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Concept
Three-state logic
Summary
In digital electronics, a tri-state or three-state buffer is a type of digital buffer that has three stable states: a high output state, a low output state, and a high-impedance state. In the high-impedance state, the output of the buffer is disconnected from the output bus, allowing other devices to drive the bus without interference from the tri-state buffer. This can be useful in situations where multiple devices are connected to the same bus and need to take turns accessing it. Systems implementing three-state logic on their bus are known as a three-state bus or tri-state bus.
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. 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 can be implemented using gates, flip-flops, or other digital logic circuits. They are useful for reducing crosstalk and noise on a bus, and for allowing multiple devices to share the same bus without interference.
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. If more than one device is electrically connected to another device, putting an output into the Hi-Z state is often used to prevent short circuits, or one device driving high (logical 1) against another device driving low (logical 0).
Three-state buffers can also be used to implement efficient multiplexers, especially those with large numbers of inputs.
Three-state buffers are essential to the operation of a shared electronic bus.
Official source
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