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What is an E-Marker in a USB Type-C Cable and How Does It Work?
Brendan Murphy

What is an E-Marker

An E-Marker (electronic marker) is a chip that is used in the latest USB connector iteration, USB Type-C, to communicate between power source and power sink devices. The chip is used to communicate with connected devices to ensure safe data and power delivery to and from the source and sink. The E-Marker provides the cable characteristics including the cable length, the maximum supported current and voltage, the type of USB signal, the vendor and product ID, any alternate mode support, and much more. An E-Marker is required on all USB Type-C cables that support 5 amps and/or exceed 60 watts of power carrying capability. USB Type-C cables that are expected to have data transfer rates above 480 Mbps, or High-speed USB 2.0, are also required to have an E-Marker chip embedded in the connectors of the cables. Applications that exceed 480 Mbps fall in the USB 3.1 realm, meaning any USB 3.1 cable is going to, with very minimal exceptions, be required, by the USB-IF community, to include an E-Marker in the Type-C cable.

black close up of usb type-c cable Image by Tomek from Pixabay

USB Type-C and Thunderbolt

Since the introduction of USB 3.0 version of the USB protocol and Type-C connectors, there has been a need to regulate how a cable works across all available applications. The Type-C cable was developed to simplify device connectivity and to create a cable that could be “universally” used across the majority of common applications. With its blazing fast transfer speeds of up to 10 Gbps, ability to continuously support up to 100W of power, and ultra-high video bandwidth capabilities, USB Type-C was built to make all other cables obsolete.

cable and power banks on a table USB Type-C cable and power adapters; Image by ready made via Pexels

With such an ambitious mission the Type-C cable had to be made in a way to work across a multitude of different applications. For instance, HDMI has been the protocol that is used primarily in the transfer of video data. The USB protocol was not originally designed to handle the display and broadcast of video streams to a monitor or TV so originally this application used, almost exclusively, an HDMI cable. Now however, USB Type-C and Thunderbolt are taking on the challenge of not only adding video streaming capabilities to the protocol but simultaneously powering the TV or monitor through a single cable input. This is an incredible feat and is extremely complex technically. With a single cable connection, one can not only stream ultra-high video content while also powering the device but they can also use that same cable to charge a smaller less demanding device such as headphones or a tablet. These two applications are governed by the same USB Power Delivery, or PD, protocol but require drastically different things from the same cable.

The Enumeration Process

Whenever a USB device is connected to a host device there is a communication that takes place called the enumeration process. The enumeration process is the initial “handshake” of the host and device. During enumeration the host device communicate with each other through the transfer of descriptors that inform the host what type of device has been connected, the device’s power requirements, and how to transfer data. For instance, when a USB mouse is plugged into a computer, during the enumeration process, that mouse sends information to the computer telling it that it is a HID (Human Interface Device) device, that it is bus powered, and that it is an optical mouse. The computer will then know how to interact with the mouse properly. All of this communication happens in a fraction of a second and is invisible to the user.

Connecting USB Type-C Devices

When a USB Type-C device is connected the initial enumeration process happens, like mentioned above, but this time there is an added step. Since Type-C devices can vary drastically in capability it is important that the cable is also included in the decision-making process. If the source and sink device are asking for 100W of power but the cable is only capable of a max of 10W then an issue arises. If the source and sink supply the full 100W of power, the cable will malfunction and things can get dangerous.  Now, prior to the enumeration process, a power delivery negotiation must occur that includes the source, sink, and the cable. This communication is made possible because of the microchip on the connector head of the cable, the E-Marker chip. The cable is able to tell the source and sink device what it is capable of doing and then the source and sink device adjust accordingly. The E-Marker acts as a middle man in the negotiation process and typically has the last word on how the source and sink device are going to communicate.

Structure of Power Delivery (PD) Transactions

When a Type-C cable is connected between a source and sink device, data packets are exchanged to define how power delivery, or PD, will be handled. PD transactions share a similar structure:

1. The Preamble allows the receiver to lock onto the carrier by sending a series of 1s and 0s enabling the receiver to sync to the transmitted clock.

2. The SOP defines the type of communication that is going to take place. This portion indicates whether a packet communication is between ports or a cable and a source. The SOP is used to identify the direction and participants in the power conversation. For transfers between two devices, SOP packets will be used.  For transfers between a source and a cable, SOP’ will be used.

