Bob Blick wrote: > Before I try, has anyone experimented with extending USB cable length > on a cheap webcam? > > If so, how far could you go? 10 feet? 20 feet? 30 feet? > > I had an old parallel port color quickcam, it seemed that 15 feet was > OK, 20 feet started to get flaky. I'd expect USB to work a little > farther, but not much. Bob: You can go five meters with just a cable, or up to 30 meters with a series of cables and hubs (or a series of active USB extension cables). 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. You can connect a series of 5 USB hubs and cables to get a maximum host-to-device length of 30 meters (the "active USB extension cables" are really just single-port hubs between two cables), but you can't use more than 5 hubs. The 5-hub limit on serially-connected hubs is a consequence of the USB spec for maximum turnaround delay between an outgoing packet and the incoming response; that spec allows for 70 ns of propagation delay through each cable/hub combination, and 30 ns through each cable alone. The sum of the propagation delays through 5 hubs and 6 cables (and back), plus the delay allowed in the device itself, beats the full-speed timeout spec by less than half a nanosecond. Adding even a few inches of cable to a full-length serial string of USB cables and hubs would violate that spec. There are more-exotic USB extenders that can give you miles of range, but they're expensive... They work by putting what looks like a USB device right next to the PC host, and what looks like a USB host right next to the device, then communicating betweeen the two using something other than USB. -Andy === Andrew Warren -- aiw@cypress.com === Principal Design Engineer === Cypress Semiconductor Corporation === === Opinions expressed above do not === necessarily represent those of === Cypress Semiconductor Corporation -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.