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I stripped a USB cable to use in a power supply and instead of the usual 4 leads inside the cable, I found only 2--red and black. I'm assuming that's 5v and ground, but I was kind of confused by what I saw. Is there another USB spec that uses only 2 leads?

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up vote 18 down vote accepted

Sadly, this seems to be common: USB cables that carry only power. To my knowledge that is not according to any USB spec. And it is quite frustrating because such a cable has no marking to distinguish it from a regular USB cable, so after connecting a device to my computer it takes me a lot of time to figure out that it is the &^%&&%$% cable. Needless to say, they end up in the trash can.

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I see. That must be why it ended up in a bin of discarded cables. Works for my purposes of building a power supply. Thanks! – user1569339 Mar 13 at 8:33
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Yes. Beware of devices that are powered by a USB port but do no actual USB. The cables that come with those are suspect. – gbarry Mar 13 at 8:44
    
I've seen some where you can look inside the plug and see only 2 contacts, so these are easier to distinguish. I mean, most people wouldn't bother looking inside the plug to see this, but if you know the possibility exists, you might do so. But yeah, some are exactly the same from all outward appearances, and you won't know until you strip it... – Darrel Hoffman Mar 13 at 14:37
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Such cables are most useful when using untrusted (eg public) USB chargers. They might be hosts performing attack via USB, so power-only cable is always in my backpack. – Agent_L Mar 13 at 16:36
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@Agent_L Or vice-versa: You want to charge your phone from your laptop, but you don't want your (potentially compromised) phone to be able to behave as, say, a keyboard or a USB-midi (e.g. CVE-2016-2384…) or so. – v6ak Mar 13 at 18:57

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