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If you connect a computer to itself using an USB cable (and 2 USB ports), are you likely to damage the device? It will recognize itself as an USB device?

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3 Answers

up vote 2 down vote accepted

Those USB-USB link cables contain a small blob of electronics somewhere in them, they're not just straight-wired cables. The Internet is full of anecdotal evidence that plugging two PCs together with their USB ports will break something.

http://www.usb.org/about/faq/ans5 is quite informative...

Q5: How can I connect two PCs to each other with USB?

A5: You need a specialized USB peripheral known as a USB bridge (sometimes called a USB to USB adapter) to do this. Without a USB bridge, connecting a USB port of a PC directly to another PC's USB port can damage the computers.

Q6: You mean I can't make a direct cable connection like a null modem?

A6: Correct. In fact, if you try this with an illegal A to A USB cable, you'll short the two PCs' power supplies together, possibly destroying one or both machines or causing a fire hazard. Even there were no danger to the machines from the problem with two power supplies, there still wouldn't be any way to get the two PCs talking to each other, since USB doesn't support that particular kind of communication. A reasonably priced solution to handle this need is the USB bridge.

Q7: So why do people make A to A cables, anyway? What kinds of cables do I need to connect USB devices together?

A7: A number of cable vendors seem to have reached the conclusion that USB is like a PC's serial port, only faster, so you need all sorts of special hardware to create the USB connection you need to make. This is completely incorrect. The only kind of cables you'll ever need to connect normal USB products are A to B cables, A to mini B cables or mini A to mini B cables of various lengths. Some special kinds of devices use nonstandard connectors and so come with their own special cable.

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I just tested this on a spare machine (as it did seem interesting)... the "Device Inserted" sound happened but nothing else.

I am guessing there are some applications that can take advantage of this for file transfer from one pc to another... but I don't have another machine here at the moment that I can test it on.

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Well, read the WP article on it, and it seems like it may depend on the type of cable you own as to whether it'll pass a host connection on both sides of the cable.

Quote: A USB connection is always between a host or hub at the "A" connector end, and a device or hub's "upstream" port at the other end. Originally, this was a "B' connector, preventing erroneous loop connections, but additional upstream connectors were specified, and some cable vendors designed and sold cables which permitted erroneous connections (and potential damage to the circuitry).

From what I know, USB is supposed to be host-mediated, so it wouldn't make sense to have a "host" connection on both sides. In all likelihood, all that will happen is that both sides will send out a handshake packet, and then move into a STALL state, because the response won't be correct.

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