## Input/Output Device Driver Reference

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### Internal/External Power

An internally powered device is one that draws current from the system unit 's power supply and has a direct line from the power supply that can be independently and uniquely controlled. An externally powered device is one that draws current from either a power supply outside the system unit's power supply or from an internal device that has no independent or unique control over the power line. For example, an asynchronous serial port (COM ) is an internally powered device, whereas the mouse connected to a COM port is considered an externally powered device because the current supply is not unique to the mouse.