Smart Technologies Smart Board 800 Series - Manual (Page 25)

USB cable length
The maximum allowable USB cable length is determined primarily by signal attenuation and
propagation delay. A USB cable also carries VBUS and GND wires on each segment to deliver power
to bus-powered devices. VBUS is nominally +5V at the source, and the GND lead provides a
common ground reference between the upstream and downstream devices. The voltage drop across
the VBUS and GND leads can also limit the maximum cable length.
You can purchase or manufacture cables using commonly available wire that conforms to USB 2.0
specifications to a maximum length of approximately 16' (5 m).
Extending USB connections
In USB topology, the hub is the only device that you can use to link USB devices. The distance
between hosts and devices, therefore, is limited by the maximum USB cable length allowed between
devices and the maximum number of hubs allowed in a path.
Basic USB connection
In the following diagram, you can see that if all USB hub connections use 16' (5 m) cables, the
maximum distance between a host’s root hub and a function is 6 × 16' (5 m) = 96' (30 m).
Root
Hub
Hub
1
Hub
2
Hub
3
Hub
4
Hub
5
Function
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 )
A P P E N D I X A
USB 2.0 specification primer
21