~jpetazzo/USB-C redux

A few months ago, I started using a 2017 12” Macbook Air. This machine has only two ports: an audio jack, and one USB-C port. That USB-C port is the only thing you have to plug external storage and monitors, network connectivity, and of course, a power supply. I had to do some research to understand how USB-C works, and find the perfect adapters (at least, the perfect adapters for what I do).

Here is a summary!


This was written in 2017. The product recommendations in this post are probably not relevant anymore. However, the general description of how USB works (in particular, USB-C alt modes) is still relevant if you’re trying to figure this out.

I also recommend to check this article which explains how USB-C data lanes get shared when using DP alt modes; it’s useful if you plan on using high resolution and high frame rates displays (think 4K at 60 fps).


If you have a machine like the one I had, with only one USB-C port, and without Thunderbolt support, I recommend to get docks and dongles similar to these ones:

Note that these products may not be available anymore today, so you might have to find other ones … Sorry!

The docking station is great. Its only downside (for me!) is that it doesn’t have a DisplayPort output; but you can get 4K on the HDMI port (if your monitor supports it; some older monitors support higher resolutions only on their DP inputs). This means that you can’t use DisplayPort MST to cascade multiple screens. However, the dock has two extra DsplayLink ports (one DVI, one HDMI) that might or might not be helpful. (I’ll cover DisplayLink briefly later.)

The adapter is great too. Its only downside (again: for me!) is that it uses a lot of power, and that’s why I ended up also getting a dock. The charger that comes with the MacBook Air delivers 40W. Once I insert the adapter between the charger and the MacBook, the latter reports that it is connected to a 13W power supply. This means that the battery will charge very slowly, or even drain slowly, if you are doing CPU intensive tasks (and that machine has a very weak CPU, so sometimes “having too many tabs open” can be CPU intensive!). Also, that means that the adapter dissipates a lot of heat. Finally, if you don’t need that plethora of connectors, there are smaller adapters that you might like more.

Both worked out of the box without installing drivers (but I had other USB adapters in the past, so perhaps at some point I installed a driver that took care of business).

If your machine supports Thunderbolt 3 (like the Macbook Pros, which also have more than one USB-C port), you can also look at this Thunderbolt 3 Docking Station. (Thanks Bryan for the recommendation!)

Finally, I’m aware that these adapters are not cheap. This is not an extorsionist move from the adapter lobby: as we will see when we dive into the details of USB-C, some features are simple, others require more complex circuitry.

If you’re on a budget, you may get similar functionalities by getting multiple cheaper adapters and switching between them when necessary. Then again, if you’re on a budget, I would humbly suggest to stay away from Apple hardware.

What I wish I had been told about USB-C

There is a lot of information out there, and it’s not easy to find palatable technical information, between marketing announcement, outdated press releases, and arcane spec sheets. This is my attempt at explaining USB-C in terms that are “just enough technical.”

According to Wikipedia:

USB-C, technically known as USB Type-C, is a 24-pin USB connector system.

(In this whole post, I am using “USB-C” for “USB Type-C”.)

So, USB-C is a connector. It’s not a protocol! The protocol would be e.g. USB 2, or USB 3, or something else.

This connector has the ability to carry many different electric signals, including:

All these electric signals can be present on a USB-C connector (but maybe not all at the same time!), and as far as I understand, none of them is mandatory.

So when you see a USB-C connector, it could be:

But it could also be (almost) all these things at the same time!

Alternate modes

The 12” Macbook Air has only one USB-C connector, but that connector can support many different electric signals simultaneously.

This is a feature in the USB-C spec, called “alternate modes” or “alt modes” in short. That’s how signals like DisplayPort, Thunderbolt, or HDMI are supported. When an “alt mode” is enabled, some “high speed lanes” (electric wires normally used for USB 3) are hijacked to transport the corresponding alt mode instead.

