System design/black box

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In system design, a "black box" is any component whose internals are not up for discussion. This may be either because they are irrelevant, because the device cannot be opened without damaging it, or because the internal functioning cannot be determined by human inspection.

A black box must be viewed in terms of a fixed set of input and output channels, where a specific set of rules is applied to the inputs in order to produce the outputs. The mechanism which executes the rules cannot be examined directly, and the rules may either be inferred from its behavior (what outputs it produces in response to a set of inputs_ or taken on faith from specifications provided by the manufacturer. In the latter case, it is often possible to run tests to confirm that the box behaves in expected ways under various conditions. If any of these tests fail, however, the entire black box must be regarded as defective, as it cannot be repaired.