hi If the termination is on the DIMM (as on SODIMMs), then the purpose is to avoid reflections. Even if the clock is unused, reflections are harmful, becuse it causes EMI radiation, and creates more crosstalk to other lines on your board. So thats why we termiante unused clocks too. Termination is to characteristic impedance of the line. The DIMM probably has Rtt of 2x39 ohms or similar, sized to the 75 ohms line impedance. If you change your diffpair to 100 ohms, then it will be not that good matched. We have lots of boards with unterminated unused clocks, we never experienced any problems in the practice, but it does not mean that there will not be problems for shure. For the used clocks, you better route them at 75 ohms, because you want them matched-terminated and clean and nice. Istvan Nagy Concurrent Tech. ----- Original Message ----- From: "Heyfitch" <heyfitch@xxxxxxxx> To: "si-list" <si-list@xxxxxxxxxxxxx> Sent: Thursday, June 04, 2009 1:17 AM Subject: [SI-LIST] DDR3 CK1/CK1# board routing > Dear SI gurus - > On single-rank DDR3 dimms - both RDIMMs and UDIMMs - the unused pair of > clocks CK1/CK1# is terminated by a 75-Ohm resistor across the pair. This > is > per JEDEC specs. > > Does this require that these clocks are routed as 75-Ohm differential on > the > mother board? Has anyone see problems arise if on a motherboard they are > routed as 100-Ohm differencial (aka loosely coupled or uncoupled)? > > Thanks. > Vadim > > > ------------------------------------------------------------------ > 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 > 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 > > > ------------------------------------------------------------------ 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 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