Link Devices with Cables
Some of the key questions you’ll have about any cable and adapter are:
What standard(s) does it support?
What’s the highest throughput I can pass across it, and between which devices can I achieve that?
What’s the highest wattage power that will pass across it, if it’s a cable that can pass power? (Most can!)
Or, more simply, the plaintive cry I hear all too often: How do I figure out what cable I have?! This section will help.
The good news is that a truly universal cable has arrived for modern devices: a combination Thunderbolt 4/USB4 cable works with all USB-C connected standards, and the price of new cables has dropped. Jump ahead to Thunderbolt Capabilities for the spoiler.
Examine the Connector
Depending on your vintage of equipment, you are likely to only see cable or adapters with the following plugs, depending on the standard (see figures earlier in this chapter):
USB: 2.0 or 3.0 Type-A, 2.0 Type-B, 3.0 Micro-B, 2.0 Micro-B, and USB-C
Thunderbolt: Original version or Thunderbolt 2 (Mini DisplayPort style) or Thunderbolt 3 or 4 (USB-C)
FireWire: Appearing on Macs and some Windows systems from the late 1990s through the 2000s; see Appendix A: FireWire
DisplayPort or HDMI: An unchanged connector type across several years, regardless of the generation of standard used
Ethernet: Standard RJ-45, common for three decades
Audio: 3.5 mm or TOSlink
Let’s break that down further into the symbols that could appear to provide additional information, the variety of output formats supported, and charging aspects (where appropriate).
USB Capabilities
To understand what USB can deliver, start by taking a look at Figure 15 for a rundown of older USB symbols:
USB 2.0: The plain USB multi-prong symbol indicates version 2.0.
USB 3.0 and 3.1: The 3.0 standard started by using the SuperSpeed label for 5 Gbps, then added SuperSpeed+ for 10 Gbps in 3.1.
USB 3.2: 3.2 swapped in an explicit 5, 10, or 20 in numerals.
USB4: A partial arc and the numbers 20 or 40 replaced the SS logo. On some cables, you might see a stylized D (3rd from right in the figure below) to indicate DisplayPort support.
Fortunately, all those logos have taken a step or two into the past with the release of simplified symbols and branding in mid-2022. Instead of SuperSpeed/SS, the USB logo, and various numbers, the USB Implementers Forum (USB-IF) managing the specification have switched to USB plus a number plus Gbps as shown in Figure 16.
USB 3.2 Gen 2x2 and USB4’s 20 Gbps speed are both shown on a cable as “20Gbps” with a USB plug—no longer the pronged symbol—while USB4’s 40 Gbps flavor is just “40Gbps” and the plug. (I explore this further in The USB Standard.)
A USB cable with any of those markings and with USB-C on both ends will offer the highest possible data rate between two jacks, a host and peripheral or two hosts, that both devices support with one exception. The minimum negotiated data rate will be 10 Gbps, although actual throughput could be substantially less depending on the devices.
Unless the cable has a peculiar design flaw or is designed only for charging, a USB-C to USB-C cable made for USB 3.1 or later connections will otherwise always work at 10 Gbps or higher rates between two Thunderbolt USB-C jacks. Let’s look at higher speeds with USB4.
USB4 over USB-C
USB4 is technically a version of Thunderbolt 3 that’s fully compatible with older USB standards, as I explain ahead in USB Standards Evolution. As a consequence, USB4 is subject to certain speed limitations related to cable length and type: you can’t get more than 20 Gbps between current USB4 controllers using a cable that’s longer than 0.8 m (31.5 inches).
Almost any USB4 cable you purchase is compatible with Thunderbolt 4 (and vice versa), as I get into in Thunderbolt 4 Capabilities; nearly all come labeled and capable for Thunderbolt 4/USB4.
But where Thunderbolt 3 and 4 can carry 40 Gbps between compatible Thunderbolt controllers over cables as long as 2 meters (6.6 feet), USB4’s 40 Gbps controllers have the 0.8 m limitation because controllers can’t use so-called active circuitry in the cables that Thunderbolt 3 and 4 can, as described in Thunderbolt 4 Capabilities. As a result, a cable longer than 0.8 m only deliver 20 Gbps between two USB4 controllers.
A change is afoot, however, with the introduction of a higher-speed USB4 data rate: 80 Gbps, technically part of USB4 2.0, though you won’t see that in any labeling. A pair of connected USB4 80 Gbps controllers will be able to transmit:
Up to 40 Gbps over existing Thunderbolt 4/USB4 cables—no hardware change out required!
Up to 80 Gbps over future active USB4 cables, not yet introduced
Further, USB4 can be configured to deliver 120 Gbps for super-high resolution DisplayPort in one direction and 40 Gbps of data in the other. See USB and Thunderbolt Data for the rest of the story.
USB over USB Type-A/USB-C
A USB cable that has a Type-A connector on one end and USB-C on the other can only ever offer the fastest rate supported by that particular Type-A standard. It could range anywhere from 480 Mbps with a USB 2.0 jack on one side through 5 Gbps or 10 Gbps with USB 3.2 (USB 5Gbps or USB 10Gbps). Obviously, those are the maximum data rates for a cable with a Type-A plug to Micro Type-A.
USB Power Limits in Cables
USB cables have varying requirements about the amount of power they have to pass. In the early days, cables didn’t need to carry more than 2.5 W; the maximum output rose to only 4.5 W over Type-A in USB 3.0. USB-C ports and cables starting with 3.2 have to support at least 7.5 W, but may allow up to 15 W, 60 W, 100 W, or 240 W. I explain this further in USB and Thunderbolt Power.
Thunderbolt Capabilities
Thunderbolt has effectively had an unchanged logo since its introduction: a simple lightning bolt 
icon, later appended with a version number (Figure 17). The plug and jack type changed from Mini DisplayPort to USB-C between versions 2 and 3, marking the only other visible difference.
Maximum data rates increased in the first three generations:
Thunderbolt (original version): 10 Gbps
Thunderbolt 2: 20 Gbps
Thunderbolt 3 and 4: 40 Gbps
Let’s first look at Thunderbolt 3, because while the standard has been superseded by Thunderbolt 4, which improves on the spec in a variety of ways, an enormous array of Thunderbolt 3 computers, peripherals, and cables remain out there.
Thunderbolt 3 Capabilities
A Thunderbolt 3 or later port has to deliver at least 15 W of power, but may output up to 100 W for charging a laptop or other connected devices.
Though the Thunderbolt 3 spec seems to require that any cable capable of more than 15 W must carry up to 100 W, you can easily find Thunderbolt 3 cables advertised for sale as only providing up to 60 W. I’ve never found an explanation for this.
One other consideration with Thunderbolt cables? The length. The original Thunderbolt specification called for extremely long cable lengths, up to tens or even hundreds of feet, but would require fiber optic strands for that to work. By the time Thunderbolt was released, Intel and partners opted for copper wiring, which reduced the length to no more than 6.6 feet (2 m) in practice.
To achieve greater lengths, Thunderbolt 3 divides cable types into two varieties: passive and active. Passive cables require less circuitry in the plugs than active in a longer cable (and are thus less expensive), but the data rate may be halved.
With Thunderbolt 3, the lengths and data rates by type are:
Passive: 40 Gbps if it’s under 20 inches (0.5 m); 20 Gbps at up to 6.6 feet (3 m)
Active: 40 Gbps up to 6.6 feet (3 m)
Active cables have another quirk to them: because of the kinds of chips available for in-cable processing at the time the Thunderbolt 3 active specification was set, they can pass only Thunderbolt 3 data at the top rate. USB data is throttled back to the 480 Mbps speed of USB 2.0.
With Thunderbolt 3 or 4 at either end of a Thunderbolt 3 cable—note emphasis—the maximum 40 Gbps Thunderbolt 3/4 data rate can be reached. But with a USB-only controller of any generation at the other end—2, 3.x, or 4—you will always max out at 480 Mbps.
That’s right: it’s the worst square in the matrix of cables, standards, and compatibility. If you need USB speeds of 5 Gbps or higher, an active Thunderbolt 3 cable will not deliver it. Fortunately, that’s all in the past—or at least only affects older cables.
Thunderbolt 4 Capabilities
Thunderbolt 4 adds requirements to controllers and cables that prevent compatibility and confusion:
Port power: The standard requires that every computer have at least one jack with 100 W power output.
Cable power: All Thunderbolt 4 cables must pass up to 100 W.
Active cables: All Thunderbolt 4 cables are active and must work at up to 6.6 feet at 40 Gbps.
Compatibility: All Thunderbolt 4 cables must be compatible with USB 2.0 to USB4 standards at up to 40 Gbps. However, cables longer than 0.8 m won’t carry USB4 at 40 Gbps until new USB4 80 Gbps controllers appear, likely in 2023.
Thus for most purposes, a Thunderbolt 4 cable will support all Thunderbolt and USB standards and offer sufficient power and throughput in nearly all cases. Let me repeat that for emphasis.
Intel wants even longer Thunderbolt cables in the future, and notes in a presentation that 16.5 feet (5 m) to 164 feet (50 m) is the target. Corning currently sells Thunderbolt cables of those lengths that use optical fibers, but they don’t pass power, support only Thunderbolt standards, and cost from $350 to $500 based on length.
Future Thunderbolt Capabilities
Hot on the heels of the USB-IF release in October 2022 of the final specification for the 80 Gbps version of USB4 (USB4 2.0), Intel announced and demonstrated an “early prototype of next-generation Thunderbolt.”
USB-IF and Intel appear to be leapfrogging each other. Just like USB4 incorporated much of Thunderbolt 3, Intel will incorporate USB4 2.0 as the basis of Thunderbolt X. I imagine it will be called Thunderbolt 5, but we’ll see if Intel can avoid messing up a straightforward naming system in use for many years.
