SN74LS257B SN74LS258B
http://onsemi.com
4
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.
When the Select Input is LOW, the I
0
inputs are selected and
when Select is HIGH, the I
1
inputs are selected. The data on
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:
When the Output Enable Input (E
0
) is HIGH, the outputs
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
Z
a
= E
0
• (I
1a
• S + I
0a
• S) Z
b
= E
0
• (I
1b
• S + I
0b
• S)
Z
c
= E
0
• (I
1c
• S + I
0c
• S) Z
d
= E
0
• (I
1d
• S + I
0d
• S)
LS258B
Z
a
= E
0
• (I
1a
• S + I
0a
• S) Z
b
= E
0
• (I
1b
• S + I
0b
• S)
Z
c
= E
0
• (I
1c
• S + I
0c
• S) Z
d
= E
0
• (I
1d
• S + I
0d
• S)
TRUTH TABLE
OUTPUT
ENABLE
SELECT
INPUT
DATA
INPUTS
OUTPUTS
LS257B
OUTPUTS
LS258B
E
O
S I
0
I
1
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)
