The various kinds of cables connecting to a Mac or PC can be confusing, as this article by Brian Livingston of the "Windows Secrets" Newsletter discusses. This might be an interesting topic for a computer club meeting.

Jim Hamm

It used to be that you could run any old USB cable between just about any two USB ports, and the devices on each end would simply work. But that hasn't been true for a long, long time.

As more and more manufacturers wanted to bring different devices with different needs to market, the standard USB-A cable was lost in the shuffle. Instead, we got a gaggle of novel USB connectors named Mini-A, Mini-B, Micro-A, Micro-B, Apple's similar-but-different Lightning, and more.

The latest connector — USB-C, finalized in 2014 by the USB Implementers Forum (USB-IF) — was supposed to bring these multiple shapes and sizes to an end. The sexy USB-C connector was the bell that got all the PC nerds salivating. USB-C was the first reversible USB connector: there was no "top" or "bottom" side for users to grapple with.

So-called ‘standard’ USB-C cables are a mess whose time is over

Unfortunately, USB-C cables are anything but identical. End users are expected to know the following differences (and more) among the cables:

Charging is supported by some USB-C cables, while others support only data.
Some USB-C cables can deliver 100 watts of power, while others deliver only 60 watts.
DisplayPort monitors are supported by some USB-C cables, but not others.
Some USB-C cables have data throughput ratings of 40Gbps, while others are rated for only 20Gpbs, 10Gbps, or even just 5Gbps. (These four speeds correspond with specs that, in reverse chronological order, are called USB4, USB 3.2 Gen 2x2, USB 3.2 Gen 2, and USB 3.2 Gen 1. Confusing, eh? Tripp-Lite has more details.)

Cheap USB-C cables gave the spec a bad name. In 2015 and 2016, Google engineer Benson Leung repeatedly posted tests showing that some laptops couldn't be charged by certain cables, contrary to what their makers claimed. One cable was so badly designed that it instantly fried a Chromebook Pixel laptop's two USB-C ports, the machine's embedded controller, and two USB power-delivery analyzers. That was enough to render each device useless, as described in an ArsTechnica article.

Even Apple was forced to recall, in early 2016, some USB-C charging cables it had shipped with MacBooks. The cables failed to actually charge the laptops, according to a Guardian article.

Faced with these public-relations disasters, USB-IF launched a new "power delivery compliance plan" in June 2016, as described in an EDN analysis. But the writing was on the wall for this kind of cable anarchy. The next stop on this train would be Thunderbolt.

Thunderbolt cables will eventually eliminate all USB cables

Thunderbolt is a data-transfer and power-delivery spec that has been shepherded into existence by Intel since Thunderbolt 1 appeared in the 2011 MacBook Pro. That version, and Thunderbolt 2 in the 2013 MacBook Pro, used a Mini DisplayPort connector. Both versions 1 and 2 were proprietary to Intel and required manufacturers to pay for a license

Thunderbolt 3 (TB3) laptops began shipping in late 2015 from Acer, Asus, HP, Dell, Lenovo, Sony, and others. Finally, in May 2017, Intel said it would make Thunderbolt 3 royalty-free. This allowed USB-IF to accept the spec in March 2019 and incorporate it into the latest version of its own standard, which is now called USB4. (There is no space between the letters and the numeral.)

Intel retained compliance rights over Thunderbolt and now attempts to ensure strict device certification for Thunderbolt 4 (TB4), the current spec. (Thunderbolt 4 cables have a symbol and the numeral "4" on the end, as shown in the photo above.) Among other things, devices with TB4 ports have the following requirements:

A maximum data-transfer rate of no less than 40Gbps, the same as Thunderbolt 3 ports. (All speeds in this article are theoretical, not actual.)
The ability to support two 4K monitors at 60Hz, which can be daisy-chained, or one 8K monitor at 60Hz (TB3 required support for only one 4K monitor).
Direct Memory Access protection, to prevent physical Thunderspy attacks that TB3 was susceptible to (this was patched in Windows 10 version 1803 in 2019 and macOS Sierra 10.12.3 in 2017).
At least one computer port must support charging (TB4 can deliver up to 100 watts).
Support for PCI Express (PCI-e), a common computer data bus, to connect to high-speed devices such as solid-state drives and video-capture devices.

Figure 1, straight from Apple's website, shows the back of a Mac Mini labeled with two "Thunderbolt/USB 4" ports. Apple can't call them "Thunderbolt 4" ports because neither port supports two 4K monitors, as required for compliance.

Figure 1. Apple labels its current Mac Mini (shown above) and MacBook Pro 13 computers as "Thunderbolt/USB 4" ports, not "Thunderbolt 4" ports. Source: Apple Mac Mini and MacBook Pro 13 Web pages

Here’s the shocker: TB3 is better than TB4 for some things

USB-IF finalized USB4 in 2019. USB4, which is a specification, should not be confused with USB-C, which is a hardware connector. USB4 is called "Thunderbolt 4 Light" by industry wags because USB4 is a "loose" standard by comparison. For instance, all Thunderbolt 4 ports must support a 40Gbps data-transfer rate. But a manufacturer can call a device "USB4" if its data-transfer rate is as low as 20Gbps. (USB4 also allows three other protocols: USB 3.2 Gen 2 supports 10Gbps, USB 3.2 Gen 2x2 supports 20Gbps, and USB 3.2 Gen 1 supports only 5Gbps.)