3. The next part of the packet is called the header sequence. The header sequence indicates what originated the request (upward or downward facing port), the ID number of the transmission, and how many data packets will follow.

4. Data packets are sent next. These data packets contain information regarding Vendor ID, Product ID, and specific capabilities. This is the information that the E-Marker contains.

5. After the E-Marker is read a Cyclic Redundancy Check (CRC) packet is sent to ensure that data being sent is not corrupted and is received properly. This check confirms the integrity of the data packets being sent over the bus. At the end of this packet, there is an “end of packet” symbol (E) that confirms the end of the PD packet being sent.

Power Delivery (PD)  Negotiation

These PD transactions make up the power delivery negotiation process. The PD negotiation process must first address the cable, this is called SOP prime. Fully featured Type-C cables are required to have an E-Marker that indicate power carrying capabilities. The SOP’ negotiation is required for any power contracts above 3 Amps or 60 watts. Once the cable capabilities are determined, the PD negotiation process can continue. The PD negotiation process can be simplified into eight stages.

  1. The Source declares its power capabilities as constrained by the cable.
  2. The Sink issues a CRC confirming receipt of the message.
  3. The Sink then issues a request for the appropriate power option.
  4. The Source issues a CRC confirming receipt of the request.
  5. The Source accepts the Sink request.
  6. The Sink issues a CRC confirming receipt of the acceptance.
  7. The Source issues the PS ready.
  8. The negotiation power level is delivered.

A device can fail at any of the above steps. If a failure occurs, power will not be delivered and the device may not be recognizable to the source or sink device. This process ensures the safety and reliability of the connected devices enabling safe usage. Luckily there are some tools to help engineers and developers ensure and guarantee that the cables they make are safe, reliable, and ready for manufacturing.

USB Type-C Testing Tools

The Advanced Cable Tester v2

Total Phase has seen the rapid adoption of the Type-C connector and has built a tool to help manufacturers ensure the safety and reliability of their cables. The tool is called the Advanced Cable Tester v2. The Advanced Cable Tester v2 is the quickest and most convenient way to comprehensively test USB, Lightning, and video cables. It provides:

  • Short testing - Dynamic visualization of test results
  • Open testing - Dynamic visualization of test results
  • Continuity testing - Dynamic visualization of test results
  • DC resistance measurement for most wires, with milliohm precision on ground and power wires, ohm resolution on most other wires, continuity check only on high-speed data lines
  • Signal integrity testing of data lines up to 12.8 Gbps per channel
  • E-Marker verification
  • Lightning plug bring up/function (Apple Lightning Cables)*
  • Over-voltage protection (Apple Lightning Cables)*
  • Voltage recovery testing (Apple Lightning Cables)*
  • Quiescent current consumption (Apple Lightning Cables)*
Advanced Cable Tester v2 face plates Advanced Cable Tester v2

The Advanced Cable Tester v2 is the fastest, most comprehensive cable tester on the market. With its rugged design, low cost per test, and easy to understand reports, the tool is ideal for lab and production environments. Regardless of the application, the Advanced Cable Tester v2 will provide high precision test coverage without the need for custom tools, highly trained personnel, or expensive oscilloscopes. This tool has the potential to save hundreds of thousands of dollars in overhead and recall costs.

The tester runs a series of different tests when a cable is connected including continuity, DC resistance, E-Marker reading, and signal integrity.  During the signal integrity test, the Advanced Cable Tester v2 is able to test signal integrity up to 12.8 Gbps per channel and render eye diagrams typically only seen with expensive scopes.

Advanced Cable Tester v2 Signal Integrity test eye diagrams Advanced Cable Tester v2 Signal Integrity test eye diagrams

Another important test ensures the presence of an E-Marker on a USB Type-C cable. The E-Marker test takes the expected values of the USB specification and compares those values with the actual values programmed into the cable’s E-Marker to ensure a match in data. If the measured values do not match, the Advanced Cable Tester v2 will fail the cable. The E-Marker test also presents a few additional fields including Product ID, Vendor ID, and Test ID.


The E-Marker is an extremely important component to be considered in all USB Type-C cables. It ensures that connected devices are operating at a level that is safe and reliable. With tools like the Advanced Cable Tester v2 and USB Power Delivery Analyzer, engineers are able to dive into test results and create the best cables for the market while ensuring safety and reliability.