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This e-mail is intended for Chris Dominico
(if it could kindly be forward to him) and could also be used as a request to
provide receiver termination as described below or explain why the receiver is
not addressed. From: David Ellington Hi Chris, I got a hold of your name by research into my situation and
am curious if you may have some thoughts. In looking at the 802.3-2005 section 3 clause 39 (39.8.6.1)
spec for 1000Base-CX, I saw a termination scheme for the driver on 50 Ohm
(single ended) systems interfacing to a 75 Ohm cable. There was no
mention to the termination on the receiver end and I am curious on why.
From what I have seen, you are an expert on cable systems and might have an
explanation. A couple thoughts I had were that (1) the termination scheme
would be the same for the receiver and that the details were not specifically
mentioned, (2) the receiver termination is going to be dependant on the cable
characteristics and length being driven. If you are interested in some background, we are helping the
avionics group at NASA with their requirement for a 1000Base-CX protocol.
Our transceivers are 50 Ohm termination at both Tx and Rx. Because they
(NASA) are using a 75 Ohm cable we need to find a resistive network to
accommodate the impedance mismatch. While we are still doing our own
modeling (hspice), we see that the IEEE already has a termination scheme in
place for the driver and
are wondering why the receiver was not covered in the spec. If you have been down this road before and have a
recommendation or know why the receiver is not mentioned, any thoughts would be
greatly appreciated. Thank you, David Ellington |