The Universal Serial Bus

USB (Universal Serial Bus) is designed to be a "plug & play" interface between a computer and add-on devices such as audio players, joysticks, keyboards, telephones, scanners, and printers. USB allows new devices to be added to computers without having to turn the computer off. USB Supports a data speed of 12 megabits per second.

Since October, 1996, Microsoft Windows has been equipped with USB drivers, however there has been a shortage of external USB devices Many new computers (Mid 1998 onwards) and peripheral devices are now equipped with a USB interface.

A significantly faster "plug-and-play" standard, Firewire IEEE1394, originated by Sony, is designed to support much higher data rates along with devices as video camcorders and digital video disk (DVD) players. USB & Firewire are not interchangeable or compatible with each other. They are expected to sit alongside each other as different standards.

Technology Overview

USB, brings Plug-and-Play technology to the external input and output devices found on today's high-performance personal computers. USB brings with it three main features:

• Ease of use through hot plugging and automatic configuration
• Standardised connection points and simplified connector design
• Simple expansion through the use of a tiered-star hub topology

USB is motivated by the need to make computers even more user-friendly. The call to provide an inexpensive, standardised interface for today's newest PC communications and I/O devices has allowed USB to emerge.

The USB specification defines four transfer types in order to enable a variety of peripherals: Control, Isochronous, Interrupt and Bulk. Every peripheral must support control transfers for configuration, command, and status information flows. Isochronous transfers provide guaranteed bus access, constant data rate and error tolerance for devices such as CTI (computer-telephone integration) and audio systems. Interrupt transfers were designed to support human input devices such as joysticks, mice and keyboards that need to communicate small amounts of data infrequently, but with bounded service periods. Bulk transfers enable devices like printers, scanners, and digital cameras to communicate large amounts of data to the PC as bus bandwidth becomes available

Topology Overview

The USB topology has three elements that work together to enable the four different transfer types: Host, Hub, and Function. Within a USB system, the host controls the flow of data and control information over the bus. This host capability is normally found on the PC motherboard, such as the integrated USB host found in Intel's PCIset chip sets. Functions provide capabilities to the host system. These can include typical PC activities such as keyboard or joystick input and monitor controls, or more advanced activities like digital telephony and image transfer.

Finally, hubs provide an expansion point for USB by supplying a connection to other USB peripherals.