Carrying DisplayPort, Thunderbolt, or HDMI signals, requires enabling the corresponding alt mode. It has to be supported on both sides of the cable (i.e. by the host and the device). You can’t connect a Thunderbolt device over USB-C to an host that doesn’t support Thunderbolt (more on that later).

If that helps, you can imagine that inside this computer, we actually have a bunch of sockets for power, USB 2, USB 3, DisplayPort, and HDMI; and all these sockets are connected to that single USB-C connector. Then you can put an (expensive) adapter or dock station, to get all these sockets back.

I’m very bad with drawing, but I found a nice diagram in this document:

Host and device and USB-C between them

The document has other schematics and explanations that you might like if you want to know more.

The 12” Macbook Air doesn’t have Thunderbolt

Just because a machine has USB-C, doesn’t mean that the machine supports all these protocols and signals. For instance, the 12” Macbook Air does not have Thunderbolt. The 13” and 15” Macbook Pros do have Thunderbolt. This means that if you plug an Apple Thunderbolt display, using Apple’s adapter, on a Macbook Pro, it will work; but if you plug the same display, with the same adapter, on a 12” Macbook Air, no dice.

To make things even more frustrating and confusing: the Thunderbolt connector is physically identical to a miniDP connector. Any other (non-Thunderbolt) display with a miniDP connector will work on any Macbook with the correct USB-C adapter (because it will use the DisplayPort protocol).

Thanks Apple, I guess.

Connecting screens over USB-C, the easy way

If we want to connect external monitors with USB-C, we have plenty of options.

Assuming that the external monitor has an HDMI (or DisplayPort) connector, the most straightforward option is to use an adapter leveraging “HDMI alt mode” or “DisplayPort alt mode”. If you have multiple USB-C ports, these adapters are a good option, because they are cheap, since the circuitry in them is pretty basic. Of course, our source (i.e. your laptop) needs to support HDMI alt mode or DisplayPort alt mode (the latter is also known as VESA alt mode, by the way).

Most laptops with USB-C ports will support these modes, but I don’t know if this is true for all laptops. (E.g. I don’t know about Chromebooks and other cheap ones.)

Phones and tablets are a totally different story! They may or may not support alt modes. I don’t expect any phone to support HDMI or DisplayPort alt modes. However, there is “MHL alt mode” which seems to be designed to carry video signals from mobile devices. I don’t have any device supporting that so I don’t know if you can use the same adapters or need different ones.

And then, there is DisplayLink.

DisplayLink is basically “video stream over USB.”

A DisplayLink adapter might look physically exactly like an alt mode adapter; except that it will work very differently. When you connect a DisplayLink adapter, instead of negotiating alt mode to allocate a few wires to HDMI signals, it will present itself as a regular USB device—i.e. one that shows up in lsusb. The driver for this USB device will behave like a graphics adapter. When you display something on this graphics adapter, the display is encoded, sent over the USB protocol, decoded by the DisplayLink adapter, and shown on the connected physical screen.

These extra steps mean that a DisplayLink adapter will use extra CPU cycles (because of the video encoding), and depending on your setup, this can add a tiny bit (or a good bit) of extra latency. Various sources recommend to NOT use DisplayLink for gaming.

Superficial research showed that there might be DisplayLink drivers available for Linux, but I didn’t try.

So far, it sounds like DisplayLink has a bunch of inconvenients: it needs a custom driver, eats CPU cycles, adds latency … But it has two advantages: you can plug as many as you want on your machine (since they’re just normal USB devices), and I saw references to Android drivers, meaning that it might work on some tablets.

This is why you can end up with an adapter that works out of the box, without drivers, on a machine; and an adapter that works almost out of the box (if, say, the driver is loaded automatically) on a tablet; but the adapters are not interchangeable (they won’t work with the other device) because they’re fundamentally different.

There is a lot of “maybe” in that section, because I didn’t take the time to try DisplayLink so far. Sorry!

One adapter to rule them all

Alright, now that we are armed with all that knowledge, let’s find the best adapter EVER.

