Tri-state buffer

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INPUT		OUTPUT
A	B	C
0	0	Z
1	0	Z
0	1	0
1	1	1

In digital circuits, a tri-state or three state buffer is used to allow multiple logic gates to drive the same output, usually a bus. This circuit is so-called because it has three output states: high (1), low (0), and high impedance (Z).

The high and low states work the same as any other logic gate, by driving the output high or low respectively. The high impedance state, however, ideally does not drive the output at all, allowing another circuit (usually another tri-state buffer) to drive it instead.

A tristate buffer can be thought of as a switch. If B is on, the switch is closed. If B is off, the switch is open.

Other implementations are possible, for example, the output may instead be enabled by a low value of B. Which is used often depends on the underlying technology.

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

Note: "Tri-State" is a registered trademark (USPTO #2138646) of National Semiconductor Corporation.