AW: [SI-LIST] : Effect of low Zo for unterminated lines

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From: Andreas Lenkisch (Trenew) (a.lenkisch@trenew.de)
Date: Thu Oct 21 1999 - 04:16:10 PDT


ABTE is mandatory for new VME developments (VME64x) and a new spec. for slightly stronger 3,3V parts is under development by the VITA and TI
The additional advantage of ABTE is the very low input capacitance of only approx. 5pF which causes a very light loading of a multipoint/multidrop bus and reduces the loaded impedance not so strong. Pl. see attached PDF file on my previous mail.

You may also click on the "Literature" button on our homepage "www.trenew.com" and download a white paper
"Signal integrity on VME backplanes - data rates of 160Mbytes/s and higher"; it deals with low impedance traces. It is now two years old, but still valid.

Andreas

----------
Von: Dan Bostan[SMTP:dbostan@ix.netcom.com]
Gesendet: Donnerstag, 21. Oktober 1999 06:43
An: si-list@silab.eng.sun.com
Betreff: Re: [SI-LIST] : Effect of low Zo for unterminated lines

Try abte devices from TI.

----- Original Message -----
From: Andreas Lenkisch (Trenew) <a.lenkisch@trenew.de>
To: <si-list@silab.eng.sun.com>
Sent: Wednesday, October 20, 1999 4:25 AM
Subject: AW: [SI-LIST] : Effect of low Zo for unterminated lines

Chris,

on usual VME backplanes you will have on a fully loaded board a impedance of
20 ..25 Ohms and sometimes maybe lower. This was well driven in the past,
but with 5MHz only and by 48mA or 65mA driver so that less power dissipation
is expected. But LVCH drive only 24mA... , do you have any comment from the
IC manufacturer?

Pl. find attached a comparison between high and low impedance
(multipoint/multidrop) traces, when they are loaded with different
capacitive loads. With some skill you can get on a low impedance trace
nearly the same loaded impedance as on usual ones.

[[ IMPEDA~1.PDF : 1803 in WINMAIL.DAT ]]

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Von: Chris Bobek[SMTP:cbobek@cadence.com]
Gesendet: Dienstag, 19. Oktober 1999 19:16
An: si-list@silab.eng.sun.com
Betreff: Re: [SI-LIST] : Effect of low Zo for unterminated lines

Hi,

I think I should restate my question. The original question wasn't just
pertaining to an LVCH245, that was just an example. I'm trying to find out
what the effects are on a driver when the Zo is low.

As a general example, assume you had a 30ohm line being driven by a driver
that was designed to drive a 50ohm load. The line is unterminated and
suppose it's a foot long. Suppose there are multiple drops on the line.
I've
already seen simulations of this, but I'm wondering if this will damage the
driver or not.

Thanks for your help,

Chris

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