Topic 3

Digital Electronics
Electronics Technology
Encoding, Decoding, and
Seven-Segment Displays

ENCODER
•ENCODER- a digital circuit that produces a
binary output code depending on which of its
inputs are activated.
•There are ONLY ONE INPUT can be active at
any given time (two or more simultaneously
active inputs would produce an undefined
output combination)

Block Diagram
BINARY CODE
OUTPUT
ONLY ONE INPUT
ACTIVATED AT A
TIME
Truth Table
(Active HIGH)

8 to 3 line encoder circuit diagram
D 7
D 6
D 5
D 4
D 3
D 2
D 1
D 0
x
D 4
+D 5
+D 6
+D 7
y
D 2
+D 3
+D 6
+D 7
z
D 1
+D 3
+D 5
+D 7

Ā 0
Ā 1
10 to 4 line
ENCODER
Ō 0
Ō 1
Ō 2
Ō 3
10 line to 4 line
Encoder
Ā 9
Ā 9
Ā 8
Ā 7
Ā 6
Ā 5
Ā 4
Ā 3
Ā 2
Ā 1
Ā 0
Ō 3
Ō 2
Ō 1
Ō 0
1 1 1 1 1 1 1 1 1 0 0 0 0 0
1 1 1 1 1 1 1 1 0 1 0 0 0 1
1 1 1 1 1 1 1 0 1 1 0 0 1 0
1 1 1 1 1 1 0 1 1 1 0 0 1 1
1 1 1 1 1 0 1 1 1 1 0 1 0 0
1 1 1 1 0 1 1 1 1 1 0 1 0 1
1 1 1 0 1 1 1 1 1 1 0 1 1 0
1 1 0 1 1 1 1 1 1 1 0 1 1 1
1 0 1 1 1 1 1 1 1 1 1 0 0 0
0 1 1 1 1 1 1 1 1 1 1 0 0 1
* An ACTIVE LOW ENCODER

10 line to 4 line Encoder
DECIMAL
BINARY (BCD)
8
4
2
1
ENCODER
9 5V
8 5V
7 5V
6 5V
5 5V
4 5V
3 5V
2 5V
1 5V
I9
I8
I7
I6
I5
I4
I3
I2
I1
74147
A3
A2
A1
A0

THE 8421 BCD CODE
• BCD stands for Binary-Coded Decimal.
• A BCD number is a four-bit binary group that
represents one of the ten decimal digits 0
through 9.
Example:
Decimal number 4926 4 9 2 6
8421 BCD coded number 0100 1001 0010 0110

ELECTRONIC ENCODER -
DECIMAL TO BCD
Decimal input
7
5
3
0
Decimal
to
BCD
Encoder
BCD output
0 10 0 1 0
1
• Encoders are available in IC form.
• This encoder translates from decimal
input to BCD output.

DECODER
DECODER- a digital circuit that converts an
input binary code into a single numeric
output.
n input
2 n output
ONLY ONE OUTPUT
ACTIVATED AT A
TIME

Example of 2 to 4 encoder
input
Logic
diagram 2 to
4 encoder
output
Truth Table

3 to 8 encoder
A 0
O 0
A 1
O 1
A 2
DECODER
3 to 8 line
Decoder
O 2
O 3
O 4
O 5
BINARY CODE INPUT
ONLY ONE OUTPUT
ACTIVATED AT A TIME
O 6
O 7
Block Diagram

4 line to 10 line Decoder
DECIMAL
9
BINARY (BCD)
8 0V
4 0V
2 0V
1 0V
DECODER
74LS42
A3
A2
A1
A0
9
8
7
6
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0

Ā 0
Ā 1
Ā 2
Ā 3
10 to 4 line
ENCODER
Ō 0
Ō 1
Ō 2
Ō 3
4 line to 10
line Decoder
(Active LOW)
Ō 9
Ā 3
Ā 2
Ā 1
Ā 0
Ō 9
Ō 8
Ō 7
Ō 6
Ō 5
Ō 4
Ō 3
Ō 2
Ō 1
Ō 0
Truth
Table
0 0 0 0 1 1 1 1 1 1 1 1 1 0
0 0 0 1 1 1 1 1 1 1 1 1 0 1
0 0 1 0 1 1 1 1 1 1 1 0 1 1
0 0 1 1 1 1 1 1 1 1 0 1 1 1
0 1 0 0 1 1 1 1 1 0 1 1 1 1
0 1 0 1 1 1 1 1 0 1 1 1 1 1
0 1 1 0 1 1 1 0 1 1 1 1 1 1
0 1 1 1 1 1 0 1 1 1 1 1 1 1
1 0 0 0 1 0 1 1 1 1 1 1 1 1
1 0 0 1 0 1 1 1 1 1 1 1 1 1

DECODERS: BCD TO
7-SEGMENT DECODER/DRIVER
BCD input
0 01 10 10
BCD-to-
7-Segment
Decoder/
Driver
Decimal output
LED
• Electronic decoders are available in IC form.
• This decoder translates from BCD to decimal.
• Decimals are shown on an 7-segment LED display.
• This IC also drives the 7-segment LED display.

TEST
Q #1- #2- #3- #4- #5- What is the decimal output from the decoder that
appears on the 7-segment display?
Answer: 30
97
6
BCD input
10 0 01 01 1
0
Decimal output
BCD-to-
7-Segment
Decoder/
Driver
?

BCD-TO-SEVEN SEGMENT DECODER DRIVER
V+
5V
74LS47
A3
A2
A1
A0
test
RBI
g
f
e
d
c
b
a
RBO
abcdef g .

