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A USB hub is a device that allows many USB devices to be connected to a single USB port on the host computer or another hub. USB hubs are often built into equipment, normally keyboards, or monitors or more rarely printers. Separate USB hubs come in a wide variety of form factors from boxes that look similar to a network hub to small designs intended to be plugged directly into the USB port on a computer (that is, without a connecting cable).
A bus-powered hub is a hub that draws all its power from the host computer's USB interface. It does not need a separate power connection. However, many devices require more power than this method can provide, and will not work in this type of hub.
USB current (related to power) is allocated in units of 100 mA up to a maximum total of 500 mA per port. Therefore a compliant bus powered hub can have no more than four downstream ports and cannot offer more than four 100 mA units of current in total to downstream devices (since one unit is needed for the hub itself). If more units of current are required by a device than can be supplied by the port it is plugged into, the operating system usually reports this to the user.
In contrast a self-powered hub is one that takes its power from an external power supply unit and can therefore provide full power (up to 500mA) to every port. Many hubs can operate as either bus powered or self powered hubs. |
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