Path: utzoo!utgpu!watserv1!watmath!att!att!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!usc!apple!bionet!hayes.ims.alaska.edu!floyd
From: floyd@hayes.ims.alaska.edu (Floyd Davidson)
Newsgroups: comp.dcom.modems
Subject: Re: Twisted pairs, was: Noisy phone lines
Message-ID: <1990Nov24.023203.25762@hayes.ims.alaska.edu>
Date: 24 Nov 90 02:32:03 GMT
References: <5072.274CAD24@zswamp.fidonet.org>
Organization: University of Alaska Fairbanks
Lines: 47

In article <5072.274CAD24@zswamp.fidonet.org> root@zswamp.fidonet.org (Geoffrey Welsh) writes:
>David Lesher,,255RTFM,255rtfm (wb8foz@mthvax.cs.miami.edu ) wrote:
>
> >All this Chubby Checker work is to cancel out the influence of external
> >magnetic fields. I can remember seeing the proof in fields class, but
> >my memory blissfully shields {booo} me from the gruesome details.
>
>   Short & simple version: picture the pair, connected at each end to some 
>kind of device, as a loop. The interference picked up is proportional to the 
>electromagnetic flux passing through the loop. Twisting the loop reduces the 
>area to negligible.
>
>   That's why signal wires should be twisted with their returns (read
>"ground" where appropriate).
 

What you are saying is true; however, the last statement is going to
lead people out to left field.  The terms "signal wires" and "returns"
may not mean the same thing to everyone.

Twisted pair works for *balanced* circuits.  It will make things
worse if the circuit is single ended.

RS-232 is single ended.  NEVER use twisted pair for RS-232.

Telephone lines are balanced circuits.  ALWAYS use twisted pair.


The terms "signal" and "return" generally refer to single ended
(unbalanced) circuits, where twisting the wires is just going to
increase the capacitive coupling between the two wires.  Any noise
picked up on one will be transfered to the other.

Balanced circuits cancel induced noise because it is common mode,
and the signal is differential mode.  Twisting the wires tends to
equalize the coupling of external fields to both of the wires by
providing equal exposure.  That maintains a higher ratio between
the common mode and the differential mode.

There is a lot more to this than that simple answer.  Anyone
who is really interested should read up on transmission lines.

Floyd
-- 
Floyd L. Davidson                             floyd@hayes.ims.alaska.edu
Salcha, AK 99714                    paycheck connection to Alascom, Inc.
 When I speak for them, one of us will be *out* of business in a hurry.