USB communications require a host computer with USB support, one or more devices with USB ports, and hubs, connectors, and cables as needed to connect the devices to the host computer.The host computer is a PC or other computer that contains a USB host controller and a root hub. The host controller formats data for transmitting on the bus and translates received data to a format that operating-system components understand. The host controller also helps manage communications on the bus.
The root hub has one or more connectors for attaching devices. The root hub and host controller together detect attached and removed devices, carry out requests from the host controller, and pass data between devices and the host controller. In addition to the root hub, a bus may have one or more external hubs.Each device has hardware and firmware as needed to communicate with the host computer. The USB specifications define the cables and connectors that connect devices to their hubs. The topology, or arrangement of connections, on the bus is a tiered star ). At the center of each star is a hub, and each connection to the hub is a point on the star. The root hub is in the host. An external hub has one upstream (host-side) connector for communicating with the host and one or more downstream (device-side) connectors or internal connections to embedded devices. A typical hub has two, four, or seven ports. When multiple hubs connect in series, you can think of the series as a tier, one above the next.
The tiered star describes only the physical connections. In programming, all that matters is the logical connection. Host applications and device firmware don’t need to know or care whether the communication passes through one hub or five.
Tags: specifications define, the cables, The USB
