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980264-001 Rev. B 2-7
CHAPTER 2
USB Concerns
(USB Specification Rev. 2.0)
1. USB-Equipped Host Computers have Hub
connection(s) and software that supports the
connections for up to 127 devices (either USB
Functions or USB Hubs), all hot swapable.
2. Hubs implement one connection path upstream
to, or toward, the Host and typically several Ports
for downstream Functions and/or other Hub
connections. Functions refer to Printers, Mice,
Keyboards, etc.
3. USB Cables e have both A- and B-type
connectors. The A connectors attach to upstream
devices, while the B connectors attach to
downstream devices. Hub loopback connections
(requiring an A-to-A cable) are not allowed.
4. System Software keeps track of the paths and path
parameters both to and from Functions (as Pipes).
System Software does not support more than five
Hubs, not counting the host, along any path.
5. A Pipe can operate at USB High Speed (480Mb/s),
USB Full Speed (12Mb/s), or USB Low Speed (1.5
Mb/s). Functions report needed parameters as part
of setup phases during operations (e.g., following
hot swaps).
6. USB Ports and Cables have a shield and four
conductors—two for data and two that can supply
5 volts to Bus-Powered Devices.
7. It is important to have cables that can support the
bandwidth requirements of a particular path.
Cables should not be used in paths that can
also operate at High/Full Speeds.
8. Cable designers must meet specified requirements
for data integrity between devices by using
materials for the lengths offered. Extension
Cables can add factors that breach critical design
elements and should not be used.
ach
Low
Speed
suitable
VBus1
D+ 2
D- 3
Ground 4
Shield
1VBus
2D+
3D-
4 Ground
Shield
Cable Wiring
4
1
3
2
4
1
3
2
End Views
A
B
Figure 2-9. USB Issues.