Hi Jim, I think you can just leave GPU's unused PCIe lanes NC. Clarify the PCIe link width that GPU supports. If it does support downgrade to PCIe x1 lane or x2 lanes, CPU and GPU will negotiation before the link set up. Above suggestion based on SAS/SATA controller and GbE controller design, I think you can find the details via GPU maker. How to terminated the unused lanes, RX pairs should be routed out from BGA to simplify Debug, no need to termination (or a capacity pad be placed as test points). For TX pairs, the terminator res can be the test point, too. Just FYI. Best regards, Yun -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of otter30 Sent: Wednesday, September 08, 2010 5:20 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Termination of unused PCIe pairs Hello Friends, I am designing a carrier pcb that provides interconnect between a CPU module and a video processing card. The video card supports 4x PCIe, but the CPU module has only 2 PCIe lanes available. Since PCIe pairs are specified to have 100ohm differential impedance, 50 ohm resistors to ground ought to work on the unused TX pairs. Do I also need a .1u cap in series with the resistors for the RX pairs? Thanks, Jim ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu