31 Dec
2016
31 Dec
'16
7:10 a.m.
Fons Adriaensen wrote:
On Fri, Dec 30, 2016 at 07:12:34AM +0100, Clemens
Ladisch wrote:
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.
(That depends on whether the device has an independent power supply.)
Regards,
Clemens
Using a common-mode choke works only for
differential signals.
True, but any signal that has both the forward and return path
in the cable is differential (even if not truly balanced). That
remains true even if the return path is the screen instead of
signal ground.
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.