USBefuddled: Untangling the Rat’s Nest of USB-C Standards and Cables

Nice. I wonder how it works when connected to a TB3 computer. The product page says it is compatible (2021 MacBook Pro M1, 2020/2019/2018/2017/2016 MacBook Pro, 2020 MacBook Air M1, 2018 MacBook Air, 2021 iMac M1, 2019/2017 iMac, iMac Pro, 2021 iPad Pro M1, 2020/2018 iPad Pro, 2020 iPad Air), but TB3 doesn’t include support for multiple downstream TB ports.

I assume that when connected to a TB3 computer, you will probably only be able to use one of the three downstream TB ports (but maybe the other two can still be used as USB-C ports).

If anybody here gets one and can test this theory, please let us know.

Yeah, good question.

Also, why do the dock brands use so many “3.2 10Gbit USB-A” ports? Why not just make two of the three into C-ports?

How many high-end users buying these docks want that many high-speed A-ports, when most gear is moving to USB-C now, if not already, and/or can be converted from C-to-A using $3 mini adapters?

To be clear, I’m not saying no legacy A-ports, but rather why only A-ports. I know the TB4 downstream ones do vanilla USB-C as well, but surely most high end users setups would likely be:

  • high speed devices via these three TB4 ports
  • slower devices via 10Gbit USB-C ports (instead of the A-ports)
  • couple of legacy devices via the remaining couple of USB2.0/3.2 A-ports
  • (optionally, convert the C-to-A with C-cables or $3 adapters)

IMO, it’s a bit weird having the 10Gbit ports being the old A-port standard connection.

My guess is support costs. If you have multiple type-C connectors, with some being TB and some being USB, you will find many users mixing them up (even if they are clearly labeled), plugging TB devices into USB ports and then assuming the dock is broken.

By making all of the type-C ports TB and using only type-A ports for USB-only connections, it eliminates the problem and associated support costs.

I didn’t get into the very tweaky stuff about “lanes” with USB and Thunderbolt. USB-C enabled dynamically configurable physical pathways for data. The connector has four pairs of pins, and each pair is a lane. Those lanes can be combined in different ways and configured dynamically for different kinds of data. With a dock, it’s dividing those lanes across the available ports, and providing an internal hub for USB 3.x.

I tried to find a good lane explainer and came up empty!

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Sure, I can see that. But really makes the docks less appealing to me. Why not add in the box an piece of paper explaining in brief terms the differences? (But then this gets complicated and/or people still won’t get it and complain/return working items, won’t they, lol!)

EDIT: Thanks @glennf - yes I knew there would be yet more to it! :joy:

This cannot be repeated enough.

The extra money you spend on a TB4 cable is still far less than what you’ll otherwise lay out for all the antidepressants you’ll be needing.

I experienced recently a very nice demonstration of this great advice. At work I received a fancy hi-res Dell 27" display (U2720Q). It comes among others with a USB-C port and advertises that connected through USB-C to a computer, it will use DP alt mode (at 60 Hz) and charge the computer as well as connect all downstream devices attached through its A-type USB 3.2 ports. Sounds great. But reality was it didn’t work that way (screen remained at measly 30 Hz) with the included USB-C cable no matter what Mac it was hooked up to or what macOS or what settings you choose on the screen. And it wasn’t just a case of crappy included cable. It didn’t work either using various high-quality USB-C cables from various reputable manufacturers all rated for Gen 2, PD as well as DP alt mode. What ultimately did work, was to connect the monitor through a more expensive Apple TB3 cable. Note though, this display knows nothing about TB. It’s pure USB-C. And yet, in order to get it t work as advertised (at least on a Mac, never bothered to try on a Linux PC), what you really need is a TB cable. Gory details can be found in this thread.

Be careful though. Most of those super cheap adapters are usually not rated for USB 3.2 Gen 2.

So if you happen to be relying on one of those to connect that super fancy PCI SSD you just put in your fancy new USB enclosure which unfortunately expects to connect to your computer through a 3.2 Gen 2 USB-A port, you could be about to turn it into the performance of a SATA-based SSD interfaced at 5 Gbps USB3.

