74AC257SC Просмотр технического описания (PDF) - Fairchild Semiconductor
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74AC257SC Datasheet PDF : 9 Pages
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Functional Description
The AC/ACT257 is quad 2-input multiplexer with 3-STATE
outputs. It selects four bits of data from two sources under
control of a Common Data Select input. When the Select
input is LOW, the I0x inputs are selected and when Select
is HIGH, the I1x inputs are selected. The data on the
selected inputs appears at the outputs in true (noninverted)
form. The device is the logic implementation of a 4-pole, 2-
position switch where the position of the switch is deter-
mined by the logic levels supplied to the Select input. The
logic equations for the outputs are as follows:
Za = OE • (11a • S + I0a • S)
Zb = OE • (11b • S + I0b • S)
Zc = OE • (11c • S + I0c • S)
Zd = OE • (11d • S + I0d • S)
When the Output Enable (OE) is HIGH, the outputs are
forced to a high impedance state. If the outputs are tied
together, all but one device must be in the high impedance
state to avoid high currents that would exceed the maxi-
mum ratings. Designers should ensure the Output Enable
signals to 3-STATE devices whose outputs are tied
together are designed so there is no overlap.
Truth Table
Output
Enable
OE
H
L
L
L
L
Select
Input
S
X
H
H
L
L
H = HIGH Voltage Level
L = LOW Voltage Level
X = Immaterial
Z = High Impedance
Data
Inputs
I0
I1
X
X
X
L
X
H
L
X
H
X
Outputs
Z
Z
L
H
L
H
Logic Diagram
Please note that this diagram is provided only for the understanding of logic operations and should not be used to estimate propagation delays.
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