Simple tests of an Ethernet port?

Lewin A.R.W. Edwards wrote:
> Hi Tauno,
> 
> 
>>The RF impedance?
> 
> 
> Well, no, DC :)
> 
> 
>>Can you get the data sheets of the PHY and magnetics?
> 
> 
> Actually I am on crack, it's been a stressful week. The PHY is
> integrated in the MAC. I meant to say the magnetics were possibly
> chosen randomly. The part on the actual board is an XFMRS
> XFATM2Y-CLYG1-4MS, 1:1. Of course, the datasheet for this cannot be
> downloaded from XFMRS' web site, it 404s. The webmaster of this site
> deserves whipping. The CLYG1-4MS suffix probably means some special
> characteristics, but without the datasheet I can only guess.

Ouch!

> 
>>Any termination / matching resistors between PHY and magnetics?
> 
> 
> Yes, but what is in the schematic does not match the board. Argh. I
> think I should abandon this project.

As a first-order approximation, the transmitter impedance inside
the PHY can be assumed as zero (much less than the matching
resistors, I hope) and the receiver impedance can be assumed
as infinite (much more than the matchin resistor).

You could calculate the impedances reflected on the network
using the above assumptions and the actual values of the
matching resistors. There are two requirements on the transmit
network: present the proper impedance and proper signal level.
For the receiver network, the impedance match usually suffices.
Some chips (e.g. LAN91C111) need attenuation of the incoming
signal, which is handled by the receive termination.

(Remember that impedance transforms in the square of the turns
ratio).

HTH

Tauno Voipio
tauno voipio @ iki fi