[LAU] USB cable with ferrite core

[Fons Adriaensen]

On Sat, Dec 31, 2016 at 07:10:16AM +0100, Clemens Ladisch wrote:

> >> But USB also uses common-mode signalling for certain states.
> >
> >If you mean signalling that depends on a ground path outside
> >the cable I doubt very much this is true.
> 
> USB does not use currents but voltage levels for signalling.
> The two data lines (D+ and D-) have controlled impedances and drive
> levels so that the effective currents are symmetrical, but when both
> lines are pulled low to signal EOP, there is not necessarily an equivalent
> return current over the ground line.

There must be. The net sum of the currents flowing through he cable
must be zero, unless some current takes a path (e.g. via protective
ground) not going via the cable.

If two devices are connected only by the USB cable there is no
such path. And if there is one (e.g. protective ground), and a
signal current takes that path rather than via the cable, then
there's something seriously wrong with the design of at least
one of the two devices.


Ciao,

-- 
FA

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