From: Peter H. Coffin (hellsop_at_ninehells.com)
Date: Fri Jan 28 2005 - 21:33:53 PST
On Sat, Jan 29, 2005 at 01:20:39AM +0100, Adriano wrote:
> Peter,
>
> your words explain why RS232 could transmit data at 230K, the same as a
> RS422
>
> As you know, RS-422 works on a binary (crossed pair in the cables)
> communication, while RS-232 runs on a single data line,
> this would explain why the entire process is more stable on this
> protocol.
Right, RS-422 carries signal on two wires, not one. But it carries
exactly the same signal on both wires, one in reverse. [1]
___
___| |___ ____ TX RS-232's one wire
|___|
Gnd 1 Gnd 0 Gnd
___
___| |___ ____ TX+
|___|
___ RS-422's single wire
___ ___| |____ TX-
|___|
All this really does is protect the signal from induced voltages from
outside sources by making sure that no matter which way the voltage is
pushed ("more positive" or "more negative") that the DIFFERENCE between
the signal wires is strong enough to see. If something induces -5V to a
RS-232, the circuitry may not be able to see the difference between that
and a chassis ground on one bit, and see a signal (even if it is twice
what it should be) on the other bit. In RS-422, you've always got 10V
difference, but the the current direction reverse between 1 and 0.
Now, this matters a hell of a lot if you're running serial lines
controlling motors the size of a VW, or around massive amounts of
switched currents, and you've only got some unsheilded wires carrying
your signal. Then, RS-422 shines, and can run at more or less full
speed.
On your desk, where induction source are lethal to disks and
everything's carefully shielded, you'll probably NEVER see a difference.
>
> If i am wrong, please tell me more about it.
>
> A big question is: why the TTL levels coming out from the Newton serial
> channel3 literary burn the 5v shifted Max232 adapters connected on it?
>
> Have you got any idea?
No clue. Never used one. Probably never even seen one. My inclination is
to declare it crap and say find a more robust adaptor. It's possible
you've got a cable that's wired wrong. I wouldn't be able to tell
without an ocilliscope, though.
[1] Yes, Dave, I know this is a massive handwave of how 232 and 422
signalling works.
-- I wish there was a knob on the TV to turn up the intelligence. There's a knob called "brightness", but it doesn't work. -- Gallagher -- This is the NewtonTalk list - http://www.newtontalk.net/ for all inquiries Official Newton FAQ: http://www.chuma.org/newton/faq/ WikiWikiNewt for all kinds of articles: http://tools.unna.org/wikiwikinewt/
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