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The internal components of the keyboard require power to operate, of course, and the presence of LED indicators on the keyboard proves that power is being provided to the component. No electronics within the PC can operate without power, obviously.
To simplify the operation of the PC, the keyboard was designed from the start to take its power from the system--more specifically, from the motherboard. Thus, two of the wires that run through the keyboard cable carry power and ground signals from the motherboard to the keyboard. This is usually a +5 volt signal, the standard for many devices within the PC.
Interestingly, some companies take advantage of this external power source, using it to power other external peripherals. For example, some storage devices that operate through the PC's parallel port include an adapter that fits between the keyboard connector and the motherboard's keyboard connector port. This "taps" some of the power being sent to the keyboard without touching the actual keyboard signals. This "parasitic" design usually does not cause any problems, but it must be kept in mind that the amount of current supplied by the motherboard to the keyboard is quite small. This means devices trying to obtain power in this manner must be very conservative in their appetites, or trouble will result.