alternatemode

Alternate mode, often written Alt Mode, is a feature of the USB Type-C specification that allows a USB-C port to carry non-USB signaling in addition to standard USB data. The most common use is to transmit video and audio signals, enabling displays and other devices to connect directly through a USB-C port without separate adapters for certain protocols.

Operation and purpose: Alt Mode is negotiated between the host (source) and the connected device (sink) via the USB-C configuration channel. When both ends and the cable are capable, the system switches the USB-C connector into a chosen alternate mode and routes the appropriate high-speed signals over the USB-C cable. The success of Alt Mode depends on device support, cable certification, and proper negotiation; incompatible cables or devices will not enter Alt Mode.

Common implementations: DisplayPort Alternate Mode is the most widely deployed Alt Mode, carrying DisplayPort video (and audio) over USB-C to a monitor or docking station. HDMI Alt Mode has been defined but has seen limited practical adoption, with many setups using adapters or native DisplayPort over USB-C instead. Thunderbolt 3 and 4 also use USB-C connectors and can transport DisplayPort and PCIe signals, but Thunderbolt is a separate technology family rather than a standard Alt Mode.

Limitations and considerations: Not all USB-C ports or cables support Alt Mode, and performance is constrained by the supported alternate mode version and the cable’s capabilities. Maximum resolutions, refresh rates, and color formats depend on negotiation between devices and the available bandwidth. Devices and cables should be certified for Alt Mode to ensure reliable operation.

See also: USB Type-C, DisplayPort, HDMI, Thunderbolt.