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Digital ElectronicsDigital ElectronicsElectronics Technology

Landon Johnson

TRISTATE BUFFERS

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Tristate CompetenciesTristate Competencies

54. Without references, the student will draw atristate buffer with an active-low enable input

with 100% accuracy.

55. Given a schematic symbol of a tristate buffer

the student will fill in the output column of atruth table for all possible combinations with100% accuracy.

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Tristate VocabularyTristate Vocabulary

TRISTATE BUFFER- commonly used to connect multiple signalsto a single bus line; three output states HIGH, LOW, & HI-Z.

HI-Z (HIGH IMPEDANCE)- when the buffer is disabled, outputis like an open circuit.

DATA BUS- the transfer of data in computers takes place over acommon set of connecting lines; the inputs and outputs ofmany devices are tied to the common bus.

BUS CONTENTION- if the outputs of two or more active devices areplaced on the bus at the same time, the result is data corruptionand damaging currents.

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TRISTATE BUFFERSTRISTATE BUFFERS

E A X

0 X HI-Z

1 0 0

1 1 1

E

XA A

E

X

A X

1 X HI-Z

0 0 0

0 1 1

E

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TRISTATE BUFFERS ICSTRISTATE BUFFERS ICS

E X

0HI-Z

1 A

E X

0HI-Z1A

E

XA

74LS12574LS125

E

XA

74LS12674LS126

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TRISTATE INVERTERSTRISTATE INVERTERS

E

XA A

E

X

E A X

X I-

A X

X I-

E

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TRISTATE BUS DRIVERSTRISTATE BUS DRIVERS

CIRCUITS THAT BUFFER THE OUTPUTS OF DEVICESCONNECTED TO

ACOMMON BUS

USED WHERE A LARGE NUMBER OF DEVICES SHARE ACOMMON BUS

Ea

A

Eb

B

Ec

C

COMMON BUS

only one Exshould be activated at any time

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EDC A B BUS

1 0 0 0 1

0 0 01 1

0 1 0 1 1

0 0 0 1 00 0 0 0 0

1 0 1 1 1

BUSDRIVERS

D

E

D

C

BUS

LINE

E

B

A

FILLIN H A LEFROM HE CIRC IT

B

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TRISTATE ICsTRISTATE ICs

Many ICs are designed with tristate outputs.

74LS374- Octal D Type Flip Flop with tristate outputs.

Decoders

Muxs

A/D Converters

Memory Chips

Micro Processors

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TEST

COMPLETE THE TRUTH TABLECOMPLETE THE TRUTH TABLE

E A X0

1

1

X HI-Z

0 1

1 0E

XA

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TEST

COMPLETE THE TRUTH TABLECOMPLETE THE TRUTH TABLE

A

E

XA X

1

0

0

EX HI-Z

0 0

1 1