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I'm having a hard time understanding the magic between DisplayPort, USB-C, and Thunderbolt. I'm trying to learn if it's supported by the protocols to have 2 monitors over USB-C both with and without Thunderbolt support. I'm not an expert on the USB-C protocols but here are my assumptions:

On the laptop with USB-C, a DisplayPort adapter is actually sending display data over the wire, not USB protocol data. The DisplayPort website hints that this functionality uses "Alternate Mode Functional Extension," and I'm assuming there's only one channel available for this. I assume that this functionality means a USB-C to DisplayPort adapter can be fully passive?

I'm assuming on the MBP, Thunderbolt just has massive raw throughput and each display adaptor is converting some thunderbolt-formatted data to the video signal being sent to the displays. These Thunderbolt to DisplayPort adapters would in that case be active adapters.

I'm assuming thunderbolt should be able to downgrade to be fully USB-C, so if I find a solution that works with regular USB-C, I should be able to use both the MacBook and PC.

I'm assuming that "Alternate Mode Functional Extension" can send video to one screen, that there should be enough bandwidth left in USB 3.0 itself to fuel another 1080p display.

I'm assuming that if I get a "dock" that has USB-C w/ Power Delivery, DisplayPort, and a USB-A 3.0 port, I can plug a USB-A 3.0 to HDMI adapter into the dock, and end up with 2 functional displays.

Is the above correct? Does this work as I'm assuming?

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  • I'm voting to close this question as off-topic because it might be better to move it to hardwarerecs.stackexchange.com
    – K7AAY
    Dec 27, 2018 at 21:09
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    Every time I see a comment like this, nsync plays in my head: "why you gotta try... to classify.. the type of thing we do." Anyways, I respectfully disagree as there's a dozen Thunderbolt/USB-C questions popping up in the "related" column to the right and the intent of my question was to clarify exactly the protocols moving across usb-c vs thunderbolt, not quite "what hardware should I buy?" However if it must be moved, that's up to your discretion, and I appreciate you spending time to moderate this forum.
    – kavisiegel
    Dec 28, 2018 at 16:32

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After a bit of digging it appears that the USB 3.0 HDMI dongles do use onboard processing, but not to a full extent. It appears it's not as readily supported as USB-C/Thunderbolt -> DisplayPort, there are drivers, data compression issues, and oddities with resuming from certain power modes. I've decided to just live with 2 cables rather than deal with software/hardware hassles.

I can conclude that what I wanted is possible, workable, but not easy enough to be worth the convenience.

I hope this information is useful to anyone in the future who's trying to peel back the oddities between Thunderbolt, USB-C, and USB 3.

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