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8/8/2019 TriState Buffers
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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