Specifications

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22
Cables and Ports
The Root USB Hub is connected directly to the USB Host, and
from there everything is done with cables. Two types of cables
can be used with USB devices: Series A and Series B. Series B
cables are limited to 3 meters in length and are for use with low-
speed (1.5 Mbps) USB peripherals such as keyboards and mice.
The UTP cable has a pair of 28 AWG wire stranded copper for
data and one pair 20-28 AWG for power.
(Series A USB Port (left) and USB Connector (right))
USB
The User-Friendly Choice
The Universal Serial Bus (USB) was developed around the idea
that users should be able to run multiple peripherals on their
computers without the hassle of physically installing boards,
manually allocating system resources, individually configuring
devices, and powering the computer up and down every time
equipment needs change. With USB, up to 127 individual
peripheral devices can be connected to a host computer using a
single interface and a system of USB hubs. (See right for a diagram
of a typical USB system.) Attaching a USB peripheral to your
computer is as easy as plugging headphones into your Walkman.
USB devices are automatically recognized and configured. They
can draw power directly from the system, from an attached self-
powered hub, or be connected to their own power supply.
USB Features
USB provides two-way communication between the PC and
peripheral devices, making it ideal for many I/O applications.
Multiple devices can connect to a system using a series of USB
hubs and repeaters. A single USB interface is attached to the
motherboard. A Root Hub with up to seven additional ports can
be integrated into the main interface, or it can be externally
connected with a cable. Each of the seven hubs on the Root
Hub can in turn be connected to seven hubs, etc. to a maximum
of seven tiers and 127 ports. A unique feature of USB is that a
peripheral device can have a hub built into it. This type of
peripheral, called "compound devices," are comprised of a
function device and one or more hubs. For example, a USB
keyboard can contain an additional USB port for a USB mouse.
USB is generally described as having a tiered star topology, however
each device communicates with the host as if it had its own
connection. This means that communication from the host
centers around a set of hubs/devices, each of which in-turn serves
as the center for another set of hubs/devices, etc. However, the
hubs are transparent to the software and the devices are addressed
individually. Cables are used to create point-to-point connections
between devices and USB ports, or to connect one USB hub to
another. The maximum cable length is five meters long. However,
a repeater hub may be used to extend the distance between the
peripheral and the host. There are also special USB repeaters
that can be used to extend the connection even further. (See
page 92 for Quatech's QExtend-4)
Operating System support for USB is built into later releases of
Windows 98 and the new Windows 2000. You can find out
whether your system supports USB by downloading an USB
evaluation utility from the official USB website (www.usb.org).
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
Peripheral
Device
HUB
USB Host
Controller
Root Hub with ports
Host
(A Typical USB System)
= USB Port
= USB Cable, max length 5 Meters
The USB Difference
USB is technically not a bus in the tradition of ISA, MCA or PCI,
as it provides more than a simple interface to the host computer.
Nor is it merely a communication protocol along the lines of
serial and parallel communication which are dependent upon
an external interface to reach the host computer. USB combines
the bus and the communication protocol into a single entity
which connects directly to a peripheral device. (Review the
diagram on page 11 for a better understanding of this difference.)
As such, the USB standard encompasses both the interface and
the method of communication. Whereas, in a PCI-based system
one standard defines the bus (eg. PCI Specification 2.1) and
another defines the communication protocol (eg. RS-232).
HUB
HUB
USB