The next-generation Thunderbolt offers everything I listed back in USB4 over USB-C for the 80 Gbps flavor of USB4: 80 Gbps of symmetrical data, an option for 120 Gbps of video in one direction and 40 Gbps in the other, and support for existing passive Thunderbolt 4/USB4 cables up to 3.3 feet (1 meter). It will also support DisplayPort 2.1, also announced in October 2022—just like USB4 2.0.
Intel’s press release is headlined “Intel Leads Industry with Next-Generation Thunderbolt.” It leads by incorporating two standards developed by other bodies!
DisplayPort and HDMI Capabilities
DisplayPort and HDMI come in only a handful of varieties and include a lot of mix-and-matching between the two standard formats (Figure 18). Cables may have a full-sized DisplayPort plug on one end and a full-sized HDMI plug on the other. You might have a device that includes a Micro HDMI jack, and you plug a Micro HDMI-to-DisplayPort adapter into that jack to connect to a display via DisplayPort. (Apple cables and jacks feature the display 
icon.)
All that sounds great! And it sometimes works as expected. But the most frustration you might experience with connecting a device to a display is that the cable you have might be inadequate to the task and you’d have no way of knowing by examining the cable.
That’s because there are several generations of DisplayPort and HDMI. Some later generations require cables with different wiring elements or minimum wiring standards even though the connectors are identical. Each generation typically jumps a level in maximum support resolution, such as from 1080p to 4K to 8K. (I explain the differences between the standards in Video and Audio.)
Many cables have no markings on them whatsoever; others may have imprinted in plastic or note the exact specifications on an attached no-tear tag. Plugging the cable in between devices can be the only way to determine whether it works or not.
Here are the minimum standards you need to be incorporated into a cable type, adapter, or multi-standard cable for each common resolution and refresh rate (in hertz):
1080p: HDMI Category 2 High Speed; DisplayPort 1.0 (Reduced Bit Rate/RBR)
4K/30 Hz: HDMI Category 2 High Speed; DisplayPort 1.0/1.1 (High Bit Rate/HBR)
4K/60 Hz: HDMI Category 3 4K (Premium); DisplayPort 1.2 (High Bit Rate 2/HBR2)
4K/120 Hz (HDMI) or 4K/144 Hz (DisplayPort), 5K/60 Hz (DisplayPort), 8K/60 Hz (HDMI/DisplayPort): HDMI Category 3 8K (Ultra); DisplayPort 2.1 (Ultra High Bit Rate/UHBR)
A USB or Thunderbolt controller can also carry video over USB-C from a host device using a special mode that I explain in USB-C’s Alt Flexibility. A few displays have native USB-C connectors for USB-based or Thunderbolt-based video connections. Where that’s not the case, typically you can just plug in an inexpensive USB-C to HDMI or DisplayPort adapter or use a video port on a USB-C dock.
HDMI and DisplayPort jacks don’t pass power to attached displays, and displays don’t provide power to attached hosts.
Ethernet Capabilities
Modern Ethernet, dating back decades, all relies on the RJ-45, a click-in standard format similar to that used for phone wiring (Figure 19). The internal copper wiring is twisted-pair, referring to how internal sets of wires are carefully spiraled in pairs around each other to cancel out electromagnetic interference.
What you need to know about Ethernet cables and compatibility is literally painted (through dot-matrix ink spray or other printing) on the side of the cable’s insulator: the category number (Figure 20). You’ll refer to this number on a cable you own or while making a purchase to determine the maximum reliable data rate you can maintain between an Ethernet switch and device port. I’ll explain these categories, go through Ethernet’s data side, and offer troubleshooting advice in Networking.
Audio Capabilities
With all the profusion of standards associated with other kinds of cables, you will be happy to learn that the typical audio cables you use are very straightforward.
You’re likely to handle 3.5 mm (⅛ inch) analog cables, a nearly universal standard for analog audio output and input, including hybrid cables and ports that pass mic input and stereo output over the same single connection.
The number of black rings on the plug helps determine whether a cable is mono, stereo, or hybrid. In Figure 21, note the plug at left has three rings and thus is a “three-pole” plug: single-channel audio input and stereo (two-channel) output are carried over that plug when connected to a compatible jack. A non-hybrid jack will still send stereo audio output “ignoring” the third ring, farthest from the pole’s tip. At right in the figure is a two-pole plug, standard for stereo audio output. Some audio devices hard-wire their plugs into a cord attached to them.
It’s unlikely these days that you’ll run across a TOSLINK (or TOSlink or Toslink) connection. These optical-digital cables and ports were used to stream lossless digital audio before that was integrated into other kinds of connections, like HDMI for TVs. The connector was largely used and may still be found on some computers, the second- and third-generation Apple TVs, add-on audio cards for computers, and plug-in USB adapters (Figure 22).
