🚀 go-pugleaf

In article <2445@tau-ceti.isc-br.com> jimc@tau-ceti.isc-br.com (Jim Cathey) writes:
>... I know that our RS-422 devices
>used +5V only on the drivers, and I seem to remember that this is
>allowed by the spec...

It's required, in fact.

>RS-423 uses the same voltage swing as RS-422...

No; correct 423 uses twice the swing, from -5 to +5.

>... Many RS-422
>driver chips have a mode pin by which you can get twice as many RS-423
>transmitters by strapping it so. 

Only if you also switch one of the power-supply pins from ground to -5.
The data sheets do not call attention to this, but it's there in the
diagrams if you look for it.

As others have mentioned, the Macs do RS423 -- swings from -5 to +5 --
for all signals, with some of the signals made available as a pseudo-422
pair of signals with opposite polarities.  The imitation is good enough
that many true 422 implementations will interoperate with it, but it is
not correct 422.
-- 
There is nothing wrong with making      | Henry Spencer @ U of Toronto Zoology
mistakes, but... make *new* ones. -D.Sim|  henry@zoo.toronto.edu  utzoo!henry