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.

Edit: Just to clarify, the question to be answered here is this: I have the pictured cable, which is a valid 3.0 USB cable and works for its intended purpose. What would happen if I, or a curious child, instead used it to connect the USB 2.0 port on one computer to another USB 2.0 port on another computer. Nothing? Smoke? Or something useful? (My guess is one of the first two.)

USB A-to-A

  • Possible duplicate of How do I work out if a USB C cable is safe to use?
    – fixer1234
    Apr 15, 2017 at 7:22
  • 3
    As 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
    Apr 15, 2017 at 7:52
  • @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
    Apr 15, 2017 at 17:21
  • 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
    May 15, 2017 at 1:05
  • 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: [media.digikey.com/pdf/Application%20Notes/…
    – user725131
    May 18, 2017 at 14:44

2 Answers 2


The devil is in the details. "Is a USB 3 Type A-to-A cable dangerous with USB 2 ports?" The emphasis needs to be on the specifying of a USB 3 cable.

A USB 3 compliant A-to-A cable will not be dangerous if used with USB 2 ports because a USB 3 compliant A-to-A cable will have only the ground contact of the USB 2 contacts connected. The D+/D- and +5v wires are not used in a USB 3 A-to-A cable and are specified to be unconnected to avoid damage to USB 2 ports.

I suspect that the A-to-A cable used for this hub is not a compliant cable because the hub will need power to operate. Without the data wires connected the hub will be unable to be used with any USB 2 devices. It's possible the hub gets power from someplace other than the USB cable to the computer but if there's no power brick required to make the hub work then it's cable to the computer must be providing power, and that would make it a noncompliant cable.

If USB 2 devices work with the hub then that's another clue it is not compliant with the USB spec. It's possible that the USB 2 data is somehow being sent over the USB 3 data lines but this is a function that is not defined in the USB spec, there would have to then be drivers installed for whatever logic is in the hub so the computer would know to look for USB 2 devices on the USB 3 data lines.

If a USB A-to-A cable does connect the +5v contacts on both ends of the cable then using it to connect two computers will hopefully just trip a self reset circuit breaker in the port on one or both computers. In some computers this can burn out a fuse or circuit board trace in the port, which would be permanent damage but not leave the port useless as devices that are powered by means other than the port should still work. Such devices that don't rely on USB power would be something like a printer. Another outcome could be the power supply protection trips and the computer shuts down.

A worst case would be something like the USB chip controlling the port being destroyed. With many computers today having a single chip for all USB ports this means the keyboard and mouse going dead. That would likely make the computer effectively dead and worthless.

  • "The D+/D- and +5v wires are not used in a USB 3 A-to-A cable and are specified to be unconnected" Aha! This was the answer I was looking for! I knew there had to be something in the USB spec to cover this scenario. (Of course, that was 3 years ago, but, eh.) I knew such cables would break stuff in USB 2, so I was wondering how USB 3 could allow this and still be backwards compatible. The insight about whether the cable is compliant is very nice as well. I'll have to see if I can find it and check.
    – jpfx1342
    Sep 2, 2020 at 18:45
  • 1
    Interesting answer. Related question: superuser.com/q/1608012/74576 :-)
    – Ryan
    Dec 8, 2020 at 20:02

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

  • 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
    May 24, 2017 at 11:34
  • 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
    Sep 1, 2017 at 1:25
  • 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
    Sep 1, 2017 at 10:16
  • 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
    Sep 2, 2017 at 20:25

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