Brian said:
USB cables are limited to 5m length.
That is why you put four 5 meter cables with active
repeaters on each, end to end.
The (5M) length limit is enforced by some time constant
in the protocol. While longer cables are offered
for sale, somehow the time of flight still has
to be taken into account. Some of the cables
cheat, by adding additional active repeaters in
the middle of the cable run. But there are some
other cables, where the wire is too long (using a
different dielectric would probably not allow the
cable length to be doubled). So they must have found
some other way around the timeout issue.
I use the 5 meter cables, because I don't like surprises.
And I was surprised how much power was available at
the end of the cable. A couple of webcams didn't complain
at all, when operated at the end of 15 meters of cable.
As an example of what you can do, this is a still picture
clipped out a movie, taken when I had a camera up behind
my bathroom wall. The object in the picture is the
valve for the shower, and it isn't leaking into the
wall (whew!). That's what owning long USB cables buys
you. Home inspection, in weird places. Some day, I'll
have a robot to climb up inside the walls and look around
http://i61.tinypic.com/vcz62h.jpg
The camera... using some ancient webcam chips (controller and sensor)
http://www.generaltools.com/DCS050--The-Seeker-050-USB-Video-Inspection-System_p_1296.html#
Paul