Modern Macs make extensive use of the USB-C connector for interconnecting with external hardware like input devices, storage devices, and displays. This connector has a lot going for it; it’s versatile, durable, symmetrical, and small. Unfortunately, with it comes a great deal of confusion and difficulty. The goal of this tutorial is to provide you with a full working knowledge of the USB-C connector and everything it can do.
Prior to the invention of USB-C, it was generally true that each type of connector found on a computer were used for one thing only, so you could just look at the connector and know what it was for. This wasn’t always true, but it was true enough to prevent mass confusion. The USB-C connector broke from this tradition rather sharply.
Before USB-C, the term “USB” referred to both a set of physical connector types and to the type of signal that was being used by those connectors. USB-C, on the other hand, is only the name of a connector. There is no protocol or signal called “USB-C.” Had they simply named it the “C” connector, or really any other name, a considerable amount of confusion could have been prevented.
On top of that, the USB-C connector is used for more than one communication protocol, unlike most of its predecessors, and the protocols themselves contain other protocols, like digital Matryoshka dolls.
To un-nest the dolls and make sense of things, we’ll discuss the physical design of the USB-C connector, and the three roles that it fills: USB, Thunderbolt, and USB Power Delivery.
USB-C connectors have 24 electrical contacts, called “pins.” These pins are connected to wires within the cable, although not all connections are mandatory:
Any USB-C cable which includes all optional connections is considered a “full-featured” cable. Cables that do not include all optional connections do not have a special name, they are just not full-featured cables.
USB, which stands for Universal Serial Bus, is a set of closely-related standards that are all, basically, ways to send data back and forth between a computer and a device connected to that computer. Ever since it was invented in the late 90s, USB has always embraced the concept of different classes of device, because the essential premise of USB is “you just plug it in, and the machines will figure out what’s supposed to happen next.” This is a worthy goal, and a frankly ambitious one. It works well most of the time; you can plug a mouse, a printer, and a synthesizer into the same computer at the same time, using the exact same type of cable, and pretty much everything works without a hassle.
When the USB-C connector is used for USB, it’s using either USB 3.2 or USB4 (note the deliberately missing space between “USB” and “4”… we don’t make these rules.) These two versions of the USB protocol are similar but not identical.
USB 3.2 was released in 2017 and supersedes all previous versions of USB. It defined four data transfer modes with confusing names:
It may be useful to know that while USB 3.2 Gen 1 and USB 3.2 Gen 2 can be used with various types of USB connectors, Gen 1x2 and Gen 2x2 can only be used with USB-C connectors.
Modes with the “x2” in their name use all four high-speed data connections, which means you’ll usually need a full-featured USB-C cable to use them. Modes without the “x2” use only two of the high-speed data connections.
USB4 was released in 2019 and is much more powerful than USB 3.2, but with great power comes great confusion. The best way to wrap your head around USB4 is to think about it from the perspective of someone planning to build a device that uses USB4. Here is a list of things that person would learn by reading the USB4 specifications:
There are two data transfer modes that are mandatory for all USB4 devices:
There are at least twelve other modes that USB4 devices may support, but don’t have to:
USB4 hosts (i.e. computers) must support at least Host-to-Host communication and DisplayPort Alternate Mode. USB4 docks and hubs must support at least Tunneled PCI Express and Thunderbolt Alternate Mode.
Clear as mud, right?
Tunneling, in this context, is the nesting protocol-within-protocol that was mentioned above. When one protocol allows tunneling of another, the data that belongs to the “inner” protocol is encapsulated into data packets belonging to the “outer” protocol and sent along with any other data that the outer protocol is sending. When the encapsulated packets get to the other end of the connection, they are de-capsulated and reassembled to re-form the stream of data that entered the tunnel. Tunneling can increase the average speed of transmission because it does not require switching the connection back and forth between modes, although tunneling usually has a lower absolute speed since it necessarily does not use the full bandwidth of the connection.
