Tri-State Buffer

A three-state gate is a digital circuit that exhibits three states. Two of the states are signals equivalent to logic 1 and 0 as in a conventional gate. The third state is a high-impedance state. The high-impedance state behaves like an open circuit, which means that the output is disconnected and does not have a logic significance. Three-state gates may perform any conventional logic, such as AND or NAND. However, the one most commonly used in the design of a bus system is the buffer gate.

Application of Tri-State Buffer: A bus system can be constructed with three-states gates instead of multiplexers too.

It is distinguished from a normal buffer by having both a normal input and a control input. The control input determines the output state. When the control input is equal to 1, the output is enabled and the gate behaves like any conventional buffer, with the output equal to the normal input.

When the control input is 0, the output is disabled and the gate goes to a high-impedance state, regardless of the value in the normal input.

Special Feature of tri state-buffer: The high-impedance state of a three-state gate provides a special feature not available in other gates. Because of this feature, a large number of three-state gate outputs can be connected with wires to form a common bus line without endangering loading effects.