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
Famous quotes containing the word power:
“The fact is that my wife if she had common sense would have more power over me than any other whatsoever, for my heart always alights upon the nearest perch.”
—George Gordon Noel Byron (1788–1824)
“But the life of Spirit is not the life that shrinks from death and keeps itself untouched by devastation, but rather the life that endures it and maintains itself in it. It wins its truth only when, in utter dismemberment, it finds itself.... Spirit is this power only by looking the negative in the face, and tarrying with it. This tarrying with the negative is the magical power that converts it into being. This power is identical with what we earlier called the Subject.”
—Georg Wilhelm Friedrich Hegel (1770–1831)
“The only government that I recognize—and it matters not how few are at the head of it, or how small its army—is that power that establishes justice in the land, never that which establishes injustice.”
—Henry David Thoreau (1817–1862)