Newsgroups: comp.dcom.telecom.tech,comp.dcom.cabling
Subject: Re: ethernet polarity
References:   <[email protected]>
"Watson A.Name - \"Watt Sun, the Dark Remover\""  writes:

> "Tomi Holger Engdahl"  wrote in message
> news:[email protected]...
> > "TerryS"  writes:
> >
> > > Are the signals on the two pairs polarity sensitive ?
> >
> >
> > At least on 10 Base-T and 100Base-TX Ethernet system the
> > parctical systems are not polarity sensitive. The Ethernet
> > devices should be built in such way that they automatically
> > correct the reversed polarity situation.
> >
> > > I never thought much
> > > on this and I don't even know what the signals are. We are
> troubleshooting a
> > > network with lots of packet errors. The customer has not noticed any
> > > degradation yet, but our monitor keeps going into alarm. Anyway, we
> found
> > > the green pair reversed, on the router to switch cable, but it must
> have
> > > been for some time, and the problem is recent. This has been
> corrected,
> > > cable tests out now. Not enough data yet to tell if that fixed it.
> Just
> > > wondering.
> >
> > Most propably your errors are caused by some other reason.
> 
> > --
> > Tomi Engdahl (http://www.iki.fi/then/)
> > Take a look at my electronics web links and documents at
> > http://www.epanorama.net/
> 
> The ethernet signal goes thru a transformer on each end, and that
> essentially makes the signal AC, eliminating any DC component.  

That's true for Ethernet systems that use twisted pair wiring 

Tere is no such transformer in original coax etherner:
Transceiver electronics is directly connected to coax, 
the AUI interface from transceiver to MAC is trransformer 
isolated (data and control signal manchester coded through 
transformers, power isolated with DC-DC converter). 

> The ethernet signals used in the original 10BaseT coax were manchester
> encoded, which is not polarity sensitive, IIRC.  

There is no such thing at 10BaseT coax. 
The 10Base-T Ethernet uses always twised pair wiring. 

The Ethernet systems that use coax have different 
names like 10Base5 and 10Base2. Coaxial Ethernet 
uses manchester encoding. The coax system is polarity 
sensitive (at least collosion detection fails if 
polarity is not right). 

> I believe the signals
> on cat5 balanced twisted pair are also manchester encoded.  I will
> crosspost to comp.dcom.cabling and for some other viewpoints.

The signals on cat5 balanced twisted pair are also manchester encoded. 
The electronics in twisted pair Ethernet transceivers are designed 
in such what that they are not polarity sensitive 
(transceiver IC can detect polarity reversal and correct it if needed). 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/