1. Code Converters, Multiplexers and Demultiplexers
Chapter 8
Code Converters, Multiplexers and Demultiplexers 1

2. Objectives You should be able to:
Use an IC magnitude comparator to perform binary comparisons.
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

3. Objectives You should be able to:
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

4. Comparators Compare two binary strings Digital comparator
Compare bit-by-bit
Outputs a 1 if they are exactly equal
Use exclusive-NOR gates
Evaluating two 4-bit numbers - see Figure 8-1 4

5. Comparators
Evaluating two 4-bit numbers – Figure 8-1 5

6. Comparators Magnitude Comparators 7485 4 bit comparator Figure 8-2
A = B
A > B
A < B
bit comparator
Figure 8-2 6

7. Comparators
Magnitude comparison of two 8-bit strings
Figure 8-3 7

8. Decoding Converting some code (binary, BCD, or hex) to a single output
BCD decoder
Figure 8-4 8

9. Decoding 3-Bit Binary-to-Octal Decoding
Truth Table for active HIGH and active LOW 9

10. Decoding
Complete Octal Decoder (active LOW out)
Figure 8-6 10

11. Decoding Octal Decoder Decoder ICs Also known as 1-of-8 decoder
Also known as 3-line-to-8-line decoder
Decoder ICs 11

12. Decoding Octal Decoder IC 74138 pin configuration and logic symbol.
Figure 8-7 12

13. Decoding Octal Decoder IC 74138 logic diagram and function table
Don’t- Care level
Figure 8-7 (continued) 13

14. Decoding BCD Decoder IC
of-10 decoder pin configuration and logic symbol – Figure 8-10 14

15. Decoding BCD Decoder IC
of-10 decoder logic diagram and function table – Figure 8-10 (continued) 15

16. Decoding Hexadecimal Decoder IC
of16 Decoder pin configuration and logic symbol – Figure 8-11 16

17. Decoding Hexadecimal Decoder IC
of16 Decoder logic diagram and function table – Figure 8-11 (continued) 17

18. Encoding Opposite process from decoding
Used to generate a coded output
Decimal-to-BCD encoder block diagram:
Figure 8-12 18

19. Encoding
Octal to binary encoder – Figure 8-12 (continued) 19

20. Encoding
The truth table can be used to design encoders using combinational logic.
See Table 8-3 in your text 20

21. Encoding
Combinational logic for decimal to BCD encoder based on truth table – Figure 8-13 21

22. Encoding Decimal-to-BCD Encoder 74147
Inputs and outputs are Active-LOW
Priority encoder - highest input has priority 22

23. Encoding Decimal-to-BCD Encoder
74147 logic symbol and function table – Figure 8-14 23

24. Encoding Octal-to-Binary Encoder 74148 Eight active-low inputs
Three active-low outputs
Priority encoder 24

25. Encoding Octal-to-Binary Encoder
74148 logic symbol and function table – Figure 8-17 25

26. Discussion Point
Explain the difference between an encoder and a decoder.
How does a priority encoder determine which input to encode if more than one is active? 26

27. Code Converters Convert a coded input into another form
Computer program (software)
MSI integrated circuits (hardware) 27

28. Code Converters BCD-to-Binary conversion Figure 8-20
weighting factor of 10
Figure 8-20 28

29. Code Converters
74184 BCD-to-Binary Converter logic symbol – Figure 8-21 29

30. Code Converters
Six-bit BCD-to-Binary Converter using – Figure 8-22 30

31. Code Converters
BCD to binary for two BCD decades – Figure 8-23(a) 31

32. Code Converters BCD to binary for three BCD decades – Figure 8-23(b)32

33. Code Converters
6 bit binary to BCD and 8 bit binary to BCD converters – Figure 8-23 (c) and (d) 33

34. 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. 34

35. Code Converters Gray Code
used to indicate angular position of rotating shafts
varies by only 1 bit from one entry to the next
Figure 8-25 35

36. Code Converters Gray Code
Comparison between regular binary and Gray code: 36

37. Code Converters
Conversion between binary and Gray code using XOR gates
Figure 8-26 and 8-27 37

38. Multiplexers
Funneling several data lines into a single one for transmission to another point
Data selector
Figure 8-30 38

39. Multiplexers 39

40. Multiplexers
Logic diagram for a four-line multiplexer:
Figure 8-31 40

41. Multiplexers 74151 Eight-Line Multiplexer logic symbol – Figure 8-32

42. Multiplexers
74151 Eight-Line Multiplexer logic diagram – Figure 8-32(continued) 42

43. Multiplexers Providing Combination Logic Functions
Multiplexers can be used to implement combinational logic circuits.
A multiplexer can replace several SSI logic gates
Example 8-12 43

44. 44

45. Demultiplexers Opposite procedure from multiplexing Data distributor
Single data input routed to one of several outputs
Figure 8-37 45

46. Demultiplexers
74139 Dual 4-line Demultiplexer logic symbol and logic diagram- Figure 8-38 46

47. Demultiplexers
74139 connected to route an input signal to the 2a output – Figure 8-39 47

48. Demultiplexers 74154 4-line-to16-line hexadecimal decoder
Used as a 16 line demultiplexer
Connected to route a signal to the 5 output – Figure 8-40 48

49. Demultiplexers Analog Multiplexer/Demultiplexer
4051, 4052, 4053 CMOS devices
Both functions
Bidirectional
Analog and digital 49

50. 4051 CMOS analog multiplexer/demultiplexer – Figure 8-4150

51. System Design Applications
The as a memory address decoder – Figure 8-42 51

52. System Design Applications
The used to encode an active alarm – Figure 8-43 52

53. System Design Applications
Serial Data Multiplexing for a Microcontroller
One serial receive line
One serial transmit line
See Figure 8-44
Analog Multiplexer
superimposed
4051
See Figure 8-45 53

54. Figure 8-44 54

55. Figure 8-45 55

56. System Design Applications
Multiplexed Display Application
Share common ics, components and conductors
Digital bus and display bus
See Figure 8-46 56

57. Figure 8-46 57

58. CPLD Design Applications
Used to simulate combinations of inputs and observe the resulting output to check for proper design operation.
See CPLD Applications 8-1 and 8-2 58

59. Summary
Comparators can be used to determine equality or which of two binary strings is larger.
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

60. 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. 60

61. Summary
Demultiplexers are used to take a single data value or waveform and route it to one of several outputs. 61