Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!zaphod.mps.ohio-state.edu!usc!ucsd!ucsdhub!hp-sdd!hplabs!hp-pcd!hpcvca!charles
From: charles@hpcvca.CV.HP.COM (Charles Brown)
Newsgroups: comp.sys.amiga.tech
Subject: Re: RS232 gender (was Re: Apple SCSI not compatible with standard SCSI?)
Message-ID: <1410040@hpcvca.CV.HP.COM>
Date: 12 Dec 89 02:47:13 GMT
References: <503@shodha.dec.com>
Organization: Hewlett-Packard Co., Corvallis, Oregon
Lines: 29

>> Differential drive is a technique of sending signals to eliminate
>> noise.  You use two wires for each signal.  One wire carries the
>> normal signal, and the other wire carries the signal after being
>> inverted.  At the recieving end, you invert the signal that was
>> inverted at the transmiter, and add them together.  Any noise on the
>> line cancels out (since each line gets the same amount of noise
>> placed on it during the traversal of the cable) and you end up with
>> a clean signal.
>>	(david williams)
> Not quite. Adding the inversion of the inverted signal to the positive
> signal will result in the doubling of the amplitude of the signal.
> However, the noise is in no way eliminated. It too in fact increseas
> in magnitude, but does not double because it is random by nature. That
> means that some times you get spikes which are as high as double in
> usual magnitude, and sometimes you get lucky ang get a cancel out.
> 	Valentin
Most of the noise that shows up in cables is EMI induced.  Thus it is
not random.  In practice noise is coupled (almost) equally into both
differential lines.  (Differential wires are frequently twisted to
encourage this.  They are then called a twisted pair.)  So the noise
DOES cancel at the receiver.

This discussion is drifting from any relevance to the Amiga.  Perhaps
we should just drop it.
(Maybe I should have already dropped it. ;-)
--
	Charles Brown	charles@cv.hp.com or charles%hpcvca@hplabs.hp.com
			or hplabs!hpcvca!charles or "Hey you!"
	Not representing my employer.