an USB port

Is the pin on the left connected directly to the ground-plane of the motherboard? Just a wire, without any resistance?

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    \$\begingroup\$ A wire has resistance too \$\endgroup\$ – PlasmaHH Nov 13 '16 at 19:42
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    \$\begingroup\$ You mean: "Is there a resistor between the Ground pin and the chassis ?" In general: no there isn't. But to be sure, get a multimeter and measure it to be sure. Not all products follow the general rule. \$\endgroup\$ – Bimpelrekkie Nov 13 '16 at 19:49

It is very unlikely that there is ANYTHING between the ground pin of the USB connector and the ground node of the computer. But it is not guaranteed. Since we don't know WHY you are asking this question, it is not clear how that is important?

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    \$\begingroup\$ There sometimes is a small choke coil in the USB +5V and in the USB GND line. \$\endgroup\$ – Janka Nov 13 '16 at 20:07
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    \$\begingroup\$ I have seen (and once destroyed) the USB ferrite ground bead on an arduino clone, I suspect it's there from EMI reasons (more to keep it out than in), no reason why people wouldn't do the same in PCs \$\endgroup\$ – Sam Nov 13 '16 at 22:39
  • \$\begingroup\$ Yes, there MAY be some kind of inductance provided between the USB ground pin and the computer ground. But it is NOT there to insert ANY resistance into the path. Ideally it will have ZERO resistance which is what @Marty is asking. \$\endgroup\$ – Richard Crowley Nov 13 '16 at 22:44

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