A USB Billboard device is a specific type of USB device, defined by the USB Billboard Device Class, used primarily in USB Type-C connections to inform a host system (like a computer) about the capabilities of a connected device, particularly regarding Alternate Modes.
Think of it as a digital sign or message board for your USB-C connection. When a USB-C device supports various functions beyond standard USB data transfer – such as carrying DisplayPort, Thunderbolt, or HDMI signals over the USB-C cable (these are "Alternate Modes") – the Billboard function is used to tell the host what modes are supported and what happens if a preferred mode cannot be activated.
Understanding the Role
The core purpose of a USB Billboard device stems from the versatility of the USB Type-C connector and the Alternate Mode feature. Not all hosts and devices support the same Alternate Modes. When you connect a device, a negotiation process occurs. If this negotiation fails for a specific mode (e.g., trying to output video via DisplayPort Alternate Mode, but the host or cable doesn't support it), the Billboard device steps in.
As defined by the standard:
The USB Billboard Device Class definition describes the methods used to communicate the Alternate Modes supported by a Device Container to a host system. This includes string descriptors that can be used to provide support details in a human-readable format.
This means the Billboard device communicates specific information to the host using predefined messages and structures. These messages can include string descriptors, which are text strings that provide details in a format easily understood by a person.
Why Are USB Billboard Devices Needed?
Before the Billboard class, if an Alternate Mode negotiation failed, the user might simply see the desired function not work without any explanation. A USB Billboard device provides crucial feedback, improving the user experience by:
- Communicating Supported Modes: Letting the host (and user) know exactly which Alternate Modes the connected device is capable of supporting.
- Explaining Failures: If a desired mode doesn't activate, the Billboard device can provide a reason (e.g., "Insufficient power," "Host does not support this mode," "Cable not compatible").
- Providing Additional Information: Including details like device identifiers, firmware versions, and links to support resources or driver downloads using those human-readable string descriptors.
How It Works
When a USB Type-C connection is established that involves Alternate Modes, the host queries the device. If the device incorporates a Billboard function, it can respond with details about its Alternate Mode capabilities.
- Device Capabilities: Lists the supported Alternate Modes (e.g., DisplayPort, HDMI, Thunderbolt).
- Status Information: Reports if an attempt to enter a specific mode failed and why.
- User Notification: The operating system on the host can then use this information, often displaying it to the user via a notification pop-up or within device properties.
Examples of Devices
You'll typically find the Billboard function integrated into devices that utilize USB Type-C Alternate Modes, such as:
- USB-C Docks and Hubs: These often support multiple Alternate Modes for external displays, networking, etc.
- USB-C Monitors: Capable of receiving video and data over a single USB-C cable using Alternate Modes.
- USB-C Adapters: Adapters that convert USB-C to DisplayPort, HDMI, or Ethernet may incorporate a Billboard function.
- Laptops and Tablets with USB-C: While often acting as the host, they also have USB-C ports that might need to communicate capabilities if acting as a device in certain scenarios, or more commonly, the peripherals connected to them use Billboard.
Benefits of the Billboard Class
| Feature | Benefit |
|---|---|
| Clear Communication | Users understand device capabilities and connection issues. |
| Improved Debugging | Easier to troubleshoot why a feature (like video out) isn't working. |
| Standardized Info | Provides a consistent way for devices to report capabilities. |
In essence, a USB Billboard device provides essential transparency in the complex world of USB Type-C Alternate Modes, acting as a crucial link between the capabilities of the device and the understanding of the host and user.
