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The Universal Serial
Bus (USB) specification, a standardized peripheral connection developed
by Intel with other industry leaders, supports multiple device connectivity,
improves the performance of the PC peripheral connection and allows for
digital multimedia integration. USB, a synchronous protocol that supports
isochronous and asynchronous data and messaging transfers, allows expandability
of the PC's capabilities via an external port, eliminating the need for
users or integrators to open the system chassis. Since USB supports multiple
peripheral devices simultaneously, it allows users to run numerous devices
such as printers, scanners, digital cameras and speakers from a single
PC. USB also allows for automatic device detection and installation, making
connectivity a true plug-and-play experience for end users.
USB's quick proliferation as the replacement of serial and other PC ports
for I/O devices in peripherals such as digital joysticks, phones, scanners
and digital cameras has accelerated the production and availability of
such devices. More than 100 were in the marketplace at the end of 1998,
with hundreds more expected during 1999. USB is already designed-in to
most new PCs being sold today so any newly purchased PC or Mac is very
likely to be USB-ready. USB is more than a plug-and-play peripheral connection.
It lets you use your PC and peripherals in some very interesting new ways.
Here are a few examples of the special benefits USB can provide:
- It is an instant,
no-hassle way to connect a new device to the computer. Adding a traditional
serial, parallel or SCSI peripheral required at least some computer
savvy in figuring out which port to use, opening the case to install
an add-in card, setting DIP switches and configuring IRQ and SCSI ID
settings. USB can replace all the different kinds of serial and parallel
port connectors with one standardized plug and port combination. USB
connections require no terminators, memory addresses or ID numbers.
They also use a new kind of cable—small (4-pin as opposed to the larger
8- to 25-pin connectors typically found on RS-232 and RS-422 serial
devices), simple, inexpensive, and easy to attach. Unlike the various
SCSI protocols, there’s only one style of cable (USB A-B). Since USB
cables are directional, the upstream connector is mechanically different
from the downstream connector. This prevents users from connecting cables
in such a way that would create a loopback connection at a hub. The
cables for high-speed and low-speed devices differ slightly in construction.
High-speed USB device cables require shielding and two twisted-pair
wires inside. One twisted pair provides power, nominally +5V (4.3 to
5.3 V at 100ma) for devices connected directly to the host, and ground.
A powered hub can provide up to 500ma of +5V. The other pair of wires
is for data I/O signals. Low-speed cables are untwisted and do not require
shielding. High-speed cables are most common, and appear as patch cables
to attach hubs to hubs, or attach high-speed devices to a hub.
- USB devices are
hot-swappable. You don't need to shut down and restart the computer
in order to attach or remove a peripheral. The PC’s USB controller automatically
detects the peripheral, determines and configures the necessary software
and bus bandwidth resources, and makes them available. This feature
is especially useful for users of multi-player games, as well as business
and notebook PC users who want to share peripherals. Likewise, on the
Mac, the appropriate USB device drivers are dynamically loaded and unloaded
as necessary by the Macintosh USB system software components.
- If you need more
than one device connected at once, USB lets you connect up to 127 peripherals
at one time (one device is taken by the root hub), through the use of
USB hubs and a series of cables—5 meter maximum for high-speed devices
and 3 meters maximum for low speed devices. Due to the fact that some
devices reserve USB bandwidth, the practical maximum of devices is less
than the theoretical maximum. Most USB PCs come with two USB ports.
USB hubs provide additional ports (usually four or seven), that let
you daisychain multiple devices together. Many USB devices also act
as hubs, providing extra outlets onboard for connecting other devices.
Apple’s current CPUs currently ship with a USB combination keyboard/hub
which provides two extra ports. PCI-USB add-in cards provide an independent
USB bus to which even more peripherals can be connected.
- USB peripherals
and hubs can be bus-powered or self-powered. A bus-powered device gets
its power from the USB bus and doesn’t need an extra power supply. USB
lets the PC automatically sense the power that's required and deliver
it to the device. The self-powered device has its own power supply.
Low-power devices such as keyboards and mice can run off the bus power
or a bus-powered hub, high power devices like digital video cameras
and scanners require their own power supply. High power devices are
devices that draw more than 100mA from the USB power line, low power
devices are ones that draw 100mA or less. The compact USB connector
provides two pins for power and two for data I/O. Power on the cable
relieves hardware manufacturers of low-power USB devices from having
to develop both a peripheral device and an external power supply, thereby
reducing the cost of USB peripheral devices for manufacturers and consumers.
- USB connections
allow data to flow both ways between the PC and peripheral. This means
you can use your PC to control peripherals in new ways. For example,
you can use your PC to automatically manage a telephone call center
to maintain voice, fax and data mailboxes, screen and forward calls,
and deliver a variety of outgoing messages. Or you can use your PC to
tune a set of USB-compliant stereo speakers to match the acoustics of
your listening environment. You need a specialized USB peripheral known
as a USB bridge to connect two PCs to each other with USB. You can't
make a direct cable connection like a null modem. USB doesn't support
this particular kind of communication.
- USB carries data
at the rate of 12 megabits per second, which is sufficient for medium
to low-speed peripherals. Full speed USB devices signal at 12Mb/s, while
low speed devices use a 1.5Mb/s subchannel. The broad range of devices
supported includes telephones, digital cameras, modems, keyboards, mice,
digital joysticks, some CD-ROM drives, tape and floppy drives, digital
scanners and specialty printers. USB's data rate also accommodates a
whole new generation of peripherals, including MPEG-2 video-base products,
data gloves and digitizers. Computer-telephony integration is expected
to be a big growth area for PCs, and USB can provide an interface for
Integrated Services Digital Network (ISDN) and digital PBXs.
The USB 1.0-specified USB peripheral controller has 4 data transfer
modes—control transfers for configuration, command and status information;
isochronous transfers for telephony and other time-critical data; interrupt
transfers to support joysticks, mice, and keyboards; and bulk transfers
for printers, scanners, and digital cameras. The USB controller is a
high performance 12MHz device with low power consumption, inherently
low noise and a mix of onboard ROM and RAM. The onboard memory enables
the chip to carry resident firmware for fast data access and minimal
space requirements. The device features four transmit FIFOs and four
receive FIFOs to support higher-speed devices like telephony and imaging.
PCIsets come in 2, 3 or 4-chip configurations, depending on the system
processor. Each PCIset includes a PCI USB host controller interface
and two USB ports connecting directly to two USB connectors on the PC
chassis without intermediate hardware. PCIset host controller circuits
execute all host functions needed to connect USB peripherals, including
supporting the 12Mbit/s data rate for each of the 4 transfer modes,
as well as detecting and interrogating peripherals as they are connected
with the bus.
- Since USB is a
cross-platform Mac/PC standard, third-party hardware is identical for
all platforms; only differing in the software required for a particular
OS. This reduces the time and cost of developing for both platforms,
and means a greater number of options will be available to the end-user.
Low-power USB devices are less expensive than serial or parallel interface
counterparts, because of the elimination of the power supply and because
the USB standard is also incorporated into PC systems developed around
the PC '98 hardware architecture. Future versions of the PC '98 compliant
operating systems will also include built-in driver support for a wide
variety of USB devices. Together these factors mean that a larger customer
base will form for USB peripheral devices, resulting in lower retail
costs of USB devices for all personal computer users
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