A tri-state buffer is like the regular buffer we discussed earlier, except that there is an additional input to control the output of the buffer. Depending on this control input, the output of the buffer can be 0, 1, or no function. So there are three kinds of output. In Figure 2.11a, if the control input C is 1 then the buffer is operating as usual. However, if the control input C is 0 then the buffer is in a no function state, there is no output signal. The symbol ? is used to indicate this no function state. Note that the ? state does not indicate 0 or 1, but indicates that there is no signal. In electronics terms this state is called high impedance. An inverse control tri-state buffer is similar to a tri-state buffer except that its control inputs are complements. See Figure 2.11b.
Figure 2.11: Tri-state buffer and Inversion control tri-state buffer
An output that is not electronically connected is different from an output that produces 0. Not electronically connected means there is no electronic signal while logic 0 is connected to GND. With a tristate buffer, it is possible to connect several outputs together without the risk of short circuits, as long as it is maintained that only one tri-state buffer is active at any one time. Tri-state buffers are important when implementing registers.
Table 2.1: Basic properties of Boolean algebra