Leonard Here are a few answers to your questions, however I would highly recommend that you engage the services of someone that has done this a few times. Differential signaling can be quite robust, as Xilinx and Dr. Johnson indicate. It is the details that either make or break a design. 1) PCB traces act as transmission lines that are capable of storing multiple bits in a data stream. It is not the delay that is important, but the loss. 2a) Differential coupling on any edge coupled differential pair is quite weak. It is permissible to break differential spacing rules in breakout. However, there are optimal and not-so-optimal ways to do this that can either enhance or reduce overall performance. 2b) There is actually quite a bit of information about differential via spacing and design out there. If I recall, there is an Ansoft article on this for small form factor plugable connectors. However, differential via design is dependent upon the via diameter, the pad diameter, the via-to-via pitch, the antipad diameter and type (dual-circular, single-rectangular, single-oval ...) etc, trace width, stackup, and trace layer entry and exit. Normally, we design and optimize using a full wave field solver, CST Microwave Studio. 2c) Breaking your spacing rule should be done in conjunction with simulation of the coupling in the channel. 500 mils may sound like a short distance, but electrically it is approximately 75 to 90 ps long. Depending upon the edge speed of the drivers, this may be enough coupled length to develop significant crosstalk. In many designs i have seen, it is not loss but crosstalk that limits performance. This crosstalk comes from the connectors, the packages, the breakout vias, the transition vias and the traces. All need to be looked at as a synergistic whole. We've done a lot of work with Samtec on their Final Inch(TM) designs for multiple connector families, including some surface mount BGA-style connectors, where we have completely modeled and simulated the breakout regions, vias, and differential traces. This is not a trivial task. 2d) Length matching of differential pairs is an interesting topic in light of the nature of FR4 laminate construction. It is my contention that extreme measures for length matching of differential pairs come to naught, unless in matching and skew control due engineering consideration is given to the nature of epoxy/fiberglass laminate weaves. Teraspeed Consulting will be presenting a paper at DesignCon 2005 on this very topic, titled: "The Impact of PCB Laminate Weave on the Electrical Performance of Differential Signaling at Mutli-Gigabit Data Rates" Another paper we are delivering is especially germane to FPGA design: "High Performance FPGA Bypass Filter Networks" best regards, scott - Scott McMorrow Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 (401) 284-1827 Business (401) 284-1840 Fax (503) 750-6481 Cellular http://www.teraspeed.com Teraspeed is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu