Over the last several months, I have been working on Dr. PD, an open-source USB Power Delivery analyzer and programmable sink. Diving deep into the world of USB-PD and USB-C has given me a unique appreciation for these rich technologies, and I want to share some of the things I have learned.

USB 3 and USB4 are designed to support a remarkably wide range of functionality. The same connector system can carry data at rates ranging from 12 Mb/s to 80 Gb/s, transport video at resolutions such as 2K, 4K, and 8K, and deliver substantial power: the latest USB Power Delivery specification supports up to 48 V at 5 A, for a maximum of 240 W.

To a much lesser extent, this was also true of USB 2.0. Its designers addressed the problem of supporting different capabilities, such as varying data rates and host-versus-device roles, by using different connector types for different applications. I think it's collectively accepted that this lead both to some of the ugliest connectors in the history of technology, and some of the most frustrated users in the history of humankind.

One plug to confuse them all

As far as USB 3 and USB4 are concerned, however, the USB Implementers Forum appears to have largely settled on USB-C as the connector that is here to stay. That creates a problem: how do you ensure that users have the right cable for a given application without making every cable unreasonably expensive?

This is not a small issue. If all USB-C cables had to support every feature of USB 3 and USB4, they would be significantly more expensive, regardless of whether they were being used to charge a $10 thermometer or connect to a $300,000 industrial machine. A cable intended to deliver, say, 30 W to an iPad has very different requirements from one designed to deliver 240 W to a high-end PC. Use conductors that are unnecessarily thick, and the cable is overbuilt for the iPad. Use wires that are too thick, and they're wasted on the iPad. Use wires that are too thin, and they are likely to melt and set a fire when you plug them into a power-hungry PC.

Thus, there is no single kind of USB-C cable. Instead, manufacturers produce different cable types for different application classes. The simplest may support only USB Power Delivery, perhaps up to 20 V at 3 A, and may not even carry USB 2.0 data. At the other end of the spectrum are cables that support USB4 data rates of up to 80 Gb/s, including active cables that incorporate signal-conditioning or retiming circuitry to maintain signal integrity over longer runs.

A computer in every cable

When you see some USB-C cables selling for $5 and others for $150, you are not necessarily being exploited by manufacturers intent on separating you from your money. Some of that price difference may well reflect marketing excess, but much of it is simply a consequence of how the USB-C ecosystem was designed.

However, this creates another problem: how can a device distinguish between different kinds of cables? If a cable rated for 100 W is connected to a source or sink capable of 240 W, there is an obvious safety concern. It would also be both impractical and unreasonable to expect users to know exactly what each of their cables can and cannot support. (I guarantee that, if you own several USB-C cables from different manufacturers, there is a very good chance that their capabilities differ substantially!)

The solution adopted by the USB Implementers Forum was to require many USB-C cables to include a small identification chip. This chip, called an e-marker, communicates over the CC lines and, when queried by a device, reports the cable’s capabilities, such as its current rating and supported data features.

Only cables designed to carry more than 60 W of power, or to support data rates above 480 Mb/s, are required to include an e-marker. This makes it possible to manufacture basic cables for low-power or low-speed applications at very low cost, while the added cost of an e-marker can be absorbed into the higher price of a more capable cable.

Interestingly, some cables contain two e-markers, one in each plug. These are used only in more specialized cable designs, such as certain active cables that include circuitry for signal conditioning or retiming over longer distances.

It's (almost) all about SOPs

If we keep peeling back the USB onion, the next challenge is one of message routing. If the cable uses the same CC line for communication as the two devices at its ends, how does everyone know who is talking to whom?

Naturally, the designers of the USB standard have thought about this, too (of course they have—otherwise, this whole thing would never work at all). Each message begins with a “Start of Packet” identifier, or SOP for short. You can see highlighted in green in this capture:

SOPs are sequences of four symbols composed of five bits (each symbol is called a K-code; the K stands for “control,” as opposed to “data” symbols). Different SOP combinations are used to identify the parties for which the message is intended:

• SOP (0x18 0x18 0x18 0x11) is used for communications between two devices.
• SOP' (SOP Prime, 0x18 0x18 0x06 0x06) is used for communication with the cable plug nearest the message originator.
• SOP'' (SOP Double Prime, 0x18, 0x06, 0x18, 0x06) is used for communication with the cable plug at the far end of the cable.

Thus, when a device wants to communicate with the cable plug nearest it, it uses an SOP′ message. The e-marker in that plug recognizes that the message is addressed to it and can respond accordingly, while the port partner ignores it. Simple.

…except that even this is not quite enough. Without some additional rules, a device could not distinguish between a cable with no e-marker at all and a cable that is supposed to have one but is malfunctioning and not responding. That would make it difficult to decide whether the cable should be treated as a lower-capability cable or as a faulty one that may be unsafe to use.

To overcome this limitation, the USB-C system adds one more layer of signaling in the analog domain. A cable that requires VCONN, such as an electronically marked or active cable, presents an Ra pull-down on the unused CC pin, nominally 1 kΩ and typically in the 800 Ω to 1.2 kΩ range. Detecting that resistor tells the VCONN source that a cable plug is present on that pin and that it should provide VCONN there, making communication with the cable electronics possible.

Some practical examples

Let’s look at what actually happens on the line when a source detects the pull-down resistor that indicates a VCONN-powered cable. As the capture below shows, once the sink attaches, the source first communicates not with the sink, but with the cable itself. That message is a Vendor Defined Message (VDM), specifically a Structured VDM such as Discover Identity, which USB Power Delivery uses to query the cable’s capabilities. Structured VDMs carry standardized formats defined by the USB PD specification, while unstructured VDMs can be used for vendor-specific communication between devices that share a proprietary protocol.

This particular Vendor Defined Message is a Discover Identity request. It tells the cable that the source wants its identity information, including the manufacturer’s USB Vendor ID (VID), the Product ID (PID), and additional data describing the cable’s type and capabilities.

The cable first responds with a GoodCRC message, which indicates that it has successfully received the message. It then sends a Vendor_Defined of its own, this time an ACKnowledgment with a lot of data about what it is and what it can do. The various bit fields in the data objects associated with this message provide a rich set of data about the cable's characteristics—which, thankfully, Dr. PD can decode for us.

As you can imagine, I have accumulated quite a collection of USB-C cables over the last few months, so let me show you what kind of information different types of cables report.

Exhibit A: The White Knight

The first cable we’re going to examine is this white oldie, which came with an Intel MacBook Pro. As you would expect from an Apple product, it is fairly well made and has survived multiple chewing attempts by several cats over the last decade.

Here's what it reports in response to an identification request:

* Structured Vendor Defined Message: DISCOVER_IDENTITY ACK
* Standard or Vendor ID: 0xFF00
* Structured Vendor Defined Message version: 0.0
* Discover Identity data:
    * USB Vendor ID: 0x05AC
    * Capabilities: modal operation
    * Certification identifier: 0x00000A26
    * USB Product ID: 0x1781
    * Device release number: 0x0100
    * Passive Cable Vendor Data Object:
        * Plug type: USB Type-C
        * Extended Power Range capable: no
        * Maximum bus voltage: 20V
        * Current capability: 5A
        * Latency: 10 ns to 20 ns (~2 m)
        * Termination: VCONN not required
        * Highest USB speed: USB 2.0 only

This cable offers a set of capabilities that is well matched to its intended use, since the MacBook Pro draws no more than 20 V at 5 A from it. It is a little disappointing that the cable supports only USB 2.0, but there is a good reason for that: higher-speed USB cables are generally limited to shorter passive lengths, after which active signal-conditioning or retiming circuitry may be needed to preserve signal integrity. USB-IF material, for example, describes passive USB4 cables as supporting up to about 0.8 m at Gen 3 rates and up to about 2 m at Gen 2 rates, while active cables are used for longer or higher-performance links. Apple therefore likely chose a reasonable tradeoff for an everyday cable: sufficient power capability, a useful length, and good mechanical durability, without the cost and complexity of active electronics.

Note that the cable correctly provides its VID (0x05AC, which is Apple's own identifier in the Vendor ID Database), PID (0x1781). To date, this is the only cable I have personally encountered that does so. As you will see below, every other cable I have examined reports 0x0000 for both fields.

Exhibit B: Dark Power

Next in our cable review is this hefty specimen from UGREEN, a company you may not have heard of but that has been around for well over a decade. This cable is marketed as capable of delivering up to 240 W, which requires a modern power supply and cable support for USB Power Delivery Extended Power Range (EPR). USB-IF states that USB PD Revision 3.1 extended the specification to support up to 240 W over USB-C.

* Structured Vendor Defined Message: DISCOVER_IDENTITY ACK
* Standard or Vendor ID: 0xFF00
* Structured Vendor Defined Message version: 1.1
* Discover Identity data:
    * USB Vendor ID: 0x0000
    * Certification identifier: 0x00000000
    * USB Product ID: 0x0000
    * Device release number: 0x0000
    * Passive Cable Vendor Data Object:
        * Plug type: USB Type-C
        * Extended Power Range capable: yes
        * Maximum bus voltage: 50V
        * Current capability: 5A
        * Latency: 10 ns to 20 ns (~2 m)
        * Termination: VCONN not required
        * Highest USB speed: USB 2.0 only

Note that the cable reports not only that it supports operation at up to 50 V and 5 A, but also that it is EPR-capable. Both matter: Under USB Power Delivery, Extended Power Range is a distinct capability. Advertising a high voltage alone is not enough; the system also needs to know that the cable is qualified for EPR operation. Without that, the connection still would be limited to Standard Power Range, with a maximum of 20 V at 5 A.

As an aside, I have found UGreen's products to be of excellent quality and offer really good value compared to their cost. This cable has a premium feel to it, and I have pushed much more than 5A over it without any problem. Their power supplies are also very, very good. (I have no affiliation whatsoever with them—I just like their products).

Unfortunately, this cable also only supports USB2, again probably because of its length.

Exhibit C: Lightning in a cable

For higher data rates, we need a different cable, again from UGREEN. This cable supports both EPR and SuperSpeed operation and is rated for data transfer at up to 40 Gb/s. That is not the highest USB data rate now defined, since the updated USB4 specification extends USB up to 80 Gb/s, but 40 Gb/s is still enough for many demanding display applications, including some dual-4K configurations depending on the transport mode and display setup. This is what it reports on the line:

* Structured Vendor Defined Message: DISCOVER_IDENTITY ACK
* Standard or Vendor ID: 0xFF00
* Structured Vendor Defined Message version: 1.1
* Discover Identity data:
    * USB Vendor ID: 0x0000
    * Capabilities: modal operation
    * Certification identifier: 0x00000000
    * USB Product ID: 0x0000
    * Device release number: 0x0000
    * Passive Cable Vendor Data Object:
        * Plug type: USB Type-C
        * Extended Power Range capable: yes
        * Maximum bus voltage: 50V
        * Current capability: 5A
        * Latency: <10 ns (~1 m)
        * Termination: VCONN not required
        * Highest USB speed: USB4 Gen3

As you can see, the much higher data rate comes at the cost of length, since this cable is only about 3 ft long. Even so, it is relatively inexpensive: at the time of writing, Amazon.ca lists it for around $12. For many applications that need USB4-class performance, such as connecting a monitor, dock, or external storage device, that length is probably acceptable. For longer runs, however, you often need an active cable or another higher-cost solution to preserve full high-speed performance, which is why longer USB4 cables can get pretty expensive.

Bonus specimen: The Red Rocket

Our final contender is this red cable, which came with a Pinecil soldering iron I bought a few years ago. It does not contain an e-marker and therefore does not respond to cable-identification messages from the source. You can tell that this is the case from the way the source advertises its power capabilities when the cable is connected.

* The source is reporting the following capabilities:
    * Supports EPR mode
    * Reports unconstrained power
* Fixed power profiles:
    * 5V @ 3A
    * 9V @ 3A
    * 12V @ 3A
    * 15V @ 3A
    * 20V @ 3A
* Programmable power profiles:
    * 5V–21V @ 3A PPS

As you can see, although the power supply is capable of delivering up to 5 A, it refuses to offer more than 3 A because the cable is not electronically marked. Under USB-C / USB Power Delivery rules, 5 A operation requires an electronically marked cable; otherwise, the source must treat the cable as a standard 3 A cable.

I should point out, incidentally, that while it's probably the least capable in electrical terms, this cable is of excellent quality; because it's designed to work with a soldering iron, it features a soft silicone sheath that is resistant to burns and bends easily to get out of your way when you're trying to reach deep into the bowels of a stubborn PCB.

It goes to show that fewer features don't necessarily translate into a less capable cable. Instead USB-C's flexibility allows manufacturers to make their investments where they matter and delivery functionality that's appropriate for the task at hand without driving the price higher.

The real problem is that, to end users, all of these cables look the same. Even when some advertise their capabilities through markings on the plugs, there is often no easy way to tell what they actually support, and troubleshooting a USB-C connection that does not work as expected can be very difficult.

Of course, that's where a project like Dr. PD comes into play! If you want your own, you can join our prelaunch page on Crowd Supply for product updates, or stay tuned here for more posts on USB-C, Power Delivery, and the hardware design behind Dr. PD.