SN74LS257B SN74LS258B

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FUNCTIONAL DESCRIPTION

The LS257B and LS258B are Quad 2-Input Multiplexers

with 3-state outputs. They select four bits of data from two

sources each under control of a Common Data Select Input.

the selected inputs appears at the outputs in true

(non-inverted) form for the LS257B and in the inverted form

for the LS258B.

The LS257B and LS258B are the logic implementation of

a 4-pole, 2-position switch where the position of the switch

is determined by the logic levels supplied to the Select Input.

The logic equations for the outputs are shown below:

are forced to a high impedance “off” 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

maximum ratings. Designers should ensure that Output

Enable signals to 3-state devices whose outputs are tied

together are designed so there is no overlap.

LS257B

LS258B

TRUTH TABLE

OUTPUT

ENABLE

SELECT

INPUT

DATA

INPUTS

OUTPUTS

LS257B

OUTPUTS

LS258B

S

Z

Z

H

X

X

X

(Z)

(Z)

L

H

X

LL

H

L

H

X

HH

L

L

L

L

XL

H

L

L

H

X

H

L

H = HIGH Voltage Level

L = LOW Voltage Level

X = Don’t Care

(Z) = High Impedance (off)