[SI-LIST] Re: Why ethernet RJ45 8P8C are wired in today's way?
- From: Julian Ferry <julian.ferry@xxxxxxxxxx>
- To: "neoserdes@xxxxxxxxx" <neoserdes@xxxxxxxxx>, "gnuarm.2006@xxxxxxxxx" <gnuarm.2006@xxxxxxxxx>
- Date: Wed, 5 Sep 2012 21:14:06 +0000
I was involved in developing some of the building wiring standards back in the
90's, which is where a lot of this started.
In a nutshell, the split pair is used because of backwards compatibility issues
with previous wiring standards. It is certainly less than optimal
electrically, and that little split caused many electrical engineers years of
frustration!
I don't want to leave a 4 page response here, but if you're interested in more
of the story, you can read about it here:
http://www.samtec.com/blog/index.php/dissecting-the-split-pair-issues-with-modular-connectors/
For the record, this info is based on experience at a previous employer.
Samtec is only marginally involved in this market.
Julian Ferry
High Speed Engineering Manager
Samtec, Inc.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Flash Neo
Sent: Sunday, September 02, 2012 9:39 PM
To: gnuarm.2006@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Why ethernet RJ45 8P8C are wired in today's way?
I took this section about RJ45 from Wiki again fromRJ11 section.
http://en.wikipedia.org/wiki/RJ11#RJ11
"The 8P8C <http://en.wikipedia.org/wiki/8P8C> (*RJ45*) jack used by
TIA/EIA-568-B physically accepts the 6-position connector used by RJ11,
RJ14 and RJ25, but only RJ11 and RJ14 have full electrical compatibility.
TIA/EIA-568 splits the third pair of RJ25 across two separate cable pairs,
rendering it unusable. This was necessary to preserve the electrical
properties of those pairs for Ethernet, which operates at much higher
frequencies than analog telephony."
RJ11 can plug into RJ45 and be usable for telephone usage. Magnetics should
prevent DC voltage from damaging the device.
However, 48V DC may burn out the magnetics itself since 48V source was
essentially shorted in DC.
Shall this be the ultimate reason?
- Neo
On Sun, Sep 2, 2012 at 3:17 PM, Rick Collins <gnuarm.2006@xxxxxxxxx> wrote:
> The residential connector is an RJ-11, but the RJ-45 is used for
> commercial PBX connections IIRC.
>
> Still, I don't see how the different wiring would "distinguish" the
> connectors in any way. They will look exactly the same from the
> outside. Does this wiring somehow prevent damage? I seem to recall
> hearing of people plugging RJ-11 phone lines into their RJ-45 network
> ports (yes, the connectors will allow that) and causing damage to
> their computers from the 48 VDC.
>
> Rick
>
>
> At 05:46 PM 9/2/2012, Flash Neo wrote:
> >Thank you for this info.
> >why people want to avoid this confusion? aren't they using different
> >connector anyway?
> >On Sep 2, 2012 2:39 PM, "steve weir" <weirsi@xxxxxxxxxx> wrote:
> >
> > > It was done back in the days of 10BT when the signal integrity impact
> > > was nil. The idea was to distinguish from telephone wiring which
> > > assigns the pairs in order.
> > >
> > > On 9/2/2012 1:30 PM, Flash Neo wrote:
> > > > Hi fellow si-listers,
> > > > I have been doing ethernet transceiver design for a while. (Not long
> > > > though.) Longly, I have a unsolved question about the way RJ45
> connector
> > > > pins are assigned.
> > > >
> > > > As shown in this wiki link,
> > > http://en.wikipedia.org/wiki/TIA/EIA-568#Wiring,
> > > > signal pair 2 is not placed next to each other, instead, pair 1
> seperated
> > > > pair 2's plus/minus pairs.
> > > >
> > > > What is the motivation to do it this way? I don't see any benefit.
> > > >
> > > > Thanks,
> > > > Neo
> > > >
> > > >
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