Page 37 - EE2449_TTL_Datasheet

SN54/74LS155 • SN54/74LS156

LOGIC DIAGRAM
E a E a A 0 A 1 E b E b
1 2 13 3 14 15

V CC = PIN 16
GND = PIN 8 7 6 5 4 9 10 11 12
= PIN NUMBERS
O 0a O 1a O 2a O 3a O 0b O 1b O 2b O 3b

FUNCTIONAL DESCRIPTION
The LS155 and LS156 are Dual 1-of-4 Decoder/Demulti- AND the minterm functions by tying outputs together. Any
plexers with common Address inputs and separate gated number of terms can be wired-AND as shown below.
Enable inputs. When enabled, each decoder section accepts
the binary weighted Address inputs (A 0 , A 1 ) and provides four f = (E + A 0 + A 1 ) ⋅ (E + A 0 + A 1 ) ⋅ (E + A 0 + A 1 ) ⋅
mutually exclusive active LOW outputs (O 0 –O 3 ). If the Enable (E + A 0 + A 1 )
requirements of each decoder are not met, all outputs of that where E = E a + E a ; E = E b + E b
decoder are HIGH.
Each decoder section has a 2-input enable gate. The E E
enable gate for Decoder “a” requires one active HIGH input A 0 O 0 A 0 O 0
A 1 A 1
and one active LOW input (E a •E a ). In demultiplexing applica-
tions, Decoder “a” can accept either true or complemented E E
data by using the E a or E a inputs respectively. The enable gate A 0 O 1 A 0 O 1
for Decoder “b” requires two active LOW inputs (E b •E b ). The A 1 A 1
LS155 or LS156 can be used as a 1-of-8 Decoder/Demulti- E E
plexer by tying E a to E b and relabeling the common connection A 0 O 2 A 0 O 2
as (A 2 ). The other E b and E a are connected together to form A 1 A 1
the common enable. E E
The LS155 and LS156 can be used to generate all four A 0 O 3 A 0 O 3
minterms of two variables. These four minterms are useful in A 1 A 1
some applications replacing multiple gate functions as shown
in Fig. a. The LS156 has the further advantage of being able to Figure a

TRUTH TABLE
ADDRESS ENABLE “a” OUTPUT “a” ENABLE “b” OUTPUT “b”

A 0 A 1 E a E a O 0 O 1 O 2 O 3 E b E b O 0 O 1 O 2 O 3
X X L X H H H H H X H H H H
X X X H H H H H X H H H H H
L L H L L H H H L L L H H H
H L H L H L H H L L H L H H
L H H L H H L H L L H H L 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

FAST AND LS TTL DATA
5-151
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