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How does RS-422 work?

Posted on 2006-07-18
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Last Modified: 2008-01-09
Suppose I have two computers connected using an RS-422 line.

Computer #1 sends three separate messages before Computer #2 reads any message.

Can Computer #2 still read all three messages or are some of them lost?


In other words, does RS-422 use buffering?

If messages are lost, how can I avoid this? Using "Ready to Send" and "Clear to Send" lines?
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Question by:rfr1tz
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scrathcyboy earned 800 total points
ID: 17134873
The specification itself does not imply buffering or include buffering, it is just a fast serial specification -
http://en.wikipedia.org/wiki/RS-422

However, it is possible to buffer the input and output on the specification, as the following examples indicate --

www.aeroflex.com/products/commtest/ber/productinfo/Parallel_Pods_Promo_030800.pdf
www.acttechnico.com/TVME322_Spec.pdf
www.bb-elec.com/sitemap.asp
www.rs485.com/psoftware.html

Note the last is a buffered driver for ethernet input/output, its speed is comparable to 10 base T.
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Assisted Solution

by:grsteed
grsteed earned 400 total points
ID: 17135247
RS-422 uses a pair of wires for each signal (TX and RX) using a differential arrangement. That is when the signal on one wire is going positive (to indicate a mark or 1) the other wire is going negative. This allows the receivers to cancel out any noise that was picked up on the line since noise would show up as a positive or negative pulse in both wires of the pair.  It can operate on a minimus of 5 wires. ( 2-TX, 2-RX and a ground)

As opposed to RS-232 which can operate on  3 wires (1-TX, 1-RX, and a ground)  The signal levels are the same as RS-232 (+/-12V)

Cheers,
Gary
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LVL 30

Assisted Solution

by:pgm554
pgm554 earned 800 total points
ID: 17135253
The buffering is done through the UART (either an 8250 or a 16550)
If you are sending serially, this is handled by the protocol.(Xmodem,Zmodem,etc)

Check this out:

http://cristal.inria.fr/~doligez/zmodem/
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by:pgm554
pgm554 earned 800 total points
ID: 17135268
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Expert Comment

by:grsteed
ID: 17135457
I guess I should have read the part about buffering.  LOL
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