Three-state logic

Tri-state buffers are commonly used in bus-based systems, where multiple devices are connected to the same bus and need to share it.

For example, in a computer system, multiple devices such as the CPU, memory, and peripherals may be connected to the same data bus.

The basic concept of the third state, high impedance (Hi-Z), is to effectively remove the device's influence from the rest of the circuit.

Three-state logic can reduce the number of wires needed to drive a set of LEDs (tri-state multiplexing or Charlieplexing).

To enable high-speed operation, the protocol requires that every device connecting to the bus drive the important control signals high for at least one clock cycle before going to the Hi-Z state.

When devices are inactive, they "release" the communication lines and tri-state their outputs, thus removing their influence on the circuit.

A three-state bus is typically used between chips on a single printed circuit board (PCB), or sometimes between PCBs plugged into a common backplane.

[2] Three-state buffers, when used to enable multiple devices to communicate on a data bus, can be functionally replaced by a multiplexer.