Is a USB 3 Type A-to-A cable dangerous with USB 2 ports?

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I recently purchased a USB 3.0 hub, and was surprised when I opened the box to find that it used an A-to-A cable for it’s main connection; see picture below. A “Type A” connector is the rectangular end that I’m sure is very familiar with.

From what I know of USB 3.0, this is basically okay, but I also know that for USB 2.0, an A-to-A cable is a big no-no, because the spec assumes that an A-plug will only be plugged into a host device. But USB 3.0 is electrically compatible with USB 2.0, so there’s nothing stopping me (or a curious user) from plugging both ends into two USB 2.0 Type A ports.

Is there something about the cable that would stop the connection, or is this cable actually dangerous if misused? For instance, could it fry a motherboard if plugged into two USB 2.0 ports? If so, doesn’t this seem like a bit of an oversight?

I’m not asking whether this particular cable is safe, rather, is a cable like this actually valid according to the spec? If so, is it safe to plug into two USB 2.0 ports or even two host USB 3.0 ports? Not that I have any plans to do it, but what would happen if someone did? Nothing? Or suddenly magic smoke appears and then yelling happens?

In any case, I’m aware that the connection wouldn’t be useful, I’m just wondering what would happen if, say, a curious child tried.

USB A-to-A

jpfx1342

Posted 2016-10-21T02:46:32.213

Reputation: 467

Possible duplicate of How do I work out if a USB C cable is safe to use?

– fixer1234 – 2017-04-15T07:22:40.770

3As the person who wrote the original question - not really a dupe. USB C has a very specific failure more I'm worried about. A to A is out of spec, sure but if something blows up, it will be a different reason. – Journeyman Geek – 2017-04-15T07:52:03.900

@JourneymanGeek, the questions are different, but your answer and Ali Chen's address this question also. If you think this shouldn't be a dupe, I'll go with your judgement. – fixer1234 – 2017-04-15T17:21:04.780

Its not a duplicate - nothing bad will happen due to crossed lines as the power and signals pins match in each type of plug/socket. However , as to if the pull up resistors would get confused and overload the sockets or cable that I cant work out. – Ross – 2017-05-15T01:05:40.477

I do not know if this is still relevant, but i came across an application note about circuit protection from TE connectivity. It's pretty interesting explaining scenarios on when and what happens when an USB is short circuited. I'm not sure if it actually answers the question, i mean we all want to see something burn, but it definitely shows the vulnerabilities in USB versus user interactions. Application note: [https://media.digikey.com/pdf/Application%20Notes/AMP%20Application%20Notes/USB_2.0_3.0_CP_Considerations.pdf](https://media.digikey.com/pdf/Application%20Notes/AMP%20Application%20Not

– None – 2017-05-18T14:44:19.767

Answers

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It is safe to use it with the hub, yes.

You are right about the host vs device protocols. The cable to the hub provides the electrical connections for data transfer, and the hub adds extra power and passes the data back and forth to the devices plugged into it.

This cable is not designed to be used between two hosts, even though the connectors would allow it to be plugged in.

Here is a good article about bridging between two hosts:

Connecting Two PCs Using a USB-USB Cable

The gist of the article is this: "if you use an A/A USB cable [between two PCs], you can burn the USB ports of your computers or even their power supplies. So, these A/A USB cables are completely useless."

It then goes on to explain that to bridge between two hosts you need a cable designed for the purpose like this:

enter image description here

SDsolar

Posted 2016-10-21T02:46:32.213

Reputation: 1 206

Thanks for the answer, though it doesn't really clear anything up for me. I'm aware that the cable is safe for use with the device it came with. (I would be very surprised if it wasn't, especially since it's plugged in right now.) Unfortunately, I can't accept this answer, since I'm trying to better understand what would happen with this cable if it was used for something other than it's design purpose. – jpfx1342 – 2017-05-24T11:34:15.367

The OP doesn't say "this cable" but rather a "cable like this" - I presumed you to be talking about the old Laplink cables. They were crossover cables, designed for the purpose of connecting two hosts with appropriate software. – SDsolar – 2017-09-01T01:25:22.273

I don't really consider the cable in this answer to be an A-to-A cable, because it actually has some hardware in the middle. My cable doesn't have any such hardware. As far as I can tell, it's just two 3.0 Type-A ends linked together. There might be some resistors and stuff in there, but I'm not really sure. (And I don't really want to take it apart to find out.) That's part of what I'm trying to get answered. Is there something in my cable that makes it safe to plug into two 2.0 ports? – jpfx1342 – 2017-09-01T10:16:48.790

Usually that will be a crossover cable. With the right software (like from a tablet to a PC) it can work, but from PC to PC it would require software designed to do the trick because they are both hosts. A to A is talking about the physical connector only. Not how it is wired. – SDsolar – 2017-09-02T20:25:06.557

Asked: 2016-10-21T02:46:32.213

Viewed: 1 321 times

Active: 2017-05-24T17:36:38.043