Hardware Reference
In-Depth Information
motherboard chipsets until 2012. Currently, most systems include four or more USB 2.0
or3.0portsasstandard,andwithhubsorportreplicators,manymorecaneasilybeadded.
USB 1.1/2.0 Technical Details
USB1.1runsat12Mbps(1.5MBps)overasimplefour-wireconnection.Thebussupports
up to 127 devices (including both functions and hubs) connected to a single root hub and
uses a tiered-star topology, built on expansion hubs that can reside in the PC, any USB
peripheral, or even standalone hub boxes.
Notethatalthoughthestandardallowsyoutoattachupto127devices,theyallmustshare
the 1.5MBps bandwidth, meaning that for every active device you add, the bus slows
down some. In practical reality, few people have more than eight devices attached at any
one time.
For low-speed peripherals, such as pointing devices and keyboards, the USB also has a
slower 1.5Mbps subchannel.
USB employs what is called
Non Return to Zero Invert (NRZI)
data encoding. NRZI is
a method of encoding serial data in which 1s and 0s are represented by opposite and al-
ternating high and low voltages where there is no return to a zero (or reference) voltage
between the encoded bits. In NRZI encoding, a 1 is represented by no change in signal
level, and a 0 is represented by a change in level. A string of 0s causes the NRZI data to
toggle signal levels for each bit. A string of 1s causes long periods with no transitions in
the data. This is an efficient transfer encoding scheme because it eliminates the need for
additional clock pulses that would otherwise waste time and bandwidth.
USBdevicesareconsideredeitherhubsorfunctions,orboth.
Functions
aretheindividual
devicesthatattachtotheUSB,suchasakeyboard,mouse,camera,printer,telephone,and
so on.
Hubs
provide additional attachment points to the USB, enabling the attachment of
additional hubs or functions. The initial ports in the system unit are called
root hubs
, and
they are the starting point for the USB. Most motherboards have two or more USB ports,
any of which can be connected to functions or additional hubs. Some systems place USB
ports in the front or sides of the computer, which is convenient for devices you use only
occasionally, such as digital cameras and flash memory card readers.
External hubs
(also called
generic hubs
) are essentially wiring concentrators, and through
a star-type topology they allow the attachment of multiple devices. Each attachment point
is referred to as a
port
. Most hubs have either four or eight ports, but more are possible.
Forfurther expandability,youcan connect additional hubstothe portsonanexisting hub.
The hub controls both the connection and distribution of power to each of the connected
functions. A typical hub is shown in
Figure 14.2
.
Figure 14.2
A typical USB hub with four ports.
