In 1994 an alliance of four industrial partners (Compaq, Intel, Microsoft and NEC) started to specify the Universal Serial Bus (USB). The USB version 1.0 was first released in January 1996. The USB version 1.1 was released in September 1998. The USB version 2.0 was announced in 1999.
The USB 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.
USB Speeds The USB has three different speeds. Low Speed (1.5Mbits/s), Full Speed (12Mbits/s) and High speed (480Mbits/s). USB Version 1.1 supported Low Speed mode (1.5MBits/s) and Full Speed mode (12Mbits/s). USB version 2.0 supported all the above modes.
USB Host Controller Most modern motherboard chipsets provide a USB host controller. Older machines which are not equipped with a USB host controller can be upgraded using a PCI cards with a host controller built in.
USB host controllers are compatible with either the Open Host Controller Interface (OHCI, by Compaq) or the Universal Host Controller Interface (UHCI, by Intel) standard. Both types have the same capabilities and USB devices work with both host controller types.
USB Connections
USB uses 4 shielded wires of which two are power (+5v & GND). The remaining two are twisted pair differential data signals. It uses a NRZI (Non Return to Zero Invert) encoding scheme to send data with a sync field to synchronise the host and receiver clocks. All devices have an upstream connection to the host and all hosts have a downstream connection to the device. Upstream and downstream connectors are not mechanically interchangeable, thus eliminating illegal loopback connections at hubs such as a downstream port connected to a downstream port. There are commonly two types of connectors, called type A and type B which are shown below.
Type A plugs always face upstream. Type A sockets will typically find themselves on hosts and hubs. For example type A sockets are common on computer main boards and hubs. Type B plugs are always connected downstream and consequently type B sockets are found on devices. Table 1 Shows standard internal wire colours which are used in USB cables.
Table 1: Standard Internal Wire Colours used in USB Cables
