Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Circuitry behind an Ethernet tap ?
References: <[email protected]>
[email protected] writes:

> Hi
> 
> I am trying to figure out what it would take to turn a homemade passive
> ethernet tap into one that has activity and collision LEDs ?  I take it
> a PHY chip would be necessary but I don't understand how these come
> into it because these taps are transparent on the network and I have
> always thought of PHY chips as negotiators of speed and duplex (if that
> makes sense).  Does anyone know what ICs higher end taps use and how
> they operate ?

There one plan for simple Ethernet tap at 
http://www.snort.org/docs/tap/ 

The higher end taps somethign like ones at 
http://www.comcraftfr.com/ethertap100tx.htm
have some more electronics to operate. 
Basically such tap for 10/100Base-T systems can 
be built by passing the two data pairs through the 
device, and having two high impedance differential 
amplifiers that "listen to" at the signals on those
data pairs, and send an amplified copy of the 
signal on those to the corresponding TAP output 
connectors. Signal from each pair gets to one 
TAP output. No matter which direction data on those 
pairs go, it gets picked up by the receiver in the 
middle of the pair.

I once looked inside one such TAP unit and found there 
several high speed differential amplifier ICs and some 
Ethernet line transformers. I do not have a circuit 
diagram of this kind of device. 


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