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Post  meodingu on Mon Jan 10, 2011 6:24 pm

The maximum length of a standard USB cable (for USB 2.0 or earlier) is 5 metres (16.4 ft).[28] The primary reason for this limit is the maximum allowed round-trip delay of about 1,500 ns. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to be 26 ns.[28] The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (192,000 km/s, which is close to the maximum achievable transmission speed for standard copper cable).[29] This allows for a five meter cable. The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification. For copper wire cabling, some calculations have suggested a maximum length of perhaps 3 m.
The data cables for USB 1.x and USB 2.x use a twisted pair to reduce noise and crosstalk. They are arranged much as in the diagram below. USB 3.0 cables are more complex and employ shielding for some of the added data lines (2 pairs); a shield is added around the pair sketched.

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