USB Hub - Power

Power

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 the hub needs one unit for itself). If a device requires more units of current than the port it is plugged into can supply, 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 500 mA) to every port. Many hubs can operate as either bus powered or self powered hubs.

However, there are many non-compliant hubs on the market which announce themselves to the host as self-powered despite really being bus-powered. Equally there are plenty of non-compliant devices that use more than 100 mA without announcing this fact (or indeed sometimes without identifying themselves as USB devices at all). These hubs and devices do allow more flexibility in the use of power (in particular many devices use far less than 100 mA and many USB ports can supply more than 500 mA before going into overload shut-off) but they are likely to make power problems harder to diagnose.

Some self-powered hubs do not supply enough power to drive a 500mA load on every port. For example, many seven port hubs have a 1A power supply, when in fact seven ports could draw a maximum of 7 x 0.5 = 3.5A, plus power for the hub itself. Designers assume the user will most likely connect many low power devices and only one or two requiring a full 500mA. On the other hand, the packaging for some self-powered hubs states explicitly how many of the ports ports can drive a 500 mA full load at once. For example the packaging on a seven-port hub might claim to support a maximum of four full-load devices.

Read more about this topic:  USB Hub

Other articles related to "power, powers":

Kawasaki Heavy Industries - Products - Energy Plants and Facilities
... an alternative to fossil fuels such as wind power generation, biomass power generation, photovoltaic systems and rechargeable batteries ... Ash handling systems Combined cycle power plants Nuclear power plant equipment Boilers ...
Factors Of Production - Historical Schools and Factors - Marxian
... made a clear distinction between labor actually done and an individual's "labor power" or ability to work ... as "effort" or "labor services." Labor-power might be seen as a stock which can produce a flow of labor ... Labor, not labor power, is the key factor of production for Marx and the basis for Marx's labor theory of value ...
Universal Serial Bus - Power - Powered USB
... references or sources Powered USB uses standard USB signaling with the addition of extra power lines ... used in retail systems and provides enough power to operate stationary barcode scanners, printers, PIN pads, signature capture devices, etc ... such that the bottom connector accepts a standard USB plug and the top connector takes a power connector ...
History of Feudalism
... rule over their allocated land and their power over the territory came to encompass the social, political, judicial, and economic spheres ... These acquired powers significantly diminished centralized power in these empires ... Only when the infrastructure existed to maintain centralized power—as with the European monarchies—did Feudalism begin to yield to this new organized power and eventually disappear ...
Optical Amplifier - Laser Amplifiers - Gain Saturation
... The inversion level of a DFA is set, primarily, by the power of the pump wavelength and the power at the amplified wavelengths ... As the signal power increases, or the pump power decreases, the inversion level will reduce and thereby the gain of the amplifier will be reduced ... – as the signal level increases, the amplifier saturates and cannot produce any more output power, and therefore the gain reduces ...

Famous quotes containing the word power:

    There is a power in love to divine another’s destiny better than that other can, and, by heroic engagements, hold him to his task.
    Ralph Waldo Emerson (1803–1882)

    I am firmly opposed to the government entering into any business the major purpose of which is competition with our citizens ... for the Federal Government deliberately to go out to build up and expand ... a power and manufacturing business is to break down the initiative and enterprise of the American people; it is the destruction of equality of opportunity amongst our people, it is the negation of the ideals upon which our civilization has been based.
    Herbert Hoover (1874–1964)

    In the United States, though power corrupts, the expectation of power paralyzes.
    John Kenneth Galbraith (b. 1908)