Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!rpi!batcomputer!riley
From: riley@batcomputer.tn.cornell.edu (Daniel S. Riley)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Apple SCSI not compatible with standard SCSI?
Summary: differential transmission, no real amiga content.
Message-ID: <9404@batcomputer.tn.cornell.edu>
Date: 9 Dec 89 17:06:03 GMT
References: <24105@cup.portal.com> <15450001@hpdml93.HP.COM> <593@bmers58.UUCP> <7388@hubcap.clemson.edu> <8910@cbmvax.UUCP>
Reply-To: riley@tcgould.tn.cornell.edu (Daniel S. Riley)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 29

In article <8910@cbmvax.UUCP> valentin@cbmvax.UUCP (Valentin Pepelea) writes:
>In article <7388@hubcap.clemson.edu> dawill@hubcap.clemson.edu (david williams) writes:
>>    Since I seem to have started this, I'll answer your question:  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.

>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. 

Random noise is not eliminated.  I suppose it increases in amplitude by
sqrt(2), or some such.  A big win in many environments is that external
interference which affects the two lines identically (emi, ground or power 
supply fluctuations, etc.) does cancel completely.  I believe it's also 
possible to reduce emi from your transmission line by using differential 
transmission, which makes it a good choice for emi sensitive applications 
as well as emi "noisy" areas.

disclaimer:  I'm not an EE, just a physicist at a lab where we have to 
deal with electronics in extremely emi "noisy" environments (including
everything from the usual sorts of digital switching spikes all the way
up to massive amounts of synchrotron radiation).

-Dan Riley (riley@tcgould.tn.cornell.edu, cornell!batcomputer!riley)
-Wilson Lab, Cornell University