What is Tri state logic?

Electronic logic uses just two states, high and low voltage, or "1" and "0". The output of a gate will always be at one value or the other. This is convenient when only a single output is used to drive a signal. In some cases, it is useful to have two or more outputs driving the same signal line. However, if two outputs are linked together, if they have different outputs, there is likely to be damage to the outputs and the level on the line will be un-predictable.

A tri-state output has the same high and low levels as standard logic outputs but it has a third state, namely high impedance. A high impedance state means that the output is not transferred to the line so effectively, the output is simply turned off. Another logic gate can now drive the line and the level is entirely predictable. Numerous outputs can now drive a single line as long as only on is turned on at any time.