Understanding USB Ports

Universal Serial Bus (USB) is an external bus technology for high-speed connection of peripheral devices. The USB specification was published in 1996 through the joint work of several major computer companies. USB support has become widespread for many reasons.

USB offers support for almost every kind of external computer device known. Monitors, mice, joysticks, printers, scanners, cameras, storage devices, modems, and more can all be connected via USB. A single built-in USB controller can support up to 127 devices. The devices can be connected via additional USB hubs, daisy-chained from one device to another, or both.

USB can transfer data at up to 12 megabits per second (Mbps). That makes it an excellent choice for data-intensive applications such as video. Slower devices such as mice operate at 1.5Mbps. In addition, USB offers the following enhancements:

• USB is capable of self-identifying devices that are attached.

• Hot-pluggable devices can be added or removed while the computer is running.

• Power can be supplied by the USB for devices that do not have a separate power supply.

How USB Works

Inside a computer that has USB ports, there is a USB controller. The controller is responsible for interfacing between software and hardware. Inside the controller, there is a host hub that contains two USB connectors, or ports. The cables that attach to the USB ports have four wires. One wire supplies power and another is used as an electrical ground. The two remaining wires are used for signaling.

When a USB device connects to the computer, it is automatically detected by the USB controller and is required to identify itself. After the device is recognized, the USB controller assigns it an ID. During this initial communication, the USB controller determines a device's bandwidth priority. Devices that require no interruption, such as video, have the highest priority. In contrast, printers, which send large amounts of data but don't care when it gets there, have the lowest priority.

Note 

USB is not supported on all operating systems. Windows NT and the first releases of Windows 95 do not support USB. The latest release of Windows 95 had limited support for USB. Currently, Windows 98, Windows 2000, Windows XP, and Windows Server 2003 have full support for USB.

electron gun

The device that shoots electrically charged particles called electrons toward the back of the monitor screen.