To make matters worse, USB4 computer ports aren't required to work with TB3 or TB4 peripherals. Support is optional. Therefore, a "certified USB4 computer" may or may not work with TB3 or TB4 monitors, docking stations, and hubs.

Buying devices that have TB3- or TB4-certified ports gives you the best shot of having a system that will keep up with greater and greater bandwidth requirements in the years to come.

IMPORTANT: TB3 is faster than TB4 at certain tasks. As mentioned above, the main reason for this is the way each spec implements its PCI Express data bus:

Both Thunderbolt 3 and Thunderbolt 4 support data transfers up to 40Gbps.
TB4 supports one PCI-e lane, allowing 700 megabytes per second.
The older TB3 supports four PCI-e lanes, allowing four times the bandwidth: 2,800 megabytes per second.
TB4 devices have fewer PCI-e lanes as a trade-off. The designers of TB4 specified only one lane of PCI-e so that TB4 devices could support as many as three directly connected devices. TB3 supports only one such device.

The speed of PCI-e makes a difference to you only if you use a Thunderbolt cable to connect a solid-state drive or other high-speed external device to a computer. However, it's likely that few manufacturers will build TB4 into such bandwidth-hungry devices. That's because USB Gen 3.2 and USB4 ports and cables can provide similar data-transfer rates at a lower hardware cost.

Sonnet Technologies, an Irvine, California, company, manufactures Thunderbolt 3 and 4 peripherals, such as the forthcoming Echo 11 Thunderbolt 4 Dock (see Figure 2). The "Echo" trademark is unrelated to the "smart speaker" of the same name that Amazon sells. Meanwhile, the "11" in the name refers to the number of ports the dock has, not the version number of the device.

"There will be no Thunderbolt 4 peripherals, except for docks and hubs, because a Thunderbolt 4 peripheral could use only one lane of PCI-e," Sonnet CEO Robert Farnsworth said in a telephone interview.

That doesn't mean either spec is obsolete, Farnsworth explains: "Thunderbolt 3 and Thunderbolt 4 each can do something that the other cannot, so we expect a long life for both."

Figure 2. The Sonnet Technologies Echo 11 Thunderbolt 4 Dock supports 11 ports, including Thunderbolt 4, USB 3.2 Gen 2, and other interfaces. Source: Sonnet Technologies product page

So what's an end user who just wants a fast, reliable system to do? Like everything else, there are complex rules to remember:

TB3 and TB4 cables are interchangeable. "You can mix Thunderbolt 3 and Thunderbolt 4 cables and peripherals in any order and they work," Farnsworth explains.
There are different lengths of Thunderbolt cables. The standards are:
— A "passive" TB cable can be only 0.8m (2.6 ft.) for 40Gbps transfers.
— A "passive" TB cable that's between 1m and 2m supports only 20Gbps.
— A TB cable with an "active" chipset can be 2m in length and still support 40Gbps.
— Fiber-optic 40Gbps, TB cables are available in 5m, 10m, and 15m lengths.
TB3 and TB4 ports are both fast enough for almost anything you might connect. Devices such as monitors, keyboards, mice, and printers won't outrun a TB3 or TB4 port.
Only truly high-speed devices — such as external graphics processing units, SSDs, and the like — need maximum bandwidth. If you buy one of these devices to connect to your computer, you'll probably find that the products on the market use some flavor of USB 3.2, USB4, or TB3.

What type of Thunderbolt port do you really have in your computer?

The final factoid you need to know is what kind of Thunderbolt ports you have in your laptop or other computing device. Just to make you crazy, Thunderbolt ports on PCs are indicated by small lightning-bolt icons, but the version (3, 4, etc.) is not always printed alongside that logo.

The basic rules for Windows PCs and Macs are:

Most Windows PCs that were sold in 2016 or later have one or more USB-C ports that will support TB3, TB4, USB 3, and USB4 devices.
All Apple computers that were sold in 2016 or later (except MacBook) have USB-C ports that will support TB3 and, when upgraded to Big Sur, will also support TB4 and USB4 devices.

You can determine the support level of a Windows PC using a simple PowerShell command. Sonnet Technologies explains this in a convenient two-page PDF. The document provides the command, as well as listing versions of Thunderbolt support for specific PCs and Macs. You can download the PDF for free.

Intel hosts a search engine at its Thunderbolt site for certified TB3 and TB4 devices, but the hits are surprisingly sparse. The search tool claims that only three laptops have been TB4-certified, for example, although more than that are compliant.

Thunderbolt 4 cables are replacing our rat's nest of old USB-A and USB-C cables, in the same way that USB thumb drives once wiped out floppy disks. Until the transition is complete, however, we need to ask a lot of questions of any provider who claims to be offering full Thunderbolt compliance.