[RFI] Ethernet RFI

Jim jvpoll at dallas.net
Sun Jan 23 15:32:25 EST 2005

Using fomulas from the theory of 'Permutations and
Combinations' the following equation determines the
number of unique groupings (irrespective of the order
or different permutations such as 1, 2  and 2, 1) or
combination relationship:

          n_C_r   =   n!  /  (n-r)! * r!

Therefore given 8 wires and choosing 2 we have:

   8_C_2 =      8!      /    (8 - 2)! * 2!

   8_C_2  =     8!      /     6!   * 2!

   8_C_2  =    40320 /    720  *  2

   8_C_2  =    28 unique combinations if my math is correct!

Jim  /  WB5WPA  /

----- Original Message -----
From: "Ed -K0iL" <eedwards at tconl.com>
To: "'RFI Reflector'" <rfi at contesting.com>
Cc: "'John Pelham'" <john at radiophile.com>
Sent: Sunday, January 23, 2005 1:18 PM
Subject: RE: [RFI] Ethernet RFI

> Wouldn't there be 8 to the 8th-power possible combinations for 8 pin
> connections?  (Someone may wish to check my math.  There are 64
> combinations if you just keep the wires in the same order just shifting
> them over!)  But only one of those would place the "twisted pairs" into
> correct EMC signal configuration.  Otherwise you might be placing
> data signals together onto the same "twist" and splitting the signal pairs
> up onto different "twists".  This would obviously work to some degree and
> for some length, but the EMC benefit of the twisted pair would then be
> lost.  It only works correctly for EMC in one configuration out of
> 16,777,216 possible combinations. (if my math is correct).
> -----Original Message-----
> From: John Pelham
> Ford wrote:
> >When you hook up the cables to keep the colors all the same on each
> >pin at each end, you will have conductivity between the correct pins,
> >and the network will work, but you completely lose any shielding benefits
> >of the twisted pairs.
> I don't follow.  I assume you mean by "colors all the same on each pin at
> each end," you mean that if at one end the green wire is on pin 6, at the
> other end the green wire is also on pin 6, for example.  It seems like
> you're saying that this is the wrong way to do it.  I've made zillions of
> Ethernet cables and have always done it this way.  I just checked my
> reference and it says to do it this way.  (I'm not talking about the
> special
> category of crossover cables, just regular Ethernet wiring.)  I don't see
> how doing it this way is wrong electrically.  In fact it seems like doing
> it
> any other way would be wrong and would likely not work at all.
> Am I misunderstanding?  If not, how would you have us wire Ethernet
> 73,
> John W1JA
> ----- Original Message -----
> From: "Ford Peterson"
> ...SNIP...
> > I was surprised to find that grounding the EMT to the #4 Cu wire that
> > comes in from outside INcreases the level of these unwanted signals by 2
> > S-units. Any ideas why, and what I could do to get rid of this RFI
> > (until I get the tower up, at which time the feed point will be much
> > farther away from any of the computer equipment)?
> >
> > Alan NV8A
> Alan,
> One of my pet peeves is when computer technicians wire cat5 cables
> incorrectly.  There is a right way and a wrong way to do it.  The right
> is the most difficult.  Guess which way most people do it?
> The 8 conductors in that cable are actually 4 pairs of twisted cables.
>  Each
> pair is a "hot" and a ground return.  When you hook up the cables to keep
> the colors all the same on each pin at each end, you will have
> between the correct pins, and the network will work, but you completely
> lose
> any shielding benefits of the twisted pairs.  You should not be hearing
> anything from that cable.
> I do not have the specification for those cable ends handy.  But as I
> recall, the ground returns are the 4 pins on one end of the connector.
> Before rewiring your house with EMT, I would make sure the connectors are
> installed correctly.
> I have one run here that an 'installation expert' installed for me.  I
> queried him explicitly on the correct wiring pattern.  A lively discussion
> ensued.  Realizing that I was arguing with an idiot with 7th grade shop
> class electrical wiring experience, and that I was paying him $2 a minute
> to
> argue with me, I let him wire it his way.  Not surprisingly, that
> particular
> 75' run of Cat5 never runs over 10mb in a 10/100 hub.  The hub shifts into
> low gear to deal with the noise.
> I'm sure short runs work just fine wired however you want.  Wired the
> 'easy'
> way, you get 2 pairs carrying all the data and two pairs acting as the
> ground return.  I'm sure there is some shielding afforded by lashing them
> into the same run of wire, but they are not acting correctly.  It takes 4
> thumbs to wire them correctly.  Find the spec and wire them right.
> Ford-N0FP
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