See also: Tri-state area and tripoint.

In digital electronics three-state, tri-state, or 3-state logic allows output ports to have a value of 0, 1, or Z. A Z output stands for the output port being disconnected from the rest of the circuit, putting the output in a high impedance state. The intent of this state is to allow multiple circuits to share the same output line or bus without affecting each other. Tri-state is a registered trademark of National Semiconductor but is often used to describe devices made by any manufacturer.

Three-state outputs are implemented in various families of digital integrated circuits such as the 7400 series of TTL gates, and often in the data and address bus lines of microprocessors. Three-state outputs may be found on individual logic gates, or in multiples in one integrated circuit package as a buffer for connection to a bus.

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

Uses of three-state logic

Three-state buffers can be used to implement efficient multiplexers, especially those with large numbers of inputs.

Three-state logic devices are used to accommodate multiple bus drivers. If the outputs of several tri-state logic are electrically connected together, only one three-state logic device may be active. The other three-state logic devices may be in the high impedance mode and thus will not affect the output of the active three-state logic device.

Three-state logic can reduce the number of wires needed to drive a set of LEDs (tristate multiplexing).

See also

• Ternary logic
• Buffer