In general logic elements, the output signal is determined by the input signal, and is in either the "1" or "0" state.
However, some logic elements have another state besides "1" and "0".
For example, the circuit in the right figure is similar to the NOT circuit, but different form the NOT circuit. The NOT circuit outputs data by inverting the input signal, this circuit outputs the input data as it is.
If the control part is set to “1” (H level), the input data is output as it is.
However, if the control part is set to “0” (L level) as shown in Figure 2, the output part will be disconnected and the data will not be output. This disconnected state is called the high impedance.
Like this circuit, a circuit that can be taken in the three types of output state such as the "1" (H level) state, the "0" (L level) state, and the high impedance state, is called a three-state or "Tri state".