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

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From: Dan Bostan ([email protected])
Date: Wed Oct 20 1999 - 21:43:26 PDT

Try abte devices from TI.

----- Original Message -----
From: Andreas Lenkisch (Trenew) <[email protected]>
To: <[email protected]>
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 ]]

----------
Von: Chris Bobek[SMTP:[email protected]]
Gesendet: Dienstag, 19. Oktober 1999 19:16
An: [email protected]
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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