What Is DisplayPort Alt Mode?

DisplayPort Alt Mode is a VESA standard, introduced in 2014, that carries a native DisplayPort video signal over a USB-C connector and cable. The USB-C plug has four high-speed lanes that normally carry USB 3 data. In Alt Mode, two or all four of them carry DisplayPort instead, while a separate pair still carries USB 2.0 and other wires carry power.

The switch happens when the cable is plugged in. The computer and the display talk over the USB-C configuration channel using USB Power Delivery messages, agree that both support DisplayPort, and reassign the lanes. The DisplayPort auxiliary channel, which carries the display’s identity and settings, runs on the two sideband pins of the connector. Nothing has to be installed or switched on in software: the port either supports Alt Mode in hardware or it does not.

For a touch monitor, that means video, touch data and, where the host port can supply enough power, the monitor’s power all travel on one cable instead of three.

The DisplayPort version carried depends on the host. Most current computers send DisplayPort 1.4 through Alt Mode, with Display Stream Compression for the highest modes, and newer hosts support DisplayPort 2.1 rates. Multi-Stream Transport also works over Alt Mode, so one USB-C port can drive two or more monitors through a dock or a daisy chain, sharing the available bandwidth between them.

How many lanes go to DisplayPort decides the balance between video and USB data on the same cable. The figures below assume DisplayPort 1.4 at its HBR3 rate and are typical, because the host’s graphics and the monitor set the real limit.

Most single-cable touch monitors use the 2-lane mode, because the touch controller needs USB data and the screen resolution is well within the limit.

Not every USB-C port carries video. A port may be USB data only, power only, or both without DisplayPort. The quickest checks are:

The cable must be a full-featured USB-C cable with the high-speed lanes wired. Cheap charge-only cables carry USB 2.0 and power but no video, which is the most common reason a monitor shows nothing. At the highest DisplayPort rates, passive cables are typically limited to around 1 to 2 m.

Adaptors work because the signal is already DisplayPort. A USB-C to DisplayPort cable is essentially passive. A USB-C to HDMI adaptor contains a small DisplayPort-to-HDMI converter, so it also relies on Alt Mode at the computer end. Our guide to USB-C explains why two identical-looking ports and cables can behave so differently, and our comparison of Thunderbolt and USB-C covers the faster port types.

USB Power Delivery can send up to 100 W over a standard USB-C cable, and up to 240 W with extended power range, in either direction. Many computer USB-C ports supply only 7.5 to 15 W, which is enough for a small, modest-brightness touch monitor but not for a larger or high-brightness one. Larger or brighter screens may still need their own supply, and some monitors power the computer instead.

One-cable wiring keeps HMI, kiosk and medical cart installations tidy and quick to fit. Our USB-C monitors run video and PCAP touch from a single USB-C port, including 1000 nit models for bright locations, and our industrial USB guide covers the wider USB story.

We supply our USB-C monitors with a host computer whose USB-C port supports DisplayPort Alt Mode as standard where the system calls for one, and we test video, touch and power over the single cable before it ships.