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The new Thunderbolt 3 connectivity standard uses USB Type-C plugs and cables. As per Intel's presentation (if I interpret slides 13-15 correctly), all Thunderbolt ports also support the DisplayPort alternate mode, so that at every Thunderbolt 3 USB-C plug, a DisplayPort monitor can be connected via an adapter.

Some USB-C alternate mode adapters use only some of the wire pairs for alternate purposes, e.g. Apple's AV multiport adapter uses two wire pairs for DisplayPort, and the remaining two for USB 3.1 transmissions at 5 Gbit/s (see the fifth slide of this presentation).

Will those "mixed alternate mode" adapters work on a Thunderbolt port as well, or are USB and DisplayPort mutually exclusive on Intel's Thunderbolt implementation (Alpine Ridge)? What about Thunderbolt ports on external docks?

If this is something that is not specified in either the DisplayPort-over-USB or the Thunderbolt 3 specs, I would also be happy about some anecdotal evidence of a working "mixed-alt-mode" adapter on a Thunderbolt-enabled USB port.

An example for an "alt-mode only" adapter is Google's USB-C to DisplayPort cable (it uses all four lanes for the DisplayPort output, and should therefore definitely work with Thunderbolt 3 ports); an example for a "mixed-alt-mode" adapter is Apple's AV Multiport adapter.

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Will those "mixed alternate mode" adapters work on a Thunderbolt port as well, or are USB and DisplayPort mutually exclusive on Intel's Thunderbolt implementation?

Apple's AV multiport adapter does work with a Thunderbolt 3 port. From this product description:

The USB-C Digital AV Multiport Adapter lets you connect your MacBook with USB-C port or MacBook Pro with Thunderbolt 3 (USB-C) ports to an HDMI display, while also connecting a standard USB device and a USB-C charging cable.

USB-C has dedicated lanes for USB 2.0 signals, so DisplayPort and USB are never mutually exclusive. One dock which does both is the HyperDrive USB Type-C Hub with Mini DisplayPort.

However, I don't believe any USB-C adapter handles DisplayPort and USB 3.1 alternate modes simultaneously. If you want both you need to go full native Thunderbolt 3.

  • If I remember correctly, Apple's multiport adapter actually provides one USB Type-A 3.1 port, so this would be an example for an adapter that uses some wires for USB (3) and others for DP. – lxgr Feb 7 '17 at 16:47
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    That seems correct; I just meant I don't think any cable offers a DP connector alongside USB 3.X – akvadrako Feb 7 '17 at 21:31
  • The table here is also enlightening: en.wikipedia.org/wiki/… – akvadrako Feb 7 '17 at 21:37

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