usb 2.0 spec

　　The original motivation for the Universal Serial Bus （USB） came from three interrelated considerations：

　　? Connection of the PC to the telephone

　　It is well understood that the merge of computing and communication will be the basis for the next

　　generation of productivity applications. The movement of machine-oriented and human-oriented data

　　types from one location or environment to another depends on ubiquitous and cheap connectivity

　　Unfortunately， the computing and communication industries have evolved independently. The USB

　　provides a ubiquitous link that can be used across a wide range of PC-to-telephone interconnects.

　　? Ease-of-use

　　The lack of flexibility in reconfiguring the PC has been acknowledged as the Achilles’ heel to its

　　further deployment. The combination of user-friendly graphical interfaces and the hardware and

　　software mechanisms associated with new-generation bus architectures have made computers less

　　confrontational and easier to reconfigure. However， from the end user’s point of view， the PC’s I/O

　　interfaces， such as serial/parallel ports， keyboard/mouse/joystick interfaces， etc.， do not have the

　　attributes of plug-and-play.