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Logics are generally binary digital circuits that know two level states: High and Low or 1 and 0. However, there are also trinary logics that distinguish three states, the tri-state logics (3-state). Like the two-state logics, the tri-state logics have the states High and Low and additionally the state "high impedance", which is marked with "Z".

The three logic states of a tri-state logic can be represented with the digits -1, 0 and 1 or alternatively with the digits 0, 1 and 2. In the trinary number system, each number consists of a combination of the digits 0, 1 and 2. The lowest digit corresponds to the logic state "untrue", the highest digit to the logic state "true" and the middle digit to the logic state "neutral". In the high state, the output level is pulled towards the positive supply voltage, while the low level is pulled towards ground. The third state represents an open, and therefore a high impedance output circuit. This state corresponds to that of an open, unconnected circuit.

Truth table and IEC circuit symbols of tri-state logic

Truth table and IEC circuit symbols of tri-state logic

Tri-state logics have a normal input "A", an enable input "C" which is used for activation, and an output "Q". If input "A" is low (0) and enable input "C" is also low (0), then output "Q" is also low. If the input "A" is High (1) and the enable input "C" is Low (0), then the output "Q" is High (1). Only when the second input "C" is activated, the output becomes high impedance (Z), regardless of the state of input "A".

Englisch: tri-state
Updated at: 02.01.2019
#Words: 248
Links: binary, digital, level, impedance (Z), logic
Translations: DE

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