>
> I'm looking at designing a multi drop comm's system for control purposes.
> I have been investigating RS485, and RS422 keeps on coming up along with it,
> so I know there can't be much difference between them, but what is the
> difference ?
>
> Thanks in advance for any responses,
>
> Wayne.

This will probably be beaten to death but.....

422 and 485 are very similar, in many cases 485 transceivers are used
in 422 applications.  The primary difference is that an RS-485 driver
can be tri-stated, allowing multiple drivers to share a line, which
allows 2-wire mode to be used.  2-wire mode ties each nodes driver
and receiver together on the same twisted pair and communicates half
duplex with up to 32 other nodes (more with special hardware).

422 can have multiple receivers sharing a driver, but since it
doesn't tri-state the drivers no two drivers may share a line.
Distances and baud rates are very similar if not identical to 485.
The input impedance of a 422 receiver is a bit lower than 485, which
allows less nodes to be hung on one driver (10 instead of 32).
RS-422 can be used in point to point systems or as the master of a
master/slave 4-wire 485 network.

We have a pretty decent app note on 422 and 485 on our web site at
www.bb-elec.com

Other good info can be found in the app notes in the back of NSC's
latest Interface Data Book.

Hope this helps.

-mike

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Mike Fahrion                mfahrion@bb-elec.com      http://www.bb-elec.com/
B&B Electronics Mfg Co      ph.(815) 433-5100 ext.215    fax (815) 434-7094
707 Dayton Road                 PO Box 1040                  Ottawa IL 61350
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