The Universal Serial Bus

In 1994 an alliance of four industrial partners (Compaq, Intel, Microsoft and NEC) started to specify the Universal Serial Bus (USB). The bus was originally designed with these intentions:

• Connection of the PC to the telephone

• Ease-of-use

• Port expansion

The specification (version 1.0) was first released in January 1996, with some changes being issued as version 1.1 in September 1998. The latest version of the specification is Version 2.0, which was release in FIXME.

The USB is strictly hierarchical and it is controlled by one host. The host uses a master / slave protocol to communicate with attached USB devices. This means that every kind of communication is initiated by the host and devices cannot establish any direct connection to other devices. This seems to be a drawback in comparison to other bus architectures but it is not because the USB was designed as a compromise of costs and performance. The master / slave protocol solves implicitly problems like collision avoidance or distributed bus arbitration. The current implementation of the USB allows 127 devices to be connected at the same time.