RE: [SI-LIST] : PCI bus impedance

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From: Greim, Michael (mgreim@sonusnet.com)
Date: Mon May 14 2001 - 10:45:08 PDT


Hi all,

This is just my 0.02, but I would say that it depends.
I have designed a number of 50 ohm intra-board pci buses without
incident into production. I think the trick is how many
loads do you have, what are you plugging into (impedance
mismatch?), and what the topology is (load spacing, stubs).
There are definately a set of cases where 50 ohm will be
more than good enough. Simulation should give you the
warm and fuzzy that you need to proceed.

Best Regards,

Michael C. Greim Sonus Networks
mgreim@sonusnet.com 978-589-8336

Making the world safe for digital signals everywhere

And all this science I don't understand
It's just my job six days a week

The time is gone. The email's over
Thought I'd something more to say......

-----Original Message-----
From: Patel, Bhavesh [mailto:bpatel@ciena.com]
Sent: Monday, May 14, 2001 1:08 PM
To: SI_LIST (E-mail)
Subject: [SI-LIST] : PCI bus impedance

Hi! Gurus, I was simulating a PCI bus with 50ohm trace impedance because we
generally try to keep on all our boards 50ohm single ended & 100ohm
differential(easy for the fab house). Now, the vendor says use 65ohm trace
impedance and I did simulations with 65ohm but I don't see much difference
i.e. not significant where I would say'YES' let's go to 65ohm. Also, if I
have to do 65ohm then it will eating up my routing layers because this bus
is confined to one section of my board.
I needed some feedback regarding this and whether it will really make or
break the PCI bus by going from 50ohm to 65ohm.
Thanks in advance
Bhavesh Patel

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