About USB
The Universal Serial Bus (USB) is a standardized intelligent
serial connection that enjoys broad acceptance in the
PC and CE industries. Two of the greatest assets of
USB are its "plug and play" support, where
peripherals are automatically detected upon connection
to a USB port, correctly identified and configured,
without having to set switches and jumpers and its "hot-swappability",
eliminating the need to reboot the system. In short,
the peripheral can simply be attached by the user and
be ready for immediate use.
USB overcame resource problems associated with legacy
PC I/O implementation, related to I/O address space,
IRQ lines and DMA channels that no longer exist with
the USB implementations. Each device residing on the
USB ports is assigned an address known only to the USB
sub-device and does not consume any system resources.
When a device is connected to a USB port of a host
computer running a Microsoft® Windows® or MacOS®, the
host PC detects the type of device connected and dynamically
determines the operating system (including driver software
and bus bandwidth) required by each peripheral. The
user installs a device by dragging its icon onto the
System Folder or by running a simple installer application.
This only needs to be done once and the device is then
available each time. USB connections require no terminators,
memory addresses or ID numbers. Cables are inexpensive
and thin with easy to attach connectors, and all in
one standard, namely USB A-B.
USB supports three transmission rates:
USB1.0 - 1.5Mbps
USB1.1 - 12Mbps
USB2.0 - 480Mbps |
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USB1.0 and USB1.1 support data transfer rates of 1.5Mbps
and 12Mbps, for low and full speed peripheral devices
respectively. USB2.0 supports more demanding PC user
applications such as digital imaging and web publishing
where multiple high-speed peripheral devices will be
running simultaneously at data transfer rates of up
to 480Mbps. In addition, USB2.0 is backwards compatible
with USB1.0 and USB1.1-enabled software and peripherals,
offering customers extensive utility with hundreds of
millions of low-speed and full-speed USB devices already
on the market. With the USB connector already ubiquitous
on PC systems, the transition to the USB2.0 specification
is proving relatively smooth.
USB can support simultaneous connection of up to 127
devices by attaching peripherals through interconnected
external USB hubs. When all the USB ports on a system
are used, one can simply attach a USB hub, providing
additional ports, and keep on plugging in more peripherals
and hubs as needed. Each USB port provides electrical
power for devices of up to 500mA. Additional power is
required for peripheral devices exceeding this power
consumption limit.
With the "hot-swap" ability of USB devices
many USB devices can be put into sleep mode by the host
computer when the computer enters a power-saving mode.
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