Now USB-C me, now you don't.
October 28, 2020 8:37 PM   Subscribe

I just bought a USB C SSD, to plug into a MacBook Air that has only USB C ports. The SSD came with a USB C->USB A cable, plus a USB A->USB C adapter. What?

Okay, so I bought this cheap USB C SSD. I am new to the world of USB C peripherals, so I expected it to come with a cable that has USB C at both ends. Instead it came with a cable that has a USB C plug at one end, and a USB A plug at the other end.

They also included an adapter that turns that USB A plug into a USB C plug. So by using the adapter, I can effectively get USB C on both ends and plug the SSD into my MacBook Air.


Doesn't the trip through USB A and back again slow things down? Don't we lose the speed advantage that USB C has over USB A if we make a round trip through USB A?

So then I looked to see what it would cost to buy a cable that has USB C at both ends. And it turns out that most of the cables for sale on Amazon are the kind I have: USB C on one end and USB A on the other. The few cables that have USB C on both ends are advertised as "charging cables", and I'm afraid using them to attach the SSD could end up frying something. (I've heard things like that can happen if you use the wrong USB C cable).

Can someone explain to me what's going on here and how this makes sense? If I'm going to start entering the world of USB C, I'd like to understand it.

Thank you.
posted by Winnie the Proust to Computers & Internet (14 answers total) 3 users marked this as a favorite

USB C took a confusing interface and increased the confusion exponentially.

Try for reasonable cables at all lengths, multiple colors, high quality, reasonable prices.
posted by blob at 8:55 PM on October 28 [1 favorite]

USB 3 can be delivered over type A, type B, or type C connections. But, there's more USB type A ports out there in the world, so it probably made more sense (i.e. it was cheaper) to put a A-to-C cable and an adapter in rather than include both a C-to-C and a A-to-C. So, it's still USB 3.whatever, but just with added bits. You need specifically a USB 3.1/3.2 type C cable if you don't want adapters. The official Apple one is the Thunderbolt 3 (USB-C) cable (though because it's both Apple and actually a Thunderbolt cable, which has tighter specs, it's pretty pricy).

Also importantly, USB type C doesn't have to be USB 3 (or a variant thereof), and a lot of the "charge cables" may actually work for data transmission, but at USB 2 speeds (i.e. slow). (Technically, the USB cable that comes with your charger is actually a USB 2.0 cable.) You do have to be careful with them, though; the power thing had more to do with dodgy power adapters and bad implementations of Power Delivery but you also don't want to get a USB 2-only cable either.

on preview: blob's adafruit link has all the details in it.
posted by mrg at 8:58 PM on October 28 [1 favorite]

USB C, A, or Micro-B refer to the shape of the connector. That is different than the underlying USB protocol that determines what features are available over that connector/cable.

USB C was designed to support a wide range of features (like power delivery or video) and data protocols (USB 2.0, USB 3.2, Thunderbolt 3, and whatever comes next). Not every cable supports every feature or protocol, so not all USB-C cables are equal and not all USB-C devices are capable of the same things. They just use a commonly shaped connector.

For example, "charging cables" generally are using the new power delivery features available over USB-C, but if you transfer a file over them it may crawl at a low speed because to make a cable that does high power delivery and delivers data at higher speed would be a lot more expensive.

Since Thunderbolt 3 cables basically do everything to high standards, they are extremely expensive and only available in USB-C connectors.

Similarly, the drive you linked is rated for max 500MB/s which is roughly USB 3.0 data transfer speed. Like mrg beat me to saying, that can be delivered over really any type of USB connector shape. They simply packaged a C-A cable with a A-C adapter because that's the cheapest thing to do for the most people to connect it to their computers. As long as you plug it into a device capable of USB 3.0-ish speeds with at least a USB 3.0 data capable cable, A or C, it should work near the advertised transfer rate.
posted by bradbane at 9:15 PM on October 28

I think you got the long answers, the short answer is that the cable you have is probably fine for transmitting at maximum speed. At least, it's possible, depending on the cable. The actual connectors don't make much difference in this case.
posted by GuyZero at 10:05 PM on October 28

Just get a USB-C cable and be done with it. It won't blow up.
posted by pompomtom at 2:22 AM on October 29 [1 favorite]

What bradbane and GuyZero said, in a sane world the cable your SSD came with should be safe and capable enough for running it at top speed; the connectors don't matter, the USB-C cable itself does.

