Tristate gates and buffers:
Tristate gate is a logic inverter, which has a tristate output. This digital circuit can be electronically connected or disconnected from the output circuit.
In other words we can say that the circuit can be electronically switched “ON” or “OFF” by an external control. The input will be passed on to the output depending upon the enable input.
Symbol and the truth table of active ‘HIGH’ tristate inverting gate:
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| Active 'HIGH' state inverting gate |
When the enable input is ‘low’ or ‘0’ the input will not be passed to the output circuit. It produces high output impedance ‘Hi-Z’ which blocks the transmission of input.
When the enable input is ‘high’ or ‘1’ the input will be passed to the output circuit. When the input is ‘1’, output will be ‘0’ and when the input is ‘0’, the output will be ‘1’.
Symbol and the truth table of active ‘LOW’ tristate inverting gate:
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| Active 'LOW' tristate inverting gate |
It is just opposite to the above tristate gate. When the enable input is ‘low’ or ‘0’ the input will be passed to the output circuit. When the input is ‘1’, output will be ‘0’ and when the input is ‘0’, the output will be ‘1’.
When the enable input is ‘high’ or ‘1’ the input will be not passed to the output circuit. It produces high output impedance ‘Hi-Z’ which blocks the transmission of input.
Application of tristate gates in buses:
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| Application of tristate buffer |
Tristate buffers are used to separate and isolate the chips between them and also from the data bus. Tristate buffers are used where multiple chips like processors, video, Ethernet in a computer which want to communicate with memory.
All these chips are connected with the shared bus with tristate buffer. Only one chip is enabled at a time and use the bus. To increase the speed the modern computers use point to point link using tristate buffer.
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