Everyone’s needs are different, but I wanted to find a way to have the following connectors on my Mac:

The latter might seem weird, but many adapters (including some from Apple) don’t pass power to the computer; and remember: that 12” MacBook Air has only one connector. You then end up with a difficult choice: do I want to connect my external monitor, or do I want to charge my battery?

I also wanted to be able to connect everything at the same time.

“Whoa, that Jérôme guy for sure is picky!”

As it happens, when I deliver a full day workshop, I need:

I also wanted to get an extra USB-C port on the adapter, because I wanted to be able to buy USB-C devices (e.g. memory sticks, security tokens…) without having to choose between the device and everything else.

It turns out that I had to drop that last requirement, as (in August 2017) I couldn’t find any adapter that would connect to a single USB-C port and then provide more USB-C ports (in addition to my other requirements).

I got this adapter. The reviews might not be stellar, but it works great for me. It also has SD and miniSD card readers (which I use once every blue moon to re-image a Raspberry Pi), and audio output (because why not). In addition to VGA and HDMI, it has a miniDP connector as well.

The adapter can also be used as a USB charger: if it is connected to the AC adapter, but not to the computer, it will still deliver power to the USB A ports.

Likewise if it is connected to the computer, but not to the AC adapter: it can charge your phone and other devices from the battery of your laptop.

Note, however, that when you plug/unplug the AC adapter, it seems to “reset” the adapter (as if you had disconnected and reconnected all the peripherals). Keep that in mind if you have a disk connected, or if you’re performing live music with a USB MIDI controller.

One dock to rule them all

I do some video editing sometimes (as well as some other CPU intensive tasks), and my adapter then has a “small” problem: the battery will charge very slowly, or even discharge if the CPU stays running at high speeds for continued periods of time. So eventually, I also got a dock. It’s not a “dock” like the docks I was used to (where you physically lock the computer to a base).

There are many docks out there, with varying options. I wanted something just like my adapter, but with full power delivery to the host, and with at least one extra USB-C port, so that I wouldn’t be constantly plugging/unplugging stuff if I decided to buy some USB-C peripheral.

I picked that dock. It might seem expensive, but the other ones that did fit my requirements were sometimes more than $300 (!). There were also a bunch of docks boasting Thunderbolt support, and I didn’t know if that meant “and also, it supports Thunderbolt!” or “and by the way, it requires Thunderbolt!” — the latter would have been a showstopper.

The dock also has mic and headphone connectors, which can be super convenient. I had a headset connected to these for a while. I never really understood how macOS picked which default output to use, but most modern conferencing software has easily accessible settings to switch audio devices to make up for that.

Having a dock also means much less plugging/unplugging: the dock can stay at home, and all my peripherals can stay connected to it, while the adapter stays in my backpack for when I’m on the go.

A couple of observations:

Wrapping it up

After spending a bunch of time reading on USB-C, trying to understand what would work, what wouldn’t, etc., I think it is pretty fantastic to be able to use a single connector for so many things. The transfer speeds are orders of magnitude faster than USB 2: with Thunderbolt 3 over a USB-C connector, you can supposedly get 40 Gb/s. You can even connect external GPUs through USB-C, because Thunderbolt can carry PCI Express lanes! (Don’t hold your breath, though: this is still pretty early stage.)

However, since all hosts (machines) and devices don’t support all these modes, it means that debugging problems gets really complicated, especially without knowing of the underlying fundamentals of USB-C, alt modes, etc.

For instance, if I plug my dock or my adapter to an Android phone with an USB-C connector, nothing happens. Obviously, I didn’t expect my phone to suddenly drive my 4K monitor, connect over my gigE interface, and mount my external disks connected to the dock. But the USB-C spec includes a lot of signaling and negotiation to let each side identify itself and its capabilities. It would be fantastic if the devices could use that and report it adequately: it would make for a much better user experience. Hopefully that will evolve in the future.

If there is an adapter or dock that you particularly like, feel free to drop me a note, I’ll add it here for others!

This work by Jérôme Petazzoni is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.