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camera, for example) and a joystick. More and more peripherals for multimedia computers arrive on
the market every day. With USB, up to 127 devices can run simultaneously on a computer.
● USB permits "hot plug-in." There's no need to shut down, plug in, reboot and run set-up to install
peripherals. And no need to go through the reverse process to unplug a device.
In short, USB transforms today's "Plug-and-Pray" into true Plug-and-Play!
Hub
A Universal Serial Bus device that provides additional connections to the Universal Serial Bus.
Hubs are a key element in the plug-and-play architecture of USB. The Figure shows a typical hub. Hubs
serve to simplify USB connectivity from the user's perspective and provide robustness at low cost and
complexity.
Hubs are wiring concentrators and enable the multiple attachment characteristics of USB. Attachment points
are referred to as ports. Each hub converts a single attachment point into multiple attachment points. The
architecture supports concatenation of multiple hubs.
The upstream port of a hub connects the hub towards the host. Each of the other downstream ports of a
hub allows connection to another hub or function. Hubs can detect, attach and detach at each downstream
port and enable the distribution of power to downstream devices. Each downstream port can be individually
enabled and configured at either full or low speed. The hub isolates low speed ports from full speed
signaling.
A hub consists of two portions: the Hub Controller and Hub Repeater. The repeater is a protocol-controlled
switch between the upstream port and downstream ports. It also has hardware support for reset and
suspend/resume signaling. The controller provides the interface registers to allow communication to/from
the host. Hub specific status and control commands permit the host to configure a hub and to monitor and
control its ports.
Device
A logical or physical entity that performs a function. The actual entity described depends on the context of
the reference. At the lowest level, device may refer to a single hardware component, as in a memory device.
At a higher level, it may refer to a collection of hardware components that perform a particular function, such
as a Universal Serial Bus interface device. At an even higher level, device may refer to the function
performed by an entity attached to the Universal Serial Bus; for example, a data/FAX modem device.
Devices may be physical, electrical, addressable, and logical.
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