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Comment by vel0city | Hacker News Reader

In the end it'll require some kind of active adapter as DP Alt Mode with USB-C doesn't support DP++. With regular DisplayPort ports supporting DP++, the port could detect HDMI and DVI connections and support those signaling protocols on that same port. This is how your passive mini display port adapter works on DisplayPort/Thunderbolt ports, as those support DP++. However, that's not the case with USB-C Alt Mode. See:

https://www.anandtech.com/show/8558/displayport-alternate-mo...

https://en.wikipedia.org/wiki/DisplayPort#Dual-mode

You might be able to get it to work with this. I've found Apple Cinema displays to be very picky in the end, so it may not work. This will require being plugged directly into a port supporting video out.

https://plugable.com/products/usbc-dvi/

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That anandtech article is from 2014. And the wikipedia article contains a similar claim, but un-citationed, as far as I can see. Do you have any links to recent articles that confirm that this is still the current state of things?

I have a chromebook that doesn't have video-out ports, but a relatively basic USB-C hub with HDMI-out port sufficed to get it connected to my HDMI monitor. Not sure if that's a counter-example, or something else entirely, but it may be relevant either way...

That Plugable adapter is interesting -- the page doesn't claim it's an "active adapter", and it only costs $15. In my mind, "active adapter" always translated to "costs $50 or more".

There just aren't enough pins. In USB Type-C DP Alternative Mode with 4xDP lanes, you've got 12 pins not including ground and power as it also does USB 2.0 in there. A single-link DVI cable or HDMI takes 19 pins. Regular DP could do it because regular DP had 20 pins to map to things. DP Alt Mode works because its pretty much just directly using the DP signaling on the four lanes and ignoring some of the other optional features of DP.

Any USB-C to HDMI adapter is an active adapter; practically nothing supports the USB-C HDMI alt-mode. That USB-C hub with HDMI is either doing things with a DisplayLink chip or it is doing USB-C DP Alt Mode and then doing an active converter from DP to HDMI. DisplayLink is pretty common on USB docks and can perform decently well for a number of tasks, especially the kind of workloads you'd expect for a Chromebook. And they'll work on pretty much any USB 3.0 (and even sometimes USB 2.0!) ports.

https://en.wikipedia.org/wiki/DisplayLink

Active adapter just means it is performing some kind of signaling modification. Chips can be quite cheap these days.

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