What Happens When You Connect a USB-C Cable

The USB Type-C® connector is designed to be very simple for the user to use: you insert it in either orientation, and a multitude of services just “work”. Though simple to use, it is a complicated connector to program and test, with a very complex system of protocols behind it. There is USB power delivery (USB-PD) and multiple rates of USB data delivery from USB 2.0 through USB4®, specified by the USB Implementers Forum (USB-IF®). There are protocols other than USB, such as DisplayPort™, High-Definition Multimedia Interface (HDMI™), Peripheral Component Interconnect Express (PCIe®), Base-T Ethernet and Thunderbolt™. 

So, what actually happens when you connect a USB-C cable? To understand that, first let’s take a look at the signals and pin assignments in the USB-C connector receptacle (Figure 1).

Figure 1: The USB-C receptacle pin assignments showing the key signals used for device-to-device communications. Related pins have matching color overlays.

USB4 Alt-Mode Testing: DPAUX and USB-PD

Figure 1: The interfaces that make up the USB-C connector and their relationships.

The USB Type-C® (USB-C) connector supports not only USB data transfers at speeds of up to 20 Gb/s but also can be reconfigured to support a wide variety of other interfaces through the USB4 provision for alternative (Alt) modes. The Alt-Mode protocols supported include Thunderbolt™, Mobile High-Definition Link (MHL), Peripheral Component Interconnect Express (PCIe®), High-Definition Multimedia interface (HDMI®) and DisplayPort™ (DP).

The diagram of the USB-C receptacle in Figure 1 shows where it potentially switches from normal operation into Alt-Mode.

Testing DisplayPort 2.0 vs. USB4 Over USB Type-C Connectors

Figure 1: The pin out of the USB Type-C connector.

DisplayPort™ 2.0 (DP 2.0) is a high-resolution video interface and USB4® is a high-speed data interface; what they have in common is the USB Type-C® (USB-C) connector. While DP 2.0 can also be deployed on the standard DisplayPort as well as mini-DisplayPort connectors, it is the USB-C connector that really excites electronics manufacturers because now they can use a single connector for high-speed data, high-resolution video and even power distribution. 

Figure 1 shows the pin assignments for a USB-C connector, which is a mechanically reversible connector that includes four, high-speed differential data lines: TX1, TX2, RX1 and RX2.

In USB4 operations, the four data lines TX1, TX2, RX1 and RX2 form a dual-lane, duplex signal path, supporting 10 and 20 Gb/s transfers on each line. When operating in USB4 Alt mode, up-to-four of these buses can be reassigned to become four DP 2.0 video lanes, which operate at 10, 13.5 or 20 Gb/s.

Testing for both interfaces over the USB-C connector is similar, but there are some notable differences.

USB Type-C Cable Detection

Figure 1: The four possible states of
connection for a USB Type-C cable

Having examined some of the basics regarding the USB Type-C connector and the Power Delivery 2.0 specification that complements the Type-C spec itself, we'll turn our attention to the topic of cable detection. The Type-C connector is reversible, which is to say there are four ways a cable can be connected between upstream- and downstream-facing ports (Figure 1). Not only that, but there are different types of Type-C cables. So how do the devices discern what's between them?

A Look at USB Type-C and Power Delivery

Figure 1: The USB Type-C
connector alongside its
Micro-B counterpart

With USB 3.1, the latest iteration of the serial-data protocol, comes a new smaller and universal connector: USB Type-C, the USB-IF's answer to Apple's Lightning connector (Figure 1). Even Apple itself has adopted USB Type-C for its latest MacBooks, a rare show of support from Cupertino for an open standard. Like Lightning, USB Type-C is reversible, but it offers other interesting features, such as the ability to handle other protocols using "alternate modes." It also incorporates the new USB Power Delivery specification for improved power-supply capabilities over USB.