. If manufacturers just did a little homework, they
could include instructions for connection to Macintoshes and immediately
open the doors to perhaps 10% more market. I still remember being so
pleased when I opened my first 68HC11 EVBU (gasp!) and found directions and
software for both IBMs and Macs.


        Jim Nagy
        Elm Electronics
  ICs for Experimenters
http://www.elmelectronics.com/