USB Hubs: The Problems of Power

USB has provided the computer user with great benefits. One of the most overlooked features on USB hubs is how it receives and distributes power. There are two power handling methods for USB; Bus Power and Self Power.

Bus Power
Power from the USB controller is allocated in units of 100 mA up to a maximum total of 500 mA per port. If more current is required by a device than the USB port can provide, the operating system usually reports this. Devices that do not get enough power may seem to power up, but will fail to function.

Self Power
A self-powered hub is one that takes its power from an external power supply. Because they are powered by an external power supply, they are able to provide full power (up to 500 mA) to every port. By having the full 500 mA available for devices, it is less likely that devices will fail because of low power issues.

Choosing Between the Two
One power plan is not inherently better or worse than the other. There are certain situations where some are better than others. Low power devices such as mice, keyboards, webcams, and some hubs do not require much power and bus power is more than sufficient for the device.
Hubs with more than 4 ports should be self powered, because if more than 4 devices are plugged in there may be power problems. Some hubs may be bus powered and provide for a power supply to be connected. This style of hub, like MANHATTAN's 7 Port, Dual Power Hub provide the user extra versatility of both power plans.
When people use active extension cables or line extenders, they should always try to use a self-powered hub at the terminating end of the extension. This ensures that any loss of power in the extension is remedied before USB devices are attached.