For the time being, where I still have a lots of devices/cables with A ports (even good 3.2 Gen 2 stuff), I actually appreciate the fact that my CalDigit TB4 hub has several C and A type ports. And of course, should I need more of either, I can just hook up either an inexpensive USB 3.2 hub, or another CalDigit TB4 hub. It’s not a cheap solution, but for the foreseeable future I’m set. :slight_smile:

There’s an excellent set of illustrations in this OWC blog entry about Thunderbolt 4 that helps visualize that! (And I only spotted it now or would have seen if I could have gotten permission to include them in the article.)

This set of cable markings is fantastic:

When there’s this “unified port” graphic that includes all the potential standards available for a USB-C port/controller pair—and then the Thunderbolt 4 logo next to a USB-C port that supports all of the above.



In general practice, your best bet might be using old-fashioned sticky labels after purchasing a cable that fits your need or opening up a cable included with a product.

This is actually what I ended up doing for all my USB cables.

I label my USB cables with their specs in terms of data rates and power. If you do it right when you receive the cable, you’ll never end up in the situaton where you have a box full of cables and no clue which one was from which order. Sure, the labeling is a kludge, but it’s still better than not getting the performance you expect from a connection and being clueless as to why. Or worse yet, spending $$$ on good cables only to end up using them for 5W charging instead of the high-performance storage access they were originally intended for.

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I ended up getting the OWC TB3 dock.

TB3 was always my favourite Thunderbird anyway.

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Maybe a 3rd party app could be written, which identifies the capabilities of each usb-c cable, if both ends were plugged into the computer.

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Thanks for this, though a bunch of it went over my head. As a total amateur who is finding herself with some apple products that come with lightning to USB-C (no brick, of course), but also with some products that are still USB-A (all for charging not data etc. transfer), I’m trying to find a solution, a non-professional level hub/docking station that will allow both USB-A and -C cables for charging. There were some recommendations in the comments here that are mighty expensive and I wonder if they do more than I need, or if they are what I need? Would welcome thoughts.

(As an aside, Xmas present for a family member was a pair of Apple Airpod Max headphones which came with lightning to USB-C. No markings on the cable at all…)

Totally appreciate that. This was a deep dive—it would be so much easier if cost weren’t an object, as I could just say “get USB 4/Thunderbolt 4 cables.”

You can get some very inexpensive USB-C hubs that have both, for sure. This Anker one looks pretty good for price and ports.


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The best USB-C cables in my office are the ones that come with our Phillips monitors. If I need to do Migration Assistant between 2 Macs with USB-C I grab one of these cables from a monitor not in use, really speeds things up. We will probably buy the USB-C 4 cables so we don’t have to worry about which cables to use with what, though perhaps a power only USB-C cable to give to staff who might be charging their MacBooks off of a public power source.

Markings are probably not necessary, since Lightning is a USB 2.0-only technology. I think any USB-C port that remotely conforms to the spec should support USB 2.0 caliber data and charging.

Where there’s the potential for confusion is things like C-C and C-A cables, where there could be a variety of different capabilities.

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There’s an Alternate Mode for DisplayPort and one for HDMI to pass video: that’s how the 12-inch MacBook could pass video over USB-C

I may be wrong but I’m pretty sure that HDMI Alternate Mode is defined in the spec but not actually implemented in the 12-inch MacBook or anywhere else at all.

USB-C to HDMI adapters are relying on DisplayPort Alt Mode or other technologies l. I’d like to understand this better since of course it bears on understanding the quality of the video signal you get and on the compatibility between a computer, cable, and display.

Yikes! I would imagine that plugging both ends of any USB-C cable into the same computer could do some damage to the computer. Am I wrong about that??

The standard anticipates that: the host has to perform certain checking before anything would happen. In the worst case, I imagine that power would flow out of one port and into the other, gradually wasting energy. (Or creating perpetual motion.)


Okay, phew; good to know…

Given the connectors all have chips on them, I’m surprised no one’s written a utility that tells you the capabilities of the cable just by plugging one end in. Unless this isn’t technically possible. But seems like it would be incredibly useful.

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