Three-state logic

tri-statetri-state bufferTri-state logicThree-state outputtri-state outputtri-statedtristate3-state buffersbuffersceasing to drive
In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels.wikipedia
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High impedance

high-impedancehigh input impedancehigh-''Z
In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels.
The combined input/output pins found on many ICs are actually tri-state capable outputs which have been internally connected to inputs (resulting in three-state logic or four-valued logic).

Charlieplexing

Three-state logic can reduce the number of wires needed to drive a set of LEDs (tri-state multiplexing or Charlieplexing).
The method uses the tri-state logic capabilities of microcontrollers in order to gain efficiency over traditional multiplexing.

Chip select

slave selectChip select/waitinput/display enable
Many memory devices designed to connect to a bus (such as RAM and ROM chips) have both (chip select) and (output enable) pins, which superficially appear to do the same thing.
Chip select (CS) or slave select (SS) is the name of a control line in digital electronics used to select one (or a set) of integrated circuits (commonly called "chips") out of several connected to the same computer bus, usually utilizing the three-state logic.

Multiplexer

demultiplexermultiplexersMUX
Three-state buffers can also be used to implement efficient multiplexers, especially those with large numbers of inputs.
The following 4-to-1 multiplexer is constructed from 3-state buffers and AND gates (the AND gates are acting as the decoder):

7400-series integrated circuits

7400 series740074 series
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits.

Open collector

open drainopen-collectoropen-drain
The open collector input/output is a popular alternative to three-state logic. Early microcontrollers often have some pins that can only act as an input, other pins that can only act as a push–pull output, and a few pins that can only act as an open collector input/output.
A three-state logic device is unlike an open collector device, because it comprises transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.

Logic level

active lowactive-lowPositive logic
In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels.
In three-state logic, an output device can be in one of three possible states: 0, 1, or Z, with the last meaning high impedance.

Push–pull output

push-pullpush-pull outputpush–pull amplifier
Early microcontrollers often have some pins that can only act as an input, other pins that can only act as a push–pull output, and a few pins that can only act as an open collector input/output.
In this state, the output is said to be floating (or, to use a proprietary term, tri-stated).

Pull-up resistor

pull-down resistorpull-uppulled up
Circuit designers will often use pull-up or pull-down resistors (usually within the range of 1–100 kΩ) to influence the circuit when the output is tri-stated.
* Three-state logic

Three-state bus

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.
A three-state bus is a computer bus connected to multiple tri-state output devices, only one of which can be enabled at any point to avoid bus contention.

Conventional PCI

PCIPCI busPeripheral Component Interconnect
The PCI local bus provides pull-up resistors, but they would require several clock cycles to pull a signal high given the bus's large distributed capacitance.
Pull-up resistors on the motherboard ensure they will remain high (inactive or deasserted) if not driven by any device, but the PCI bus does not depend on the resistors to change the signal level; all devices drive the signals high for one cycle before ceasing to drive the signals.

Three-valued logic

ternary logicthree-valuedlaw of excluded fourth
The term tri-state should not be confused with ternary logic (3-value logic).

Electronics

electronicelectronic equipmentelectronic device
In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels.

Logic gate

digital logiclogic gateslogic circuit
In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels.

Processor register

registersregistergeneral purpose register
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits.

Bus (computing)

buscomputer busdata bus
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits. This allows multiple circuits to share the same output line or lines (such as a bus which cannot listen to more than one device at a time).

Flip-flop (electronics)

flip-flopflip-flopslatch
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits.

4000-series integrated circuits

4000 seriesCD40004000
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits.

Integrated circuit

integrated circuitsmicrochipchip
Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits.

Microprocessor

microprocessorsprocessorprocessors
Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals.

Computer memory

memorymemoriesmain memory
Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals.

Peripheral

peripheralsperipheral devicecomputer peripheral
Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals.

Capacitance

capacitiveself-capacitanceelectrical capacitance
The PCI local bus provides pull-up resistors, but they would require several clock cycles to pull a signal high given the bus's large distributed capacitance.

Leakage (electronics)

leakage currentleakageleakage currents
This way, the pull-up resistors are only responsible for maintaining the bus signals in the face of leakage current.

Low Pin Count

LPCLPC busLPC bridge
Intel refers to this convention as "sustained tri-state", and also uses it in the Low Pin Count bus.