SN74LS251 查看數據表（PDF） - ON Semiconductor

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SN74LS251

8-Input Multiplexer with 3-State Outputs

ON Semiconductor 

SN74LS251

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, S0, S1, S2. Both assertion and

negation outputs are provided. The Output Enable input

(EO) is active LOW. When it is activated, the logic function

provided at the output is:

Z = EO ⋅ (I0 ⋅ S0 ⋅ S1 ⋅ S2 + I1 ⋅ S0 ⋅ S1 ⋅ S2 + I2 ⋅ S0 ⋅ S1 ⋅

Z = EO ⋅ S2 + I3 ⋅ S0 ⋅ S1⋅ S2 + I4 ⋅ S0 ⋅ S1 ⋅ S2 + I5 ⋅ S0 ⋅

Z = EO ⋅ S1 ⋅ S2 + I6 ⋅ S0 ⋅ S1 ⋅ S2 + I7 ⋅ S0 ⋅ S1 ⋅ S2).

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

E0 S2 S1 S0 I0 I1 I2 I3 I4 I5 I6

H X X X X X X X XX X

L

L L L L X X X XX X

L

L L L H X X X XX X

L

L L H X L X X XX X

L

L L H X H X X XX X

L

L H L X X L X XX X

L

L H L X X H X XX X

L

L H H X X X L XX X

L

L H H X X X H XX X

L

H L L X X X X LX X

L

H L L X X X X HX X

L

H L H X X X X XL X

L

H L H X X X X XH X

L

H H L X X X X XX L

L

H H L X X X X XX H

L

H H H X X X X XX X

L

H H H X X X X XX X

H = HIGH Voltage Level

L = LOW Voltage Level

X = Don’t Care

(Z) = High impedance (Off)

I7 Z Z

X (Z) (Z)

XHL

X LH

XHL

X LH

XHL

X LH

XHL

X LH

XHL

X LH

XHL

X LH

XHL

X LH

L HL

H LH

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