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• The curiously complex taxonomy of USB plugs

  Marco Tabini • 14 hours ago • 0 comments

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  In the last decade or so, USB-C has become the dominant standard for power and data transmission. Despite the occasional attempt at proprietary formats (Lightning, anyone?), practically every device these days comes with the ubiquitous rounded port.

  Without a doubt, this success is well deserved: USB-C cables are easy to acquire and inexpensive, compatible power supplies can be had for less than the cost of an espresso beverage, and inserting a plug into a receptacle does not require a degree in quantum physics.

  These capabilities, however, come at a cost. USB-C receptacles and cables are surprisingly complex, and designing hardware around them can be challenging even if you decide to use off-the-shelf parts.

  Anatomy of a USB-C port

  Let's start by looking at a “full” USB-C port and its signals:

  Let's go through these in detail:
  

  • GND and VBUS carry power to and from a device. By default, only 5V is available, with analog signaling on CC1 and CC2 used to advertise or detect up to 3A of current. By using Power Delivery, however, devices can negotiate various voltage and current combinations up to 48V and 5A—a whopping 240W of power.
  • CC1 and CC2 can be used for four (!) purposes:
  • By manipulating the analog characteristics of these lines, a device can advertise 5V at either 500mA, 1.5A, or 3A without the need for more advanced digital negotiation. Default USB current corresponds to the legacy USB current level, typically 500 mA for USB 2.0 or 900 mA for USB 3.x.
  • One of the two lines can be used to provide VCONN, a special 5V power rail capable of providing up to 1W of power that can be used to power accessories independently of VBUS. This is useful because an accessory may need 5V to function, but then request a higher (or lower) voltage to be delivered on VBUS to power some of its peripherals or another device. Notably, VCONN does not normally pass through the cable end-to-end; it is used to power circuitry in the cable or plug assembly and is typically not presented as a usable rail at the far end.
  • The other line can be used for Power Delivery signaling. By using a dedicated serial protocol, devices can exchange information, negotiate a power contract to deliver a voltage different from the standard 5V alongside specific power and current capabilities, or enter Alternate Modes that repurpose some of the data lines in the cable for something other than ordinary USB communication. For example, monitors use an Alternate Modes to receive video data from a host device.
  • Because each line takes on a different role, their relative position is used to determine the orientation of the cable in the receptacle.
  • The SBU pins are used for “sideband” data. They provide an independent low-speed (up to 1MHz) channel that can be used for custom out-of-band communication between compatible devices. For example, DisplayMode Alternate Mode uses them for AUX signaling, and USB4 can also use them for sideband functions during link management and initialization.
  • D+ and D- are traditional USB 2.0 data lines. These represent a single differential pair and run as a twisted pair inside the physical cable, and are capable of serial speeds up to 480Mbps.
  • TX+, TX-, RX+, and RX- are SuperSpeed differential pairs. In a full-featured USB-C connection, these high-speed pairs can be used for protocols such as USB 3.x, USB4, Thunderbolt, or Alternate Modes. In the newest USB4 mode, the total link rate can reach 80 Gb/s symmetrically or 120 Gb/s in one direction with reduced bandwidth in the other. As I mentioned above, these lines can also be repurposed to carry non-USB data through Alternate Modes.

  If this seems complicated… well, it is. A good, if somewhat reductionist, way of looking at how all these lines relate to each other is:

  • D+ and D− are legacy USB 2.0 data lines. They are...

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