Ports are connectors provided on a computer to comunicatewith peripheral devices such as modems and printers. TheRS232 serial port introduced in 1969 could transfer dataat a speed of 20 Kbps (kilobytes per second). The parallelport introduced in 1981 could transfer data at a speed of150 Kbps. Later improvements increased the speed of theparallel port to 4 Mbps (megabits per second). Serial portsand parallel ports are frequently subject to resourceconflicts.
In 1996 the Universal Serial Bus (USB) was developed toreplace the serial and parallel ports. Today, the USB portis used to connect scanners, digital cameras, and externalstorage devices like CD-RW drives. USB 1.1 can transferdata 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 thatyou can plug and unplug devices without restarting thecomputer. The controller will sense the device andreconfigure. Installing a USB device is as simple asplugging it into a USB port.
Another feature of USB is that a single controller cansupport up to 127 devices simultaneously. A USB device canprovide a port for another device, allowing you to "daisychain" devices together, or you can use a stand alone hub.USB uses a single IRQ, I/O address range, and DMAcontroller to poll the devices for data.
The USB cable has four wires, two for communications, andtwo that can provide up to 500 mA (milliamps) of power tothe USB devices. This means devices that only need a smallamount of power don't need to have a separate power supplyand power cord.
As you can see from the above specifications, USB 2.0 is40 times faster than USB 1.1. But most operating systemsdo not yet have native support for USB 2.0. Microsoft hasreleased USB 2.0 drivers for Windows XP. You can downloadthem from the Windows Update site:http://windowsupdate.microsoft.com.
You can plug a USB 2.0 device into a USB 1.1 port, but youwill only get the 12 Mbps transfer speed. Similarly, youcan plug a USB 1.1 device into a USB 2.0 port, but you willonly get the 12 Mbps transfer speed.
If your computer was built in 1997 or later, you probablyhave USB 1.1 support. The USB Implementers Forum providesa free utility download that examines your hardware todetermine its USB capability. You can download it from: http://www.usb.org/data/usbready.exeIf you have a computer with USB 1.1 support, you can use anexpansion card that provides USB 2.0.
Manufacturers of USB 2.0 devices provide drivers that workwith Windows 98 or higher. First you insert a CD thatcomes with the product to install the driver. Then, when youplug in the USB cable, the port will automaticallyconfigure for the device.
The USB standard requires cables to operate at a length ofup to five meters, but many hardware manufacturersguarantee their products to work only with the much shortercable that they provide.
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