Now how many USB-C™ to USB-C™ cables are there? (USB4™ Update, September 12, 2019)

The answer so far is 8, btw
posted by Bangaioh at 2:24 AM on October 29 [1 favorite]

USB C-C cables labeled as for charging don’t have the superspeed pairs so are limited to 40MB/s. They won’t break anything.

You can use what it came with, which will be fully functional, or buy a full-featured USB SuperSpeed Type C cable (it’s a bit scary I need to use that many words to describe it) such as this. This particular one also supports full-current charging (5A), which is another selectable option (!).
posted by doomsey at 6:06 AM on October 29 [1 favorite]

Thanks for all of these great explanations. The thing I hadn't gotten was the distinction between connectors (A, B, C) and protocols (2, 3). I didn't realize those were somewhat orthogonal. So now I know the cables I have will work fine, and hopefully I'll be less confused buying stuff in the future.
posted by Winnie the Proust at 6:31 AM on October 29

I wouldn't assume that because the protocol supports X MBps and the SSD is X MBps that I am getting maximum speed from the drive. There's considerable overhead, the top-line number is definitely the "not to exceed" number. What you get depends on the quality of the connection (connectors firmly seated, grounding), the USB chips, whether you have a hub in between, etc. We're talking about an SSD, an enclosure, and a cable for $40 here, the default cable is going to be as cheap as possible. Presumably the reason you bought an SSD instead of a vastly larger spinning disk HDD is to get a peppier response time. You might see if you can borrow a high-quality Thunderbolt cable from someone and see whether it makes a big difference. Run a disk benchmmark, or copy a directory of 1000 photos and see whether the time is vastly different.
posted by wnissen at 9:16 AM on October 29

You might want to see if you can borrow a high-quality Thunderbolt cable from someone and see whether it makes a big difference

So this is actually a big gotcha - any Thunderbolt 3 cable >1 m has active circuitry in it, and won’t support USB SuperSpeed (or other alt modes). So in this particular case (USB SuperSpeed SSD) it’ll actually be subject to the same 40MB/s (roughly) limit as a USB charging cable!

You are in a maze of twisty little cables, all alike
posted by doomsey at 10:38 AM on October 29 [3 favorites]

Wait wait wait, what? They're not backwards compatible with USB 3.0? Now that is truly insane. I get that you can have a charging-only cable that doesn't support any kind of fast data, but those Thunderbolt 3 cables are designed for high-speed data. I do admit that one of the silver linings of my 15" MacBook Pro with no USB-A ports is that I can plug in a dongle and get two-way power and data. Your caveat is not listed in the Adafruit guide, do you have an explainer handy?
posted by wnissen at 2:35 PM on October 29

This article sort of explains it but the deal is that longer TB3 cables are "active" - there's a chip inside each one. I can't actually find a reference to explain what it does, but it is either a marker chip to indicate that the cable is tested to the much higher TB3 speeds (40Gbps vs 20 Gbps for USB 3.2) or it may be an actual amplifier to help make sure these higher speed signals make it to the other end without attenuating too much. But it's probably the latter as it interferes with USB 3.2 signaling and is definitely supposed to be labelled as a different type of cable.
posted by GuyZero at 2:51 PM on October 29

Hah, longer TB3 cables actually have both - they have an e-marker chip (which is a chip that talks to the USB-C port controller and tells it what kind of cable it is), and an active amplifier/driver chip, on both ends, that provides the actual data connectivity. The reason the cable can’t pass USB3.x signals is that the active amplifier is designed for Thunderbolt protocol specifically and can’t pass the USB 3.x signal which is electrically distinct. So the system ends up not being able to connect the USB 3.x / SuperSpeed wires and falls back to the 2.0 pair - 480mbit/s - which is required to be present in all USB-C cables.

Many USB-C cables have the e-marker chip. Anything that supports 5A must; the spec is somewhat unclear on whether USB 3.x capable cables must have an e-marker, but I think most do. Those marker chips don’t interfere with protocols running over the cable, though, so they’ll work with USB data and the vast majority of alternate modes (like HDMI and DisplayPort). But there also exist USB SuperSpeed cables with redrivers, typically very long ones, and if you get one of those it’ll only work with USB SuperSpeed and no other USB C mode!

So, yeah, fun stuff. Very flexible. Very confusing.
posted by doomsey at 4:18 PM on October 29 [1 favorite]

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