Andrew Warren wrote: > 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). And the absurd thing is that the LOW SPEED USB devices have a much SHORTER cable limit than the high speed ones? Any rational explanation for that would be appreciated. > 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. It take it that the USB designers never heard of 'terminations' to prevent reflections? Or 'transient clamp diodes'? > 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. Would it not help to use a 'low capacitance' cable, which has a higher propagation velocity? I've read of a videomaker in Montreal who runs firewire links to his camera that are WAY over spec (150' he claims) by using Cat5 wire. I've been meaning to do this with my firewire camera, but have not been able to find a Male-Female firewire extension cable I could hack up. > 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. So what's the point of using USB then? How long before we see ethernet mice/keyboards and cameras? Looks to me like a market opportunity, particularly for cameras. If I can get a 100BT card for under $10, putting the chip IN the camera can't be THAT expensive. Robert -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.