USB A 3.1 has in total 9 pins.

USB C 3.1 has in total 24 pins.

So how are cables like USB A to USB C built? Which of the 9 pins from USB A goes to the equivalent 9 pints in USB C?

USB A 3.1 also supports 10gbps, how is this possible with fewer pins than USB C 3.1?


Like USB 3.0, USB 3.1 uses one legacy USB 2.0 differential pair, and two SuperSpeed differential pairs (for a total of 6 pins), plus pins for power and ground. Wikipedia has the pinout details.

USB C for USB 3.0/3.1 was made to work no matter which way you plug it in, so it uses roughly half the available pins, plus two pins to select which ones. Wikipedia as the USB 3.0/3.1/3.2 mode assignments, as well as the pinout.

From that, it should become clear which pins are connected to which, which play a special role, and why not all USB C pins are used.

The difference in speed between USB 3.1 and USB 3.0 is because USB 3.1 allows an additional encoding scheme over the same two SuperSpeed pairs, which increases the signal rate. Again Wikipedia has details.

And you didn't ask, but USB 3.2 can also use all four differential SuperSpeed pairs of USB C connectors over an USB C cable, which is why it can achieve greater throughput using such a cable. Again Wikipedia has details.

  • You beat me to it. So I'll just add a couple of points. In addition to extra pins for reversibility, USB C has some other intended uses, and additional pins for those uses. Not all of the pins get used for each application. Also, USB C is a smart connector with an embedded chip. So you can't create a simple A to C cable by just connecting the right pins, at least if the C connector is going to function as intended. – fixer1234 Jan 9 '19 at 11:56
  • You might clarify if you mean USB 3.1 Gen 1 or USB 3.1 Gen 2 when you refer to "USB 3", "USB 3.1". – Ramhound Jan 9 '19 at 12:50

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