DisplayPort Alt Mode USB-C Pin Assignment

DisplayPort Alternate Mode (DP Alt Mode) allows a USB Type-C connector to transmit native DisplayPort audio and video signals directly to displays while preserving power delivery and legacy data transmission. By negotiating through USB Power Delivery (USB PD), the host and display dynamically repurpose the high-speed differential data lanes and sideband pins within the symmetrical 24-pin USB-C connector. The physical reversibility of the cable is managed by identifying the plug's orientation via the Configuration Channel (CC) pins before any DisplayPort lanes are mapped.

Orientation Detection and Configuration

A standard USB Type-C connector features 24 pins arranged in two opposing rows of 12 (designated as rows A and B). Because the cable is mechanically reversible, the connection is not mirror-symmetric; instead, it is rotationally symmetric.

When a cable is inserted, the host detects orientation using the Configuration Channel pins (CC1 at A5 and CC2 at B5). The pin that establishes a direct connection becomes the active CC line used for USB PD communication, while the alternate pin is repurposed as VCONN to power active cable circuitry. Once orientation is established, the multiplexers within the host and device route the DisplayPort signals to the correct physical pins regardless of which way the cable is plugged in.

High-Speed Lane Mapping Configurations

The USB-C interface contains four high-speed differential pairs primarily designed for SuperSpeed USB data:

  • Lane 0: TX1+ / TX1- (Pins A2, A3)
  • Lane 1: RX1+ / RX1- (Pins B11, B10)
  • Lane 2: TX2+ / TX2- (Pins B2, B3)
  • Lane 3: RX2+ / RX2- (Pins A11, A10)

Under the VESA DisplayPort Alt Mode standard, these four pairs are reassigned into specific configurations depending on bandwidth needs:

1. Four-Lane DisplayPort (Pin Assignment C and E)

In this full-bandwidth configuration, all four high-speed differential pairs are reassigned to carry DisplayPort data:

  • ML0 (Main Link Lane 0): TX1+ / TX1- (A2, A3)
  • ML1 (Main Link Lane 1): RX1+ / RX1- (B11, B10)
  • ML2 (Main Link Lane 2): TX2+ / TX2- (B2, B3)
  • ML3 (Main Link Lane 3): RX2+ / RX2- (A10, A11)

This mode yields maximum display resolution and refresh rates (such as high-refresh 4K or 8K video). Because all high-speed lanes are allocated to video, USB 3.x/4 data is unavailable, though USB 2.0 remains operational.

2. Two-Lane DisplayPort with USB 3.x (Pin Assignment D and F)

In this shared configuration, often used by docking stations and monitors with integrated USB hubs:

  • ML0 and ML1: Mapped to two differential pairs (Pins A2/A3 and B11/B10).
  • USB 3.x Data: The remaining two pairs (Pins B2/B3 and A10/A11) continue to operate as a standard bidirectional SuperSpeed USB data lane.

This enables multi-gigabit USB data transfer and native DisplayPort video to travel over the same cable concurrently.

AUX Channel and Sideband Use (SBU)

DisplayPort requires an auxiliary channel (AUX CH) for link management, Extended Display Identification Data (EDID) handshaking, and HDCP authentication. DP Alt Mode maps this bidirectional communication channel to the USB-C Sideband Use pins:

  • AUX+: Assigned to SBU1 (Pin A8) or SBU2 (Pin B8), depending on cable orientation.
  • AUX-: Assigned to the opposing SBU pin.

Hot Plug Detect (HPD), which alerts the host when a display is connected or disconnected, is signaled either via logical interrupts transmitted over the AUX channel or through USB PD messaging over the CC line.

Retained Dedicated Pins

Throughout any DP Alt Mode configuration, the core legacy pins remain untouched:

  • Power (VBUS and GND): Pins A4, A9, B4, B9 (VBUS) and A1, A12, B1, B12 (GND) continue to handle USB Power Delivery up to 240W.
  • USB 2.0 (D+ and D-): Pins A6, A7, B6, and B7 remain dedicated to standard USB 2.0 data, ensuring low-speed peripherals (like mice and keyboards) function uninterrupted regardless of the video configuration in use.