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Chapter 8
Code Converters, Multiplexers and
Demultiplexers
1
Objectives
You should be able to:
Describe the function of a decoder and an encoder.
 Design the internal circuitry for encoding and
decoding.
 Use manufacturers’ data sheets to determine
operation of IC decoder and encoder chips.

2
Objectives
(Continued)
Explain the procedure involved in binary, BCD, and
Gray code conversion.
 Explain the operation of code converter circuits
built with SSI and MSI ICs.
 Describe the function and uses of multiplexers and
demultiplexers.
 Design circuits that employ multiplexer and
demultiplexer ICs.

3
Decoding


Process of converting some code (binary,
BCD, or hex) to a
single output
4-bit BCD decoder

Comprised of a
combination of
logic gates
8
Decoding

3-Bit Binary-to-Octal Decoder Truth Tables
9
Decoding

Complete Octal
Decoder (active
LOW outputs)
10
Decoding

Octal Decoder
Also known as 1-of-8 decoder
 Also known as 3-line-to-8-line decoder


Decoder ICs
11
Decoding

Octal Decoder IC

74138 pin configuration and logic symbol.
12
Decoding

74138 logic diagram and function table
13
Decoding

7442 BCD-to-DEC Decoder IC

1-of-10 decoder pin configuration and logic symbol
14
Decoding

7442 BCD-to-DEC Decoder IC

1-of-10 decoder logic diagram and function table
15
Decoding

74154 Hexadecimal Decoder IC

1-of-16 Decoder pin configuration and logic
symbol
16
Decoding

74154 Hexadecimal Decoder IC

1-of-16 Decoder logic diagram and function table
17
Encoding


Opposite process from decoding
Used to generate a coded output from a
singular active numeric input line.
18
Encoding

Octal-to-binary and decimal-to-BCD encoders
19
Encoding

A truth table can be used to design
encoders using combinational logic.
20
Encoding

A basic decimal-to-BCD encoder
21
Encoding


74147 Decimal-to-BCD Encoder
Inputs and outputs are Active-LOW


Note bubbles on logic symbols
Priority encoder - highest input has priority
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Encoding

74147 Decimal-to-BCD Encoder logic symbol and
function table
23
Encoding

74148 Octal-to-Binary Encoder
Eight active-low inputs
 Three active-low outputs
 Priority encoder

24
Encoding

74148 Octal-to-Binary Encoder logic symbol
and function table
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Code Converters


Convert a coded input into another form
BCD-to binary is a good example
Computer program (software)
 MSI integrated circuits (hardware)

27
Code Converters

Software BCD-to-Binary conversion
Must develop an algorithm
 Weighting factor of 10

28
Code Converters

74184 BCD-to-Binary Converter logic symbol

Hardware approach
29
Code Converters

Six-bit BCD-to-Binary
Converter using 74184
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Code Converters

BCD-to-binary
for two BCD
decades
31
Code Converters

BCD-to-binary for
three BCD decades
32
Code Converters

6 bit binary-to-BCD
and 8 bit binary-toBCD converters
33
Code Converters

BCD-to-Seven Segment Converters
4-bit BCD into a 7-bit code to drive display
segments
 Useful in calculators and any application that
requires a 7 segment display.
 The 7447 is a commonly used IC

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Code Converters

Gray Code is used to indicate the angular
position of a rotating machine.

Varies by only 1 bit from one entry to the next
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Code Converters

Gray Code
36
Code Converters

Conversion between binary and Gray code
using exclusive-OR gates
37
Multiplexers


Funneling several data
lines into a single one
for transmission to
another point
Data select control
determines which input
is transmitted
38
Multiplexers

S0 and S1 determine which data is transmitted
39
Multiplexers

Logic diagram for a four-line multiplexer
Figure 8-31
40
Multiplexers

Some popular TTL and CMOS multiplexers
40
Multiplexers

74151 Eight-Line Multiplexer logic symbol

Note 3 data select inputs for 8 inputs (23 = 8)
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Multiplexers

74151 Eight-Line Multiplexer logic diagram
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Multiplexers

Provide Combination Logic Functions
Multiplexers can be used to implement
combinational logic circuits.
 A multiplexer can replace several SSI logic gates

43
Demultiplexers



Opposite procedure
from multiplexing
Data distributor
Single data input
routed to one of
several outputs
45
Demultiplexers

In this simulation file, the D waveform is routed to the Y
outputs as determined by the data select inputs (S0 and S1).
45
Demultiplexers

74139 Dual 4-line Demultiplexer logic symbol
and logic diagram
46
Demultiplexers

74139 connected to route an input signal to
the 2a output
47
Demultiplexers

74154 4-line-to16-line hexadecimal decoder
Used as a 16 line demultiplexer
 Connected to route a signal to the 5 output

48
Demultiplexers





Analog Multiplexer/Demultiplexer
4051, 4052, 4053 CMOS devices
High-speed CMOS also available (74HCT4051)
Function as either device since inputs and
outputs are bidirectional
Analog since outputs can be any value between
positive and negative supplies (not just 0 and 1)
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Demultiplexers

4051 CMOS analog
multiplexer
/demultiplexer
50
System Design Application

The 74138 as a memory address decoder
51
System Design Application

The 74148 used to encode an active alarm
52
74HCT4051 Staircase Generator
55
74HCT4051 Staircase Generator

The voltage divider at the Y-inputs creates
outputs at 8 different voltage levels.
Multiplexed Display Application

Allows sharing of common ICs, components, and
conductors
Four-digit registered are multiplexed to a single-digit
bus
 Display bus is multiplexed into the four-digit displays

56
Multiplexed Display Application
57
Summary


Decoders can be used to convert a binary
code into a singular active output
representing its numeric value.
Encoders can be used to generate a coded
output from a singular active numeric input
line.
59
Summary



ICs are available to convert BCD to binary and
binary to BCD.
The Gray code is useful for indicating the
angular position of a shaft on a rotating device,
such as a motor.
Multiplexers are capable of funneling several
data lines into a single line for transmission to
another point.
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Summary

Demultiplexers are used to take a single data
value or waveform and route it to one of several
outputs.
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