SN74LS138

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4

FUNCTIONAL DESCRIPTION

The LS138 is a high speed 1-of-8 Decoder/Demultiplexer

fabricated with the low power Schottky barrier diode

process. The decoder accepts three binary weighted inputs

function allows easy parallel expansion of the device to a

1-of-32 (5 lines to 32 lines) decoder with just four LS138s

and one inverter. (See Figure a.)

The LS138 can be used as an 8-output demultiplexer by

using one of the active LOW Enable inputs as the data input

and the other Enable inputs as strobes. The Enable inputs

which are not used must be permanently tied to their

appropriate active HIGH or active LOW state.

TRUTH TABLE

INPUTS

OUTPUTS

H

X

X

X

X

X

H

H

H

H

H

H

H

H

X

HX

X

XX

H

HH

HH

HH

H

X

XLX

XX

H

HH

HH

HH

H

L

LHL

LLL

HH

HH

HH

H

L

LH

H

LL

H

L

H

HH

HH

H

L

LHL

HLH

H

L

HH

HH

H

L

LH

H

HLH

HH

L

H

HH

H

L

LHL

LH

H

HH

H

L

HH

H

L

LH

H

LH

H

HH

HH

L

H

H

L

LHL

HHH

HH

HH

H

L

H

L

L

H

H

H

H

H

H

H

H

H

H

H

L

H = HIGH Voltage Level

L = LOW Voltage Level

X = Don’t Care

H

LS04

E

LS138

LS138

123

1 2 3

123

12 3

Figure a

LS138

LS138

E

E

E