Andrew, the drivers are required to limit Tr / Tf in full-speed mode to no less than 4nS. The stub that you are introducing should not be any consideration as long as you limit it to a couple of inches. I don't see a good reason for the resistors at that slow an edge rate. If you are really worried, put a series damping resistor in each the + and - line at the Y divide. But I really do not see that as necessary. Steve. Andrew Morley wrote: > Hi all, > > There are plenty of guidelines for high-speed (480Mb/s) USB PCB > layout, but I can't find any for full-speed (12Mb/s). Obviously > full-speed is much less demanding, and for what I'm considering doing, > it will need to be. > > I would like to provide a standard USB 'B' socket (full-speed, *NOT* > high-speed). That all sounds pretty straightforward. However I'd also > like to connect the same USB transceiver to a proprietary connector > (that carries USB and other signals). The system would be > (mechanically) arranged so that it would not be possible to use both > sockets. With the proprietary connector disconnected, the user would > be able to use the standard USB; with the proprietary connector mated, > USB would be carried over the connector and the user will not be able > to use the standard connector. It is important that the standard USB > 'B' port should pass USB compliance. > > What I'm considering doing is having the transceiver connected to each > port by a set of impedance-matching resistors (i.e. each port has its > own resistors). > > For high-speed (480Mb/s) USB, paralleling-up the two ports in this way > would be completely unacceptable. For full-speed (12Mb/s) and below, > I have been unable to find a reason why this shouldn't work. However > I'm still rather uneasy about it. > > Has anyone tried this? Did it work? Were there any problems > (especially passing compliance)? Any thoughts? > > I am aware that there are reasonably cheap analog switches that can be > used for USB - but I'm sure you know how the pressures on BOM cost are. > > Many thanks for all the provocative discussion over the last few years, > > Andrew > > ------------------------------------------------------------------ > 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 > > > > > -- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 California office (408) 884-3985 Business (707) 780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ------------------------------------------------------------------------------------------------------ Teraspeed(R) 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 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