Cables
The
maximum length of a standard USB cable (for USB 2.0 or earlier)
is 5.0 metres (16.4 ft). 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 speed
for standard copper cable).
[29]
This allows for a 5 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 3m. No fiber optic cable
designs are known to be under development, but they would be likely to
have a much longer maximum allowable length, and more complex
construction.
Maximum useful distance
USB 1.1 maximum cable length is 3 metres (9.8 ft).[30]
USB 2.0 maximum cable length is 5 metres (16 ft). Maximum permitted
hubs connected in series is 5. Although a single cable is limited to 5
metres, the USB 2.0 specification permits up to five USB hubs
in a long chain of cables and hubs. This allows for a maximum distance
of 30 metres (98 ft) between host and device, using 6 cables 5 metres
(16 ft) long and five hubs. In actual use, since some USB devices have
built-in cables for connecting to the hub, the maximum achievable
distance is 25 metres (82 ft) + the length of the device's cable. For
longer lengths, USB extenders that use CAT5 cable can increase the distance between USB devices up to 50 metres (160 ft).
[링크 :
http://en.wikipedia.org/wiki/Universal_Serial_Bus]
결론만 말하자면 허브로 연장해도 30m, 연장하지 않고 단일로 사용하면 5m가 한계이다.
요즘에 USB 1.1 장비는 많이 사용하지 않으니까 무난하게 사용하려면 3m가 한계라고 보면된다.
