From: Mike Detlefsen (mdetlefsen_at_mac.com)
Date: Mon Sep 13 2004 - 10:24:12 PDT
RS-422 connections use two lines for Transmit and two for Receive,
whereas RS-232 uses one line for each plus a ground reference. The 232
signals are expressed by a voltage relative to ground, the 422 signals
are expressed by a voltage differential between the two transmit or
receive lines.
You can make a 422 device communicate with a 232 device most of the
time by not using two of the 422 connections, the RxD+ and the TxD-.
Doesn't always work. There are companies that sell RS-232 to RS-422
converters. In fact, the company I used to work for had to provide
these for customers to communicate to several products that used RS-422
for internal communications.
Contributing to the mess is the tendency of some manufacturers to play
fast and loose with the hardware handshaking lines. Back in the late
80's and early 90's, there were several companies that had to include
special cables for Mac with their modems, because their "standard
RS-232 serial ports" wouldn't work with a Mac. Ask how I know this,
LOL. I think I still have one of those in my junk bin.
It's sort of almost nearly backwards compatible mostly by accident, not
design. There's not a requirement in the standard for interoperability,
to my recollection (could be wrong, it's been a while).
Mike
On Monday, September 13, 2004, at 02:51 AM, Martin Joseph wrote:
>>
> RS 422 using different number of signal wires? Not quite. It's a
> serial standard, and it's backwards compatible with RS-232. It did
> allow for longer line lengths, more devices and greater speeds though.
> Personally I have never seen an RS 422 implementation that had any
> issues doing RS 232.
>
> Here a web page I googled with some interesting info too.
>
> http://wi-fiplanet.webopedia.com/TERM/R/RS_232C.html
>
> Marty
>
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