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The Universal Serial Bus
By Stephen Bucaro
Ports are connectors provided on a computer to comunicate
with peripheral devices such as modems and printers. The
RS232 serial port introduced in 1969 could transfer data
at a speed of 20 Kbps (kilobytes per second). The parallel
port introduced in 1981 could transfer data at a speed of
150 Kbps. Later improvements increased the speed of the
parallel port to 4 Mbps (megabits per second). Serial ports
and parallel ports are frequently subject to resource
conflicts.
In 1996 the Universal Serial Bus (USB) was developed to
replace the serial and parallel ports. Today, the USB port
is used to connect scanners, digital cameras, and external
storage devices like CD-RW drives. USB 1.1 can transfer
data at a speed of 12 Mbps. USB 2.0, introduced in 2000,
can transfer data at a speed of 480 Mbps.
Besides much higher speed, a main feature of USB is that
you can plug and unplug devices without restarting the
computer. The controller will sense the device and
reconfigure. Installing a USB device is as simple as
plugging it into a USB port.
Another feature of USB is that a single controller can
support up to 127 devices simultaneously. A USB device can
provide a port for another device, allowing you to "daisy
chain" devices together, or you can use a stand alone hub.
USB uses a single IRQ, I/O address range, and DMA
controller to poll the devices for data.
The USB cable has four wires, two for communications, and
two that can provide up to 500 mA (milliamps) of power to
the USB devices. This means devices that only need a small
amount of power don't need to have a separate power supply
and power cord.
As you can see from the above specifications, USB 2.0 is
40 times faster than USB 1.1. But most operating systems
do not yet have native support for USB 2.0. Microsoft has
released USB 2.0 drivers for Windows XP. You can download
them from the Windows Update site:
http://windowsupdate.microsoft.com.
You can plug a USB 2.0 device into a USB 1.1 port, but you
will only get the 12 Mbps transfer speed. Similarly, you
can plug a USB 1.1 device into a USB 2.0 port, but you will
only get the 12 Mbps transfer speed.
If your computer was built in 1997 or later, you probably
have USB 1.1 support. The USB Implementers Forum provides
a free utility download that examines your hardware to
determine its USB capability. You can download it from:
http://www.usb.org/data/usbready.exe
If you have a computer with USB 1.1 support, you can use an
expansion card that provides USB 2.0.
Manufacturers of USB 2.0 devices provide drivers that work
with Windows 98 or higher. First you insert a CD that
comes with the product to install the driver. Then, when you
plug in the USB cable, the port will automatically
configure for the device.
The USB standard requires cables to operate at a length of
up to five meters, but many hardware manufacturers
guarantee their products to work only with the much shorter
cable that they provide.
Resource Box:
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