USB cable length and shirt-pocket drives?

[phiscock-g851W1bGYuGnS0EtXVNi6w at public.gmane.org]

A useful rule of thumb is that the cable design becomes critical when it's
electrical length - the time it takes for a pulse to propagate from one
end to the other - is comparable to the width of the data pulses. In
electrical parlance the cable becomes a transmission line and echoes from
either the destination and source ends become significant. So it may be
that reflections in the cable are corrupting the data. I assume you are
using high-speed USB, in which case this effect could be important.

At USB 2.0 speeds, two metres is a significant length of cable, so it's
not entirely surprizing that this is a problem. If this is a really
important problem to solve, you might want to check out the Black Box
catalogue and look for USB 2.0 cables and extenders.

If you wanted  to verify that this is the problem, get a wide-bandwidth
oscilloscope and check out the eye diagram created by the pulses.

Peter

>   Using an approx 2 metre long USB connector cable, my card reader works
> OK, but a 40 gig shirt-pocket drive flakes out, with error messages in
> my logs.  Replace the cable with a 2-foot cable, and the shirt-pocket
> drive works fine.  Is this an attenuation issue, or are there extra
> wires in the short cable.  There is a jack for a 5 volt dc supply.
> However, I'm running the drive strictly off the USB port for power.
>
> --
> Walter Dnes <waltdnes-SLHPyeZ9y/tg9hUCZPvPmw at public.gmane.org> In linux /sbin/init is Job #1
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-- 
Peter Hiscocks
Professor Emeritus,
Electrical and Computer Engineering,
Ryerson University
416-465-3007
www.ee.ryerson.ca/~phiscock

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