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I want to have a laptop supporting external 4K@60Hz output, so I think it needs either HDMI 2.0, DisplayPort 1.2 or DP ALT Mode over USB-C.

I think DisplayPort over USB-C and Thunderbolt 3 over USB-C are different standards, and Asus customer service guys confirm it: both Asus T303UA and T305CA have a TB3 port, but only T303UA supports DP over USB-C.

However, I found a "Thunderbolt 3 to Dual DisplayPort Adapter", supporting 4K@60Hz: https://www.startech.com/AV/Converters/Video/thunderbolt-3-to-dual-displayport~TB32DP2 It doesn't require the laptop supporting DP ALT Mode and a TB3 port is the only thing it needs.

SO...does Thunderbolt 3 itself support 4K@60Hz output? I'm really confused.

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One can almost say that there is no such thing as "Thunderbolt 3 over USB-C", because Thunderbolt simply changed the physical connector it uses from mini DisplayPort to USB-C since version 3.

So the question is rather, whether a USB-C port is a Thunderbolt 3 port, or whether a mini DP is a Thunderbolt 1 / 2 port, and that pretty much depends on whether a Thunderbolt controller exists on a motherboard and whether a port is routed to it.

The spec of T305CA listed a "USB TYPE C" port, while that of T303UA listed a "Thunderbolt" port (USB-C "physically", which makes it a TB3). That actually looks to me that T305CA does NOT have a TB3 controller. But if it does, then I see no reason that the USB-C/TB3 port on it cannot provide DP 1.2 connection.

Thunderbolt 3 (and 2) supports DisplayPort 1.2 connection, which supports 4K@60Hz. However, this is merely referring to the capability of the Thunderbolt controller. The actual display resolution that it can provide ultimately depends on the graphics unit that the Thunderbolt controller is routed to, which is probably always the one in the CPU.

For the T303UA, all the CPUs it may use support 4K@60Hz (in fact the actual maximum is slightly higher, which is 4096x2304@60Hz, while "4K" usually refers 3840x2160 these days):

http://ark.intel.com/products/88193/Intel-Core-i5-6200U-Processor-3M-Cache-up-to-2_80-GHz

http://ark.intel.com/products/88194/Intel-Core-i7-6500U-Processor-4M-Cache-up-to-3_10-GHz

P.S. Apparently there is "pure" DP over USB-C solution that does not involve a Thunderbolt controller. I am not sure how common it is on PC though. Maybe for routing a port directly to a dedicated GPU that is not involved hybrid graphics (which is itself a rare case these days)? Since AFAIK Thunderbolt controller cannot get access to a dedicated GPU directly.

  • Thank you for explanation! I will check T305CA out in a local store later. – CatoPX Sep 24 '16 at 10:36

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