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In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the output from the circuit. This allows multiple circuits to share the same output line or lines (such as a bus which cannot listen to more than one device at a time). Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits. Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals. Many devices are controlled by an active-low input called (Output Enable) which dictates whether the outputs should be held in a high-impedance state or drive their respective loads (to either 0- or 1-level). The term ''tri-state''〔''Tri-state is a registered trademark of National Semiconductor but is often used to describe devices made by any manufacturer.''〕 should not be confused with ternary logic (''3-value'' logic). == Uses == The whole concept of the third state, high impedance (Hi-Z), is to effectively remove the device's influence from the rest of the circuit. If more than one device is electrically connected, putting an output into the Hi-Z state is often used to prevent short circuits, or one device driving high (logical 1) against another device driving low (logical 0). Three-state buffers can also be used to implement efficient multiplexers, especially those with large numbers of inputs. Three-state buffers are essential to the operation of a shared electronic bus. Three-state logic can reduce the number of wires needed to drive a set of LEDs (tristate multiplexing or Charlieplexing). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Three-state logic」の詳細全文を読む スポンサード リンク
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