TEST
A 0
A 1
A 2
A 3
A 4
ENCODER
O 0
O 1
O 2
O 3
A 5
A 6
A 7
A 8
A 9

TEST
A 0
O 0
A 1
O 1
A 2
A 3
DECODER
O 2
O 3
O 4
O 5
O 6
O 7
O 8
O 9

Question
• IC 74138 is a 3 to 8 decoder. The input is given by a
pin number A, B and C. State the binary code needed
to activate the output device Y 0, Y1, Y5, Y6 and Y7.
A B C Output
Y0 -Dot Matrix
Y2-Laser
Y5-Desk Jet
Y6-Plotter
Y7-Serial
Port

Multiplexer /
Demultiplexer
Digital Electronics

What is a Multiplexer (MUX)?
• A MUX is a digital switch that
has multiple inputs (sources)
and a single output
(destination).
• The select lines determine
which input is connected to the
output.
• MUX Types
2-to-1 (1 select line)
4-to-1 (2 select lines)
8-to-1 (3 select lines)
16-to-1 (4 select lines)
Inputs
(sources)
Multiplexer
Block Diagram
2 N
MUX
N
Select
Lines
1
Output
(destination)
23

Typical Application of a MUX
Multiple
Sources
Selector
Single
Destination
MP3 Player
Docking Station
Laptop
Sound Card
D0
D1
D2
D3
MU
X
Y
Surround Sound System
Digital
Satellite
Digital
Cable TV
B A Selected Source
0 0 MP3
0 1 Laptop
1 0 Satellite
1 1 Cable TV
24

4-to-1 Multiplexer (MUX)
D0
D1
D2
D3
MUX
Y
B A Y
0 0 D0
0 1 D1
1 0 D2
1 1 D3
B A
25

4-to-1 Multiplexer Waveforms
D0
D1
D2
Input
Data
D3
A
B
Y
D0 D1 D2 D3 D0 D1 D2 D3
Select
Line
Output
Data
26

Medium Scale Integration MUX
4-to-1 MUX
8-to-1 MUX
16-to-1 MUX
Inputs
Output (Y)
(and inverted
output)
Select
Enable
27

What is a Demultiplexer (DEMUX)?
• A DEMUX is a digital switch with
a single input (source) and a
multiple outputs (destinations).
• As an electronics switch (data
distributer) to route an incoming
data signal to one of the several.
• The select lines determine which
output the input is connected to.
• DEMUX Types
1-to-2 (1 select line)
1-to-4 (2 select lines)
1-to-8 (3 select lines)
1-to-16 (4 select lines)
Input
(source)
Demultiplexer
Block Diagram
1
N
DEMUX
Select
Lines
2 N
Outputs
(destinations)
28

Typical Application of a DEMUX
Single Source
Selector
Multiple
Destinations
B/W Laser
Printer
X
DEMUX
D0
D1
D2
D3
Fax
Machine
Color Inkjet
Printer
B A Selected Destination
0 0 B/W Laser Printer
0 1 Fax Machine
1 0 Color Inkjet Printer
1 1 Pen Plotter
Pen
Plotter
29

1-to-4 De-Multiplexer (DEMUX)
X
DEMUX
D0
D1
D2
D3
B A
B A D0 D1 D2 D3
0 0 X 0 0 0
0 1 0 X 0 0
1 0 0 0 X 0
1 1 0 0 0 X
30

1-to-4 De-Multiplexer Waveforms
X
S0
S1
Input
Data
Select
Line
D0
D1
D2
Output
Data
D3
31

Medium Scale Integration DEMUX
1-to-4 DEMUX
1-to-8 DEMUX
1-to-16 DEMUX
Select
Input
(inverted)
Outputs
(inverted)
Note : Most Medium Scale Integrated (MSI)
DEMUXs , like the three shown, have outputs that
are inverted. This is done because it requires few
logic gates to implement DEMUXs with inverted
outputs rather than no-inverted outputs.
32

Seeing Is NOT Always Believing
• Our lives are filled with electronic
signs that display the time,
temperature, or ball game score.
However, what we see is not
always what is really happening.
• In fact for most displays, the
individual display segments are
cycled through so that only one
display is on at any given time.
brgprecision.com
• The cycle speed is so fast that the
human eye perceives that all
segments are on.
33

Simple Message: All Segments On
• The circuit to the right uses
four 7-segment displays to
display the word CIAO. In
this circuit all displays are
continuously illuminated,
each displaying one letter in
the word.
• Though this method works, it
is a VERY inefficient use of
power. To illuminate the
simple message CIAO in this
way, 18 segments must be
continuously on.
• Can you think of another way
to display this message that
would use less power?
would use less power?
34 34

TEST
ANSWER THE FOLLOWING QUESTIONS WITH ONE OR MORE
OF THESE WORDS: MUX, DEMUX, ENCODER, DECODER.
A. Has more inputs than outputs.
B. Uses select inputs.
C. Can be used in parallel-to-serial conversion.
D. Produces a binary code at its output.
E. Only one of its outputs is activated at one time.
F. Used to route input signals to one of several outputs.
G. Used to generate arbitrary logic functions.
H. 3 line-to-8 line or binary to octal.
I. Data Selectors are also

TEST (ANSWER)
ANSWER THE FOLLOWING QUESTIONS WITH ONE OR MORE
OF THESE WORDS: MUX, DEMUX, ENCODER, DECODER.
A. Has more inputs than outputs. ENCODER, MUX
B. Uses select inputs. MUX, DEMUX
C. Can be used in parallel-to-serial conversion. MUX
D. Produces a binary code at its output. ENCODER
E. Only one of its outputs is activated at one time. DEMUX, DECODER
F. Used to route input signals to one of several outputs. MUX
G. Used to generate arbitrary logic functions. MUX, DEMUX
H. 3 line-to-8 line or binary to octal. DECODER
I. Data Selectors are also MUX.