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

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## Chapter 8

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**Chapter 8**Code Converters, Multiplexers and Demultiplexers 1**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**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**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**Comparators**• Evaluating two 4-bit numbers – Figure 8-1 5**Comparators**• Magnitude Comparators • A = B • A > B • A < B • 7485 4 bit comparator • Figure 8-2 6**Comparators**• Magnitude comparison of two 8-bit strings • Figure 8-3 7**Decoding**• Converting some code (binary, BCD, or hex) to a single output • BCD decoder • Figure 8-4 8**Decoding**• 3-Bit Binary-to-Octal Decoding • Truth Table for active HIGH and active LOW 9**Decoding**• Complete Octal Decoder (active LOW out) • Figure 8-6 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. Figure 8-7 12**Decoding**• Octal Decoder IC • 74138 logic diagram and function table • Don’t- Care level • Figure 8-7 (continued) 13**Decoding**• BCD Decoder IC • 7442 1-of-10 decoder pin configuration and logic symbol – Figure 8-10 14**Decoding**• BCD Decoder IC • 7442 1-of-10 decoder logic diagram and function table – Figure 8-10 (continued) 15**Decoding**• Hexadecimal Decoder IC • 74154 1-of16 Decoder pin configuration and logic symbol – Figure 8-11 16**Decoding**• Hexadecimal Decoder IC • 74154 1-of16 Decoder logic diagram and function table – Figure 8-11 (continued) 17**Encoding**• Opposite process from decoding • Used to generate a coded output • Decimal-to-BCD encoder block diagram: Figure 8-12 18**Encoding**• Octal to binary encoder – Figure 8-12 (continued) 19**Encoding**• The truth table can be used to design encoders using combinational logic. • See Table 8-3 in your text 20**Encoding**• Combinational logic for decimal to BCD encoder based on truth table – Figure 8-13 21**Encoding**• Decimal-to-BCD Encoder • 74147 • Inputs and outputs are Active-LOW • Priority encoder - highest input has priority 22**Encoding**• Decimal-to-BCD Encoder • 74147 logic symbol and function table – Figure 8-14 23**Encoding**• Octal-to-Binary Encoder • 74148 • Eight active-low inputs • Three active-low outputs • Priority encoder 24**Encoding**• Octal-to-Binary Encoder • 74148 logic symbol and function table – Figure 8-17 25**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**Code Converters**• Convert a coded input into another form • Computer program (software) • MSI integrated circuits (hardware) 27**Code Converters**• BCD-to-Binary conversion • weighting factor of 10 • Figure 8-20 28**Code Converters**• 74184 BCD-to-Binary Converter logic symbol – Figure 8-21 29**Code Converters**• Six-bit BCD-to-Binary Converter using 74184 – Figure 8-22 30**Code Converters**• BCD to binary for two BCD decades – Figure 8-23(a) 31**Code Converters**• BCD to binary for three BCD decades – Figure 8-23(b) 32**Code Converters**• 6 bit binary to BCD and 8 bit binary to BCD converters – Figure 8-23 (c) and (d) 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. 34**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**Code Converters**• Gray Code • Comparison between regular binary and Gray code: 36**Code Converters**• Conversion between binary and Gray code using XOR gates • Figure 8-26 and 8-27 37**Multiplexers**• Funneling several data lines into a single one for transmission to another point • Data selector • Figure 8-30 38**Multiplexers**39**Multiplexers**• Logic diagram for a four-line multiplexer: Figure 8-31 40**Multiplexers**• 74151 Eight-Line Multiplexer logic symbol – Figure 8-32 41**Multiplexers**• 74151 Eight-Line Multiplexer logic diagram – Figure 8-32(continued) 42**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**Demultiplexers**• Opposite procedure from multiplexing • Data distributor • Single data input routed to one of several outputs • Figure 8-37 45**Demultiplexers**• 74139 Dual 4-line Demultiplexer logic symbol and logic diagram- Figure 8-38 46**Demultiplexers**• 74139 connected to route an input signal to the 2a output – Figure 8-39 47**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**Demultiplexers**• Analog Multiplexer/Demultiplexer • 4051, 4052, 4053 CMOS devices • Both functions • Bidirectional • Analog and digital 49

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