Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

The new USB Type C connector doesn't have a physical reverse polarity protection anymore. You can plug it in any way you want on both ends, there is also no A and B end anymore, it's all the same.

So how does this new USB type handle that the polarity doesn't end up being reversed? Do the devices have to agree on something in hardware and route the connection appropriately?

Or is there some sort of routing magic going on in the connector and the devices don't have to handle anything and can be sure the polarity is always correct?

Type C connector and receptor

share|improve this question
geometrical symmetry. – Vladimir Cravero Aug 13 '14 at 15:27
It's obviously more than just that. – Professor Sparkles Mar 12 '15 at 15:31
up vote 27 down vote accepted

Below is the pinout for the receptacle:

GND  TX1+ TX1- Vbus CC1   D+   D-  SBU1 Vbus RX2- RX2+ GND
 |    |    |    |    |    |    |    |    |    |    |    |
 |    |    |    |    |    |    |    |    |    |    |    |
GND  RX1+ RX1- Vbus SBU2  D-   D+  CC2  Vbus TX2- TX2+ GND

You will note that all the pins are rotationally symmetric, so if you flip the connector, TX1+ connects to TX2+, TX1- connects to TX2-, etc. and most importantly, Vbus and GND always match up.

The trick lies in the controller and cable -- the CC pins are used to detect orientation, at which point the controller routes appropriately:

2.3.2 Plug Orientation/Cable Twist Detection

The USB Type-C plug can be inserted into a receptacle in either one of two orientations, therefore the CC pins enable a method for detecting plug orientation in order to determine which SuperSpeed USB data signal pairs are functionally connected through the cable. This allows for signal routing, if needed, within a DFP or UFP to be established for a successful connection.

Source: blogspot link Source: blogspot link

As you might imagine, the cables are going to be a fair bit heftier due to the extra wires.

  • A minimum of 15 wires plus braid required for full-featured Type-C (i.e. USB 3.1 -- recommended 4-6mm outer diameter)
  • 10 wires plus braid for legacy Type-C USB 3.0/3.1 cables (intended to connect to Type-A or Type-B on the other end -- recommended 3-5mm outer diameter)
  • For USB 2.0 or earlier, whether connecting to Type-C or a legacy type on the other end, the usual four wire configuration is permitted (recommended 2-4mm outer diameter)

Source: USB 3.1 Specification @ usb.org -- specifically, the Universal Serial Bus Revision 3.1 Specification PDF available for download at the top of the page)

Also a great blog post explaining all the details about the Configuration Channel pin:


Archive.org (in case it goes offline)

share|improve this answer
Why not have it exactly rotationally symmetric and not have to worry about orientation whatsoever and cut down on pin count?? – ACD Aug 13 '14 at 18:25
@ACD to do that, you'd have to add four more wires after removing the two CC wires, which is two more than the wiring that detects orientation. – Funkyguy Aug 13 '14 at 18:36
@Funky I meant why care about orientation at all. If you made the connector like this instead: imgur.com/VKqyvJg it's the same number of pins and no need to have a controller change routing if it's plugged in one way or the other. – ACD Aug 13 '14 at 18:57
@ACD: Yes, as he mentioned earlier add four more wires after removing the two CC wires uses more pins and more importantly more wires (which makes the cable much thicker which is a bigger problem). This scheme is simply to reduce the number of pins by 2. I honestly don't see that as too much but the designers certainly have a different opinion. – slebetman Aug 13 '14 at 19:57
<s>The pins are rotationally symmetrical, so why does either end care which orientation it is? The CC pins aren't needed?</s> Ohh, because there are 2 transmit pairs and 2 receive pairs. – endolith Nov 18 '14 at 21:45

Since the cables are passive and are meant to be backwards compatible, the signals are duplicated top and bottom. This has the advantage of doubling the power pins and thus increasing current capacity.

share|improve this answer
So you also have every cable twice? Doesn't that make the cables pretty thick? Is that also the reason why they simply doubled the data rate for 3.1? They just have the double amount of everything? – Professor Sparkles Aug 13 '14 at 14:35

protected by Nick Alexeev Aug 13 '14 at 20:26

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.