USBcable length

Themaximum allowable USB cable length is determinedprimarily by signal attenuationand
propagationdelay. A USB cable also carries VBUS and GND wires on each segment to deliver power
to bus-powereddevices. VBUS is nominally+ 5V at the source, andthe GND leadprovides a
commongroundreference betweenthe upstream anddownstream devices. The voltage dropacross
theVB US and GND leads can also limit the maximum cable length.
Youcan purchase or manufacturecables using commonly availablewire that conforms to US B 2.0
specifications to a maximum lengthof approximately 16' (5 m).
Extending USB connections
In USB topology, the hubis the only device that you can use to link USB devices. The distance
betweenhosts and devices, therefore, is limited by the maximum USB cable length allowedbetween
devices andthe maximum numberof hubs allowedin a path.

BasicUSB connection

In the followingdiagram, you can see that if all USB hubc onnectionsuse 16' (5 m) cables, the
maximumdistance between a host’s root hub anda function is 6 × 16' (5 m) = 96' (30 m).
Root
Hub
Hub
1
Hub
2
Hub
3
Hub
4
Hub
5Function
Host
Host (Tier 1) Tier 2 Tier 3 Tier 4 Tier 5 Tier 6 Tier 7
16' (5 m ) + 16 ' ( 5 m ) + 16 ' (5 m ) + 16 ' (5 m) + 16' ( 5 m ) + 16 ' (5 m) = 96 ' ( 30 m )
AP P E N DI X A
USB2.0 specification primer
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