From the nimble fingers of John H. Guillory (johng@...)
(20/2/2002 6:18 PM) came...

> Hmmm, I faintly remember a little about the RS-422 if it is what I'm
> thinking... (It was 422, 428, or something in that area....)  Used for
> some of the really old Gang Eprom Programmers on PC's... They where
> basically a RS-232 that supported multiple devices... Something I had
> a hard time to really understand how they could do, but I guess by
> multiple devices they mean the daisy chaining of Apple Talk network on

!! You can do multi drops on RS232 (or 422/423 for that matter) only if you
can selectively turn the remote transmit ports off: otherwise 2 (or more)
transmitters are grabbing the wire at the same time, resulting in either
fire or indeterminate data. Multi-dropping 232 is, essentially, just not
done.

422/423 is distinct from 232 only in that differential lines are used:
transmit plus (Tx+) and transmit minus (Tx-), and the corresponding Rx+ and
Rx-. On differential lines, one sucks while the other blows, and a certain
amount of error voltage common to both is allowed. RF problems are reduced
with balanced lines. It's no better at multi-dropping than 232.

Old Appletalk used RS485 which was a true multi-drop bus, using only a
single differential pair (of wires) with an Appletalk adaptor, and about 210
kbaud. 485 is still used in lots of factory automation systems for its quite
spectacular noise and common-mode rejection. Spectacular, that is, for a
slow (1 Mbaud), simple bus that doesn't recover all that neatly from data
collisions.
 Point-to-point wiring was possible between 2 (and only 2!) Appletalk ports
with the cross-over ImageWriter cable; the cable with 8 wires looked more or
less the same as a pair of adaptors connected by 2 wires.
-- 
Ted Spencer; ted@...
--
Work is the greatest thing in the world, so we should save some of it for
tomorrow.
(Don Herold)