Alternate modes, by contrast, achieve the same end result as tunneling (allowing more than one type of communication along a single connection) but instead of the inner/outer protocol concept, they electrically switch one or more of the high-speed data pairs for dedicated use by the alternate mode. As a result, using an alternate mode necessarily reduces the amount of bandwidth available for a regular data connection, since some of that bandwidth is being dedicated to the alternate mode connection. In exchange, they have the highest possible potential speed.
The USB specification allows for any number of alternate modes in theory, but there are only five that exist as of the writing of this manual:
Thunderbolt is similar to USB on its face; it’s a way to connect devices to a computer. Thunderbolt has a fundamental conceptual difference, however, which might feel technical and vague at first, but which is essential to unraveling the mysteries of the USB-C connector.
As you may be expecting at this point, Thunderbolt is not really a protocol in and of itself; rather, it’s a set of hardware that combines two preexisting protocols plus a supply of electrical power into a single cable. The two protocols that it combines are PCI Express and DisplayPort which can each be described briefly, mercifully:
Thunderbolt allows high-bandwidth devices which previously could only be built as PCIe cards and installed in desktop computers to be built as portable stand-alone devices powered by the same cable that provides the data connection.
Thunderbolt version 1 and version 2 used the Mini DisplayPort connector. Thunderbolt version 3 and version 4 use the USB-C connector only.
Thunderbolt 3 specifies:
Thunderbolt ports on a device are connected to the internal circuitry of that device through a Thunderbolt controller chip. Controllers come in three forms: a double port controller, a single port controller, and a low power controller.
If a Thunderbolt connection is made between a computer and a non-display device, like a hard disk or an audio interface, then the full 40 Gbps is available to the data connection. If a display is connected to the same Thunderbolt controller, however, the amount of bandwidth available for general data is limited to whatever’s left over after the display gets what it needs. Note that these limitations are per-controller, not per-port.
Finally, USB-C connectors are used by the USB Power Delivery standard, which requires hosts to supply a minimum of 1.5 amps at 5 volts DC per physical port.
Hosts can optionally (of course there are options) supply higher amperages and voltages as well, all the way up to 5 amps at 48 volts (240 watts).
The Power Delivery specification includes a system of signaling and physical wiring which prevents devices from receiving more power than they are designed for. This is accomplished by always supplying the base amperage and voltage, and only raising the amount of supplied power if the device requests it.
Separately from the three main uses of the USB-C connector, there are also two “accessory modes” which are supported directly by the USB-C hardware design and which do not involve software at all:
Any device which supports all optional USB4 functionality is, officially, a Thunderbolt 4 device. Since one of the USB4 options is Thunderbolt Alternate Mode, which is Thunderbolt 3, the definition of Thunderbolt 4 is therefore: Thunderbolt 3 + USB4.
One of the stated goals of the convergence of USB4 and Thunderbolt was to reduce confusion. It is left to the reader to decide whether this goal has been met, or even approached.
If your head is spinning by this point, you are not alone. One connector is used for multiple protocols, each of which contains other protocols, many of which include multiple different ways to achieve the exact same results using either identical protocols implemented in different ways, or different protocols whose differences are necessarily invisible to the end user. It is mind-boggling. In the hopes of assembling some order out of this chaos, the following recommendations can be used:
When shopping for docks or hubs:
When shopping for video adapters (DisplayPort, HDMI, MHL, DVI, or VGA):
When shopping for ethernet adapters:
Gbps stands for gigabits or billion (1,000,000,000 or 109) bits per second. Why is speed expressed in bits when file sizes are expressed in bytes? The reason is that while there are typically 8 bits to a byte in file storage, data transfer protocols can use different numbers of bits to make up a byte, and have different quantities of overhead that eat up portions of the total capacity. Measuring data transfer protocols in bits, not bytes, per second makes it easier to compare any two protocols accurately, even though it makes it harder to understand how long it will take to transfer a file of a known size.↩
Evidently, as of January 11, 2023, HDMI Alternate Mode has been discontinued. Apparently no actual USB-C cables or adapters using HDMI Alternate Mode were ever manufactured; all USB-C adapters which output HDMI use DisplayPort in some form. As such, we likely will never know just how useful HDMI Alternate Mode might have been.↩
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