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- Newsgroups: sci.electronics
- Path: sparky!uunet!utcsri!utzoo!henry
- From: henry@zoo.toronto.edu (Henry Spencer)
- Subject: Re: RS-422A and RS-423. Whats the diff?
- Message-ID: <Brz8nB.71L@zoo.toronto.edu>
- Date: Sun, 26 Jul 1992 03:35:34 GMT
- References: <92Jul21.174720.26587@acs.ucalgary.ca> <Brsw0q.FKD@zoo.toronto.edu> <2445@tau-ceti.isc-br.com>
- Organization: U of Toronto Zoology
- Lines: 27
-
- 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
-