DisplayPort USB-C Pin Assignment Modes C and D

This article provides a technical overview of how DisplayPort Alternate Mode reconfigures USB Type-C connectors under Pin Assignment Mode C and Pin Assignment Mode D. It details the precise mapping variations across high-speed differential pairs, sideband lines, and data channels, explaining how each configuration balances display resolution capabilities with concurrent USB SuperSpeed data transfer.

The USB Type-C High-Speed Architecture

A standard USB Type-C receptacle contains four high-speed differential pairs capable of transmitting data:

• TX1 / RX1: Top-row differential pairs (Pins A2/A3 and B10/B11)
• TX2 / RX2: Bottom-row differential pairs (Pins B2/B3 and A10/A11)

In native USB operation, these pairs carry bidirectional USB 3.x/4 data. When a DisplayPort Alternate Mode (DP Alt Mode) connection is negotiated through USB Power Delivery (USB-PD), the VESA standard repurposes these high-speed pairs for DisplayPort main link lanes, while using the Sideband Use (SBU) pins for the DisplayPort Auxiliary (AUX) channel.

Pin Assignment Mode C: 4-Lane DisplayPort

Pin Assignment Mode C allocates all four high-speed differential pairs exclusively to DisplayPort video transmission.

Lane Allocation and Pin Mapping

• DP Lane 0: Mapped to TX1+/- (Pins A2/A3)
• DP Lane 1: Mapped to RX1+/- (Pins B11/B10)
• DP Lane 2: Mapped to TX2+/- (Pins B2/B3)
• DP Lane 3: Mapped to RX2+/- (Pins A11/A10)
• AUX Channel: Mapped to SBU1 (Pin A8) and SBU2 (Pin B8) as AUX+ and AUX- (orientation-dependent)
• Hot Plug Detect (HPD): Managed virtually via USB-PD messaging over the Configuration Channel (CC1/CC2)
• USB Data: Limited strictly to USB 2.0 (High-Speed/Full-Speed) via dedicated D+ / D- pins (Pins A6/A7 and B6/B7)

Operational Characteristics

Mode C offers the maximum possible DisplayPort bandwidth over a single USB-C cable (up to full 4-lane DP 1.4 or DP 2.1 speeds). Because all high-speed lanes are allocated to video, concurrent USB 3.x SuperSpeed data transfers are disabled.

Pin Assignment Mode D: 2-Lane DisplayPort + USB 3.x

Pin Assignment Mode D splits the high-speed differential pairs evenly between DisplayPort video transmission and USB 3.x SuperSpeed data.

Lane Allocation and Pin Mapping

• DP Lane 0: Mapped to TX1+/- (Pins A2/A3)
• DP Lane 1: Mapped to RX1+/- (Pins B11/B10)
• USB 3.x TX: Retained on TX2+/- (Pins B2/B3)
• USB 3.x RX: Retained on RX2+/- (Pins A11/A10)
• AUX Channel: Mapped to SBU1 (Pin A8) and SBU2 (Pin B8) as AUX+ and AUX- (orientation-dependent)
• Hot Plug Detect (HPD): Managed virtually via USB-PD messaging over CC pins
• USB Data: Supports simultaneous USB 3.x SuperSpeed (up to 5 Gbps, 10 Gbps, or higher depending on the host/hub controller) along with USB 2.0 via D+/D-

(Note: Depending on cable plug orientation, the exact assignment of the pairs can flip symmetrically via the internal multiplexer, but the 2-lane DP and 2-lane USB allocation remains constant.)

Operational Characteristics

Mode D halves the maximum video bandwidth compared to Mode C. It is commonly implemented in multi-port USB-C docks, hubs, and monitors with built-in data hubs, where maintaining 5 Gbps or 10 Gbps USB peripheral connectivity alongside monitor output is required.

Summary of Differences