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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.