5-429
FAST AND LS TTL DATA
SN54/74LS257B SN54/74LS258B
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:
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
)
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.
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 L L H
L H X H H L
L L L X L H
L L H X H L
H = HIGH Voltage Level
L = LOW Voltage Level
X = Don’t Care
(Z) = High Impedance (off)
GUARANTEED OPERATING RANGES
Symbol Parameter Min Typ Max Unit
V
CC
Supply Voltage 54
74
4.5
4.75
5.0
5.0
5.5
5.25
V
T
A
Operating Ambient Temperature Range 54
74
–55
0
25
25
125
70
°C
I
OH
Output Current — High 54
74
–1.0
–2.6
mA
I
OL
Output Current — Low 54
74
12
24
mA
