Thee DocKtor's Blog

All About Docks

DisplayPort Alternate Mode vs. DP Alt. Mode

Thee DocKtor realizes that DisplayPort Alternate Mode and DP Alt. Mode may appear to be the same and just spelled differently, and this was done to get your attention. Now that I have it, please read further so that I may explain.

DisplayPort Alternate Mode (aka DP Alt. Mode) can be designed with two, three, or four lanes of DisplayPort 1.3 HBR data, the latter supporting 4K @ 60Hz, see https://en.wikipedia.org/wiki/DisplayPort#Support_for_DisplayPort_Alternate_Mode .

Therefore, many USB Type-C DP Alt. Mode Downstream Facing Ports (DFP) on host devices (i.e., the Dell Latitude™ 5285) contain four lanes of DisplayPort 1.3 data capable of 4K @ 60Hz. This is acceptable for directly connecting to a 4K 60Hz capable DisplayPort monitor Upstream Facing Port (UFP) to render uncompressed video right from the host’s graphics adapter.

However, docking requires functionality  in addition to the display monitor graphics. GbE needs 1Gbps of the available USB Type-C bandwidth, and USB SuperSpeed is also necessary for the USB hub in a dock to function at a reasonable level of performance (close to 5Gbps). Therefore, USB Type-C Alternate Mode docking stations like the Targus DOCK410 are implemented with only two lanes of DP 1.3 HBR data capable of 4K @ 30Hz.  This leaves bandwidth available for the other docking station’s input/output (I/O) including GbE  and connected USB peripherals.

Categories: Did You Know

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