5-411
FAST AND LS TTL DATA
SN54/74LS251
FUNCTIONAL DESCRIPTION
The LS251 is a logical implementation of a single pole,
8-position switch with the switch position controlled by the
state of three Select inputs, S
0
, S
1
, S
2
. Both assertion and
negation outputs are provided. The Output Enable input (E
O
)
is active LOW. When it is activated, the logic function provided
at the output is:
Z = E
O
⋅ (I
0
⋅ S
0
⋅ S
1
⋅ S
2
+ I
1
⋅ S
0
⋅ S
1
⋅ S
2
+ I
2
⋅ S
0
⋅ S
1
⋅
Z = E
O
⋅ S
2
+ I
3
⋅ S
0
⋅ S
1
⋅ S
2
+ I
4
⋅ S
0
⋅ S
1
⋅ S
2
+ I
5
⋅ S
0
⋅
Z = E
O
⋅ S
1
⋅ S
2
+ I
6
⋅ S
0
⋅ S
1
⋅ S
2
+ I
7
⋅ S
0
⋅ S
1
⋅ S
2
).
When the Output Enable is HIGH, both outputs are in the
high impedance (high Z) state. This feature allows multiplexer
expansion by tying the outputs of up to 128 devices together.
When the outputs of the 3-state devices are tied together, all
but one device must be in the high impedance state to avoid
high currents that would exceed the maximum ratings. The
Output Enable signals should be designed to ensure there is
no overlap in the active LOW portion of the enable voltage.
TRUTH TABLE
E
0
S
2
S
1
S
0
I
0
I
1
I
2
I
3
I
4
I
5
I
6
I
7
Z Z
H X X X X X X X X X X X (Z) (Z)
L L L L L X X X X X X X H L
L L L L H X X X X X X X L H
L L L H X L X X X X X X H L
L L L H X H X X X X X X L H
L L H L X X L X X X X X H L
L L H L X X H X X X X X L H
L L H H X X X L X X X X H L
L L H H X X X H X X X X L H
L H L L X X X X L X X X H L
L H L L X X X X H X X X L H
L H L H X X X X X L X X H L
L H L H X X X X X H X X L H
L H H L X X X X X X L X H L
L H H L X X X X X X H X L H
L H H H X X X X X X X L H L
L H H H X X X X X X X H L H
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 4.75 5.0 5.25 V
T
A
Operating Ambient Temperature Range 0 25 70 °C
I
OH
Output Current — High –2.6 mA
I
OL
Output Current — Low 24 mA
