In message <Pine.SOL.3.96.1000105162334.25095D-100000@marcy.nas.nasa.gov>,Bill 
Studenmund writes:
>It's been a while since I played with RS442/485, but I think the signaling
>is different for them. I'm not sure if just a cable will work. RS485 is
>the differential version of RS442.

rs422 is just differential rs232.  in the case of the macintosh most cables
just connect the tx+ and rx+ to ground and drive the tx- and rx- 
with the rs232 tx and rx in order to connect an rs232 device to the
mac's rs422 port.  if the other device uses an 8-pin mini-din you just
need a plain macintosh serial cable and you should be fine.

[on a side note, some of the larger sgi's have a special ioctl for
opening their serial ports in order do rs422.  since these boxes
have db9s, you need some way of saying enable the tx+ and rx+ pins
on the connectors]

btw, i thought rs485 was a bus version of rs422.  it uses differential coding
like rs422 but you can have multiple senders and receivers on the same
set of wires.  i believe rs485 specifies some way of determining when it
is clear to access the 'bus'