It is actually outside the USB spec to make any 'extension cord' or overlong cable. For example a A-male to A-female or B-male to B-female cable are "illegal" because they could be used to make a connection that is too long. The design decisions made in the USB spec were intended so that no matter what you did, if you could hook something up, it would work. (The only exception is that you could hook up a chain of more than five hubs, which is relatively unlikely to happen). Another design decision involved having low speed devices requiring captive cables (or cables with a custom (non-USB connector) end at the device) that are not longer than a certain limit. I forget the details on this, but I do remember that it was a requirement to ensure reliable operation (otherwise the user could use a cable designed for full speed operation and get problems). And yes, low speed devices are limited to shorter cables than full speed devices because of the simpler signaling scheme used. Unfortunately idiotic companies don't follow the spec properly, leaving support folks to deal with the grief. Bob Ammerman RAm Systems ----- Original Message ----- From: "Josh Koffman" To: Sent: Friday, January 09, 2004 5:30 PM Subject: Re: [EE:] USB cable length > I guess this is mainly a question for Andy, as he's likely pretty up to > date on the USB standard. There is a local store here that I've seen USB > A-B cables in 25 foot lengths. Now, the packages could be mislabeled, > I'm not sure. These are the no name, made in China imported jobbies. > These cables would seem to be definately out of spec. They are not > powered extensions, they don't have a big enough "hump" on the cable. If > I read your post correctly, having a cable that long could damage the > drivers, correct? So why would anyone make one that long? Is it possible > that it might work? > > Josh > -- > A common mistake that people make when trying to design something > completely foolproof is to underestimate the ingenuity of complete > fools. > -Douglas Adams > > Andrew Warren wrote: > > USB cables are limited to 5 meters because that length is short > > enough to allow reflections from the far end of the cable to settle > > between bits; if a longer cable is used, reflections won't be damped > > as well, and the line voltage could rise high enough to cause > > physical damage to the drivers. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.