Monday, June 15, 2020

Computer ports: making sense of USB and Thunderbolt

Do you want to know how to connect an older peripheral to your computer? Are you confused by how Thunderbolt relates to USB-C? This tech article is designed to shed light on this acronym-ridden realm.

Universal Serial Bus (USB) is a series of standards that govern how peripherals (external drives, phones, printers, pointing devices, etc.) will connect to a host (computer, gaming console, etc.). USB consists of two distinct parts. The first part of the specification consists of numbered protocols that indicate the available features and transfer speeds. The second part consists of lettered standards specifying the physical ports.

USB Protocols

Here's a list of the historically available protocols, with their specification dates. Note that these protocols are backwards compatible. For example, USB 3 supports USB 2 features and transfer speeds. You can plug an older device into a newer USB port and it will function.

USB 1.0 (1996) used Type-A ports and implemented the so-called Low Speed of 1.5 Mbps.

USB 1.1 (1998) added a Full Speed of 12 Mbps and could also be found in Mini A and Mini B ports.

USB 2.0 (2001) added a Hi-Speed mode of 480 Mbps and implemented battery charging for the first time, supporting a maximum power of 5V at 1.8A. Micro USB ports began being used for smaller peripherals.

USB 3.0 (2011) added a SuperSpeed mode of 5 Gbps. Confusingly, this standard is also named "USB 3.1 Gen 1" and "USB 3.2 Gen 1".

USB 3.1 (2014) is also known as "USB 3.1 Gen 2" and "USB 3.2 Gen 2." It added a SuperSpeed+ mode of 10 Gbps. Three power profiles are available: 2A at 5V, 5A at 12V, 5A at 20V. This protocol is almost always implemented using Type-C ports. Where implemented, an "Alt Mode" allows connection to a DisplayPort monitor. Other features are found in proprietary implementations, such as Samsung's device daisy-chaining. This allows peripherals to be connected one to the next, instead of each connecting back to the host device.

USB 3.2 (2017) is also known as "USB 3.2 Gen 2 x 2" (no, really). It added a transfer rate of 20 Gbps that is nonetheless also labelled SuperSpeed+. You'd think people writing standards could do a better job of nomenclature!


On the Apple side of the fence, Thunderbolt was implemented in three versions. The original Thunderbolt (2011) used a physical configuration known as Mini DisplayPort and supported speeds up to 10 Gbps. Thunderbolt 2 (2013) increased the maximum rate to 20 Gbps.

Thunderbolt 3 (2015) was designed by Intel and first surfaced on Windows laptops. This was effectively a superset of the existing USB functionality, using the USB-C port. A rate of 40 Gbps is now available in cables under 50cm, or longer cables only if they are active (e.g. powered). Connection to two 4K DisplayPort monitors is supported at a refresh rate of 60Hz. Daisy chaining up to 6 devices is supported. You can connect PCIe devices such as external graphics cards.

The USB 4 protocol is the next standard to be rolled out. This integrates a superset of Thunderbolt and USB features, using Type-C ports exclusively. All Type-A and Type-B ports will be deprecated, as will the name Thunderbolt. USB ports will also be increasingly used for audio connections, powering, and other tasks.

Physical Ports

The original USB physical system used Type-A plugs for the host side of the cable and Type-B plugs on the peripheral side. Mini and Micro variants were added later, to facilitate connection to smaller devices. See the diagram at the top of this article for illustrations of the most common variants.

With the introduction of USB 3.0, both plugs and sockets were (usually) coloured blue to indicate support for the faster speeds.

Many companies created proprietary cables, with a Type-A plug on one side, customised connectors on the other. A common example was the 30-pin cables for iPhones and iPads.

Type-C connectors attempted to harmonise this disarray, by introducing a smaller, more powerful system. For the first time both plug ends are identical, so that cables are reversible. Type-C USB is used for both USB 3.1 and Thunderbolt 3. Sometimes a lightning bolt icon is used to indicate support for the extra features of Thunderbolt 3.

Mini DisplayPort is an obsolete video port created by Apple, used for both video connections and Thunderbolt 1 and 2 connections. Because there are two distinct functions for the same port, Thunderbolt support was once again indicated by a lightning icon.

Final Notes

When buying USB cables, be sure they support the standards you require. Cables that look identical may be electronically distinct. This problem rears its head in two common cases. First, support for different USB 3.x speeds requires specific Type-A cables. Second, only those Type-C cables that are certified with Thunderbolt 3 support should be used for that task.

If you have a computer with only Type-C ports, you can nonetheless get a Type-C to Type-A adapter cable, in order to use older peripherals with Type-B ports.

You cannot use a Thunderbolt 3 device in a standard USB Type-C socket. If you have a Type-C port on a current Apple computer, it will support Thunderbolt 3. If you have a Type-C port on a current Windows laptop, it will only support Thunderbolt 3 if this is explicitly specified. Read your docs.

Because both Thunderbolt and generic USB Type-C can be used for video signals, you can also find cables such as USB Type-C to HDMI.

Original images by Fred the Oyster, CC BY-SA 4.0


No comments:

Post a Comment