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I was wondering why almost all USB cables that exist are USB-A to (Micro-USB | USB-C | USB-B | etc.)

Why was USB-A to USB-A not a popular thing? Is there a technical reason this cannot work?

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    \$\begingroup\$ i have one of those ... it is just a USB extension cord ... it works just fine \$\endgroup\$ – jsotola Jun 19 '19 at 0:54
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    \$\begingroup\$ USB A male to USB A male -> Methaphor: Consider a mains plug male to be the plug on a power cord that plugs into a female wall power socket. Now make a cord with one of these on each end. It will do SOMETHING but maybe not what you expect or want and it has the potential (pun noticed) to be dangerous in some cases. VERY dangerous in the case of mains power appearing on the pins of a male plug. \$\endgroup\$ – Russell McMahon Jun 19 '19 at 6:40
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    \$\begingroup\$ @RussellMcMahon But this isn't a mains plug, so your comment isn't valid or helpful. The reason a male-to-male mains plug is dangerous is that it's easy to touch pins at high voltage. USB is low voltage (5V max), so there is no safety risk to users. USB A connectors also cover their connections within a housing, so there is no risk of the connectors inadvertently touching anything else and damaging the USB circuitry or the other equipment. \$\endgroup\$ – Graham Jun 19 '19 at 10:57
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    \$\begingroup\$ @RussellMcMahon Who said anything about male-to-male? \$\endgroup\$ – David Richerby Jun 19 '19 at 11:32
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    \$\begingroup\$ @Graham Your comment is not valid or helpful :-). A surprising proportion of people are unaware of how critical comments come across and a smaller but deplorably significant proportion of others don't care. Starting off with a manifestly obvious statement followed by a couple of low grade denigrations is a strange way to start out. See following ... \$\endgroup\$ – Russell McMahon Jun 19 '19 at 12:24
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USB-A male to USB-A female is fine. They exist, and are relatively common. They work just fine as USB extension cables, provided the overall length is not too long. For longer distances, active cables exist that contain an internal repeater or USB hub to regenerate the signal. However, you may run in to power delivery issues due to voltage drop.

USB-A male to USB-A male is dangerous. It is also not compliant with the USB specification. They also exist, though I am not really sure why. Perhaps because the USB-A connector is shorter than the USB-B connector, so chinese external hard drive enclosure vendors decided to use it because it was a half a cent cheaper than the proper connector. A USB-A male to USB-A male cable is similar to a line cord with a male plug on both ends. You should never, ever use one of these. There are several problems. First of all, USB does not allow two USB hosts to be connected to each other. The software stack does not support it. You cannot simply connect two computers to each other via USB and have them talk to each other without inserting some component in the middle, such as two back-to-back USB to Ethernet converters or similar. USB on-the-go is a slightly different story as a USB OTG device will switch roles between host and device. USB OTG also uses a different connector entirely, an A/B variant that accepts both A and B type connectors, which only exists in the smaller sizes.

But this is not what makes USB-A to USB-A cables dangerous. What makes them dangerous is that the USB cable provides power, and USB hosts are in general not designed to accept power coming in the wrong way. If you connect two computers together with a USB-A to USB-A cable, their 5V supplies are almost certainly not at the same voltage, so one of them will power the other one through the USB cable. This in and of itself is not really a major problem, provided the source has appropriate current limiting to prevent the cable heating up and starting a fire. The problem really comes when one of the devices is turned off. Now, you have an external 5V supply coming in to a powered-off device that isn't designed to handle it. This can damage components on the motherboard in one or both devices. Also, USB devices are in general designed to draw less than 500 mA, but whatever is hanging off of the 5V rail inside a host could draw orders of magnitude more than that.

USB-C is a different story as is specifically designed to have identical connectors on both ends. Not only that, it is explicitly designed to support charging a host device via a USB C connector. As a result, USB C devices have to be designed to handle this situation appropriately.

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  • \$\begingroup\$ Excellent comment, thanks for explaining the history behind this and also delving into why USB-C to USB-C is so common. I appreciate it! \$\endgroup\$ – NessDan Jun 19 '19 at 2:56
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    \$\begingroup\$ I own a device (a weather station) with an USB-A socket. So to connect it to my computer I need an USB-A-male to USB-A-male cable. \$\endgroup\$ – guntbert Jun 19 '19 at 10:57
  • \$\begingroup\$ USB-A male to USB-A female cables (without an integrated hub) is not defined by the USB specification, but are produced nevertheless. Since USB supports only a maximum cable length because of signal integrity, and such USB-extension cords make it possible to exceed that length, they might not work. USB was (originally) intended such that anything that fits mechanically does work. Non-Standard extension cords (without an integrated hub) subvert this. \$\endgroup\$ – Erlkoenig Jun 19 '19 at 11:09
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    \$\begingroup\$ @guntbert -- yeah sighs some derps put USB-A sockets on their devices, either out of cluelessness, or for mechanical design raisins (and not good ones either) \$\endgroup\$ – ThreePhaseEel Jun 19 '19 at 11:40
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    \$\begingroup\$ @ThreePhaseEel Mechanical design raisins are my least favorite dried fruit. \$\endgroup\$ – Tashus Jun 19 '19 at 14:33
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Because it's against the standard. The USB standard says that type A is for hosts, and type B is for devices.

In practical terms, a device with a type A connector is expected to provide power, so if you connect two of them together, they will attempt to power each other, leading most likely to Bad Things Happening. Type C gets around this by a negotiation mechanism.

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    \$\begingroup\$ Brownie points if you can list the spec :) That is, if you want to pay a million dollars for the entire document LOL. \$\endgroup\$ – KingDuken Jun 19 '19 at 1:01
  • \$\begingroup\$ @KingDuken Somehow I don't think that crowdfunding campaign will take off. \$\endgroup\$ – Hearth Jun 19 '19 at 1:02
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    \$\begingroup\$ You can just download the spec from the USB-IF for free: usb.org/documents \$\endgroup\$ – Erlkoenig Jun 19 '19 at 11:39
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USB pre-3.0 has a host side and a slave side. USB-A connectors are for the host, USB-B are for the slave.

USB 3.0 and above is peer-peer, and the USB-C connector supports that.

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  • \$\begingroup\$ "USB 3.0 and above is peer-peer" - no, the structural topology of USB3 is the same as of USB2: a star with one host and multiple devices, possibly connected through hubs. \$\endgroup\$ – Dmitry Grigoryev Sep 26 '19 at 9:18
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USB-A to USB-A cables are in fact legal. It's in the USB 3.x specification and it has been since at least 2011. I clipped the image below out of the USB 3.0 specification document published in June 2011. This cable is safe to connect two hosts together because the VBUS line is not connected. The D+ and D- lines used for USB 1.1/2.0 backward compatibility is also not connected, making this a USB 3.x-only cable. The "superspeed" data lines are crossed over for 5Gbps or 10Gbps data transfer.

It is true that the type A connector is for hosts but what has not been true for years is that host to host connections are not allowed by the USB spec. I do wish this kind of cable was more popular since this would provide a very fast connection with a relatively inexpensive cable. This would be quite useful as well since on some newer computers there aren't many ports except USB. All it should take to make this work for connecting two computers to create a serial-like or Ethernet-like connection is the right software.

USB A to A cable assembly wiring

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