Usb Devices and Transfer Characteristics

There are a wide range of USB devices intended for a wide range of purposes, and this means that implementation details can vary widely.

A device can be self powered, bus powered or both. The USB can provide a power supply up to 500mA for its devices. If there are only bus powered devices on the bus the maximum power dissipation could be exceeded and therefore self powered devices exist. They need to have their own power supply. Devices that support both power types can switch to self powered mode when attaching an external power supply.

Even the maximum communication speed can differ for particular USB devices. The USB specification differentiates between low speed and full speed devices. Low speed devices (such as mice, keyboards, joysticks etc.) communicate at 1.5MBit/s and have only limited capabilities. Full speed devices (such as audio and video systems) can use up to 90% of the 12Mbit/s which is about 10Mbit/s including the protocol overhead.