Re: [SI-LIST] : Timing on a large board

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From: Juergen Hannappel (hannappe@physik.uni-bonn.de)
Date: Fri Oct 06 2000 - 10:32:15 PDT


Michael Nudelman <mnudelman@tellium.com> writes:

> Juergen,
>
> How many stubs do you have on any given LVDS line on your backplane? If it is not
> point-to-point, in the first place equalizing LVDS diff pairs on the board would be
> a bad idea, since you want to keep stubs on your boards 1" or shorter. Again, only
> if it is not point-to-point. Otherwise it's fine.

The LVDS outputs are point-to-point. On the edge of the board i have a
connector, and from there the signals go via twisted pair flat cables
to other boards wich each have input termination.

>
> The 7ns difference would account for 42 (3.5 feet) inches, and you said your board
> is roughly foot and a half. If the traces do not wiggle, and it is not some kind of
> reflection you see - where the 7ns skew comes from? YOu should see no more than 3ns
> skew worst case..

I am also astonished by this, but the pattern of delays is the same
for a number of boards (4), so it is not an erratic effect and also
cannot be caused by the discriminator modules.

>
> The driver of 10125 can drive 50 Ohm line; if you have space problem - use 75/100

Thanks for that info, i was unable to extract it from the data sheet...

> Ohm lines (will make traces thinner and you could pack more of them), and since you
> are point-to-point (TTL-to-LVDS input), use series termination; it will save you
> instant drive current versus end-AC termination and power consumption versus
> Thevenin split.

It would not be point-to-point, but rather a 3 to 4 drop
configuration, because i need 3 or 4 LVDS versions of the same TTL signal.

>
> Mike.

-- 
Dr. Juergen Hannappel  Office: W148 Phone: +49 228 73 2447  FAX +49 228 73 7869
mailto:hannappel@physik.uni-bonn.de        Physikalisches Institut der Uni Bonn
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