Universal Serial Bus USB

The world of consumer computers has spawned numerous technologies for connecting CPUs to each other and to other peripherals. Some of these technologies have been very successful. These include the RS-232 serial link, the IEEE-1284 parallel link, the Apple ADB link and more.

In the course of their existence, these technologies have undergone sometimes drastic transformations. For example, the parallel link, originally unidirectional and not very efficient, is now sometimes used as an ersatz SCSI connection to connect CD-ROM burners.

At the other end of the spectrum, more ambitious technologies have made it possible to connect computers to each other in a high-performance and reliable way, but not always easily. For example, while the SCSI standard is renowned for its efficiency, it remains tricky to maintain in a configuration that often varies. It does not support untimely connections or disconnections. Computer interconnection via Ethernet is also reliable and now very fast, but it is still prohibitively expensive (not least because of the transformer required) to replace simple links such as serial or parallel.

These high-performance technologies have also evolved, and have been the subject of many different versions. In some cases, it has proved difficult to ensure backward compatibility. For example, the SCSI standard has adopted four different versions of physical connectors, requiring the use of complex and costly adapters.

All these factors explain why these technologies are not widely used in consumer applications.

What's more, the communication protocols used on top of these technologies rarely have the flexibility and robustness required for consumer products. The versatility of personal computers means that peripherals of very different types and characteristics (e.g. scanners, printers, modems, digital cameras, etc.) can be connected to them. The mobility of some of these peripherals means that the mini-network formed by the computer and its technological environment must be dynamically reconfigured on an almost permanent basis. Finally, users, who generally refuse to read the documentation accompanying products, are above all looking for simple solutions that "resist their ignorance", or even anticipate future needs. The expected result must be obtained whatever the conditions of use.

The needs that prompted the initial development of the Universal Serial Bus by Compaq, Intel, Microsoft and NEC were :

• easy connection with coding ;

• (almost) automatic configuration and reconfiguration ;

• replacing the multiple standard connection technologies...