[SI-LIST] Re: PCIe routing

Hello Bernd--
A few suggestions for you:

1.  You are correct, putting TX and RX on different layers would
alleviate near-end crosstalk, but then you have an issue with the
far-end crosstalk, which will in most cases be a larger problem,
especially on the outer layers.  This paper may help:
http://www.xilinx.com/publications/xcellonline/xcell_58/xc_pdf/p074-076_
58-mentor.pdf

There is also a PCI-Express Design Kit with lots of helpful tips on
Mentor's SupportNet if you are a customer. 

2.  The are many straight connectors which support multi-Gbs signals (if
your boards are parallel).  Any of the major connector websites should
have details.  Or if you do have to cable the signals, I would stay away
from a flat ribbon cable, but something like the twinax of a SATA cable
might work well and is relatively flat.

--Pat

Patrick Carrier
Technical Marketing Engineer
High Speed Design
Mentor Graphics Corporation
Patrick_Carrier@xxxxxxxxxx
ph. (512) 425-3015

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of bernd schuster
Sent: Saturday, January 09, 2010 4:49 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] PCIe routing

Hi,
I`ve a few questions about using PCIe (using an four layer FR4 board).

1) Is it a good recommendation to route for example the RX pairs on the
bottom layer and the TX pairs on the top layer to reduce near-end
crosstalk?


2) Do you know any board-to-board (e.g with flat cable) connectors (not
for PCIe slots) which are recommend to use for PCIe signals? I`ve one
motherboard and a board with all the outputs installed (which are not
perpendicular to each other)

3) Are there any recommendation I have to know adding a PCIe switch? The
first thing I recognized was the delay of 128ns - what about the PCIe
clock signal?



best regards
Bernd


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