5-2

FAST AND LS TTL DATA

SN54/74LS155

• SN54/74LS156

LOGIC DIAGRAM

Ea Ea

A0

A1

EbEb

O0a

O1a

O2a

O3a

O0b

O1b

O2b

O3b

14

12

6

7

3

4

5

9

11

12

10

13

15

VCC = PIN 16

GND = PIN 8

= PIN NUMBERS

FUNCTIONAL DESCRIPTION

The LS155 and LS156 are Dual 1-of-4 Decoder/Demulti-

plexers with common Address inputs and separate gated

Enable inputs. When enabled, each decoder section accepts

the binary weighted Address inputs (A0, A1) and provides four

mutually exclusive active LOW outputs (O0–O3). If the Enable

requirements of each decoder are not met, all outputs of that

decoder are HIGH.

Each decoder section has a 2-input enable gate. The

enable gate for Decoder “a” requires one active HIGH input

and one active LOW input (Ea•Ea). In demultiplexing applica-

tions, Decoder “a” can accept either true or complemented

data by using the Ea or Ea inputs respectively. The enable gate

for Decoder “b” requires two active LOW inputs (Eb•Eb). The

LS155 or LS156 can be used as a 1-of-8 Decoder/Demulti-

plexer by tying Ea to Eb and relabeling the common connection

as (A2). The other Eb and Ea are connected together to form

the common enable.

The LS155 and LS156 can be used to generate all four

minterms of two variables. These four minterms are useful in

some applications replacing multiple gate functions as shown

in Fig. a. The LS156 has the further advantage of being able to

AND the minterm functions by tying outputs together. Any

number of terms can be wired-AND as shown below.

f = (E + A0 + A1) ⋅ (E + A0 + A1) ⋅ (E + A0 + A1) ⋅

(E + A0 + A1)

where E = Ea + Ea; E = Eb + Eb

Figure a

E

A0

A1

E

A0

A1

E

A0

A1

E

A0

A1

O0

O1

O2

O3

E

A0

A1

E

A0

A1

E

A0

A1

E

A0

A1

O0

O1

O2

O3

TRUTH TABLE

ADDRESS

ENABLE “a”

OUTPUT “a”

ENABLE “b”

OUTPUT “b”

A0

A1

Ea

Ea

O0

O1

O2

O3

Eb

Eb

O0

O1

O2

O3

X

X

L

X

H

H

H

H

H

X

H

H

H

H

X

XX

HH

HH

H

X

HH

HH

H

L

LH

LL

HH

H

L

LL

HH

H

H

LH

LH

LH

H

L

LH

LH

H

L

HH

LH

HL

H

L

LH

HL

H

H

H

H

L

H

H

H

L

L

L

H

H

H

L

H = HIGH Voltage Level

L = LOW Voltage Level

X = Don’t Care