Three-state logic

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Categories: Electronics stubs | Digital electronics

In electronics a three-state or 3-state digital logic gate is one in which the output circuit can be completely switched off, putting the output in a high-impedance state. The output asserts neither a logic 1 nor logic 0 voltage level and aside from stray capacitance and leakage currents puts no load on the circuits connected to the output. A common synonym is tristate logic, which is a registered trademark of National Semiconductor. Such output circuits are commonly used when many devices must share a common bus, such as for memory or input/output circuit boards.

3-state outputs are implemented in various families of digital integrated circuits such as the Texas Instruments 7400 series of TTL gates, and often in the data and address bus lines of microprocessors.

This form of gate should not be confused with 3-value logic (ternary logic).