Three-state logic

In digital electronics, a tri-state or three-state buffer is a type of digital buffer that has three stable states: a high voltage output state (logical 1), a low output state (logical 0), and a high-impedance (Hi-Z) state. In the Hi-Z state, the output of the buffer is effectively disconnected from the subsequent circuit.

Tri-state buffers are commonly used in bus-based systems where multiple devices are connected to the same shared bus, because the Hi-Z state allows other devices to drive the bus without interference from the tri-state buffer. For example, in a computer system, multiple devices such as the CPU, memory, and peripherals may be connected to the same data bus. To ensure that only one device can transmit data on the bus at a time, each device is equipped with a tri-state buffer. When a device wants to transmit data, it activates its tri-state buffer, which connects its output to the bus and allows it to transmit data. When the transmission is complete, the device deactivates its tri-state buffer, which disconnects its output from the bus and allows another device to access the bus. Tri-state buffers are also useful for reducing crosstalk and noise on a bus.

Tri-state output can be incorporated into various logic gates, flip-flops, microcontrollers, or other digital logic circuits.

Operation

A tri-state buffer behaves either like an open switch (i.e. presenting a Hi-Z output) when the enable signal B is off or as a regular non-inverting buffer (which duplicates and boosts the input onto the output) when the enable signal is on:

Unlike a simple transmission gate, a tri-state buffer when enabled additionally provides voltage level restoration to boost the input to be well within its valid logic voltage range. A tri-state buffer's behavior is given by the following truth table: