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# Note for Digital Electronics - DE By RAHUL MAHENDRAKAR

• Digital Electronics - DE
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#### Note for Digital Electronics - DE By RAHUL MAHENDRAKAR

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SANDUR POLYTECHNIC, YESHWANTNAGAR DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING Course Material DIGITAL ELECTRONICS [15EC32T] Unit I Combinational Logic circuits Course Coordinator Rahul S Mahendrakar. B.E. (E&CE)

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SANDUR POLYTECHNIC YESHWANTNAGAR DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING TOPIC WISE QUESTION BANK WITH ANSWERS Semester Course Unit title III Course code 15EC32T DIGITAL ELECTRONICS Credits 1. Combinational logic circuits Questions to be set for SEE R 17% Weightage Course Coordinator 05 U 10 04 Planned hours A Total marks Mr. Rahul S Mahendrakar. Lecturer Dept. of ECE 09 10 25 B.E.(E&CE) Topic wise Questions Sl Cognitive level Marks Question Introduction 1 Define combinational logic circuits with examples. Remember 05 2 List any five combinational logic circuits and their functions. Remember 05 Multiplexers: definition, expression, truth-table, realization of simple (2:1) multiplexer using gates, and applications 3 Define multiplexer and list its applications. Remember 05 4 Describe the multiplexing process with suitable digital multiplexer Understand 05 circuit. 5 Calculate how many control lines required for 2:1 mux, 4:1 mux and Application 05 8:1 mux and sketch their logic symbols. With logic symbol, circuit, truth table, explain the working of 2:1 6 Understand 05 multiplexer. With logic symbol, circuit, truth table, explain the working of 4:1 7 Understand 05 multiplexer. Application of multiplexers to implement logic gates and simple sum-of-product equations Explain how to implement the Boolean expression Y= ̅B+A ̅ using multiplexer. Or 8 Application 05 Convert multiplexer into logic gate. Or Show how to implement NAND function using multiplexers. ̅ ̅ C+AB. 9 Illustrate use of multiplexer to realize Y= ̅ ̅ Application 05 Realization of higher-order multiplexer using lower-order multiplexer ICs Illustrate how to realize higher order multiplexers using lower order 10 multiplexers. Or Application 05 Show how to realize 4:1 multiplexer using 2:1 multiplexers. Multiplexer ICs and their features 11 List multiplexer ICs and their features. Remember 05 Demultiplexers: definition, expression, realization of simple (1:2) demultiplexer using gates, truthtable and applications, and demultiplexer ICs and their features. 12 Define demultiplexer and list its applications. Remember 05 13 Describe the demultiplexing process with the help of any demux circuit. Or Understand With logic symbol, circuit, truth table, explain the working of 1:2 demultiplexer. SANDUR POLYTECHNIC 15EC32T Digital Electronics 05 1

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14 15 16 17 18 19 20 21 22 23 24 25 26 27 With logic symbol, circuit, truth table, explain the working of 1:4 demultiplexer. Discuss the role of control lines in demux circuits. Or Calculate how many control lines required for 1:2 demux, 1:4 demux and 1:8 demux and sketch their logic symbols. List demultiplexer ICs and their features. List the similarities and dissimilarities between mux and demux. Decoders and encoders Define decoders and list its applications. With the help of logic symbol, logic circuit and truth table explain the working of BCD to decimal decoders. With the help of logic symbol, logic circuit and truth table explain the working of BCD to 7-segment decoders. Define encoders and list its applications. With the help of logic symbol, logic circuit and truth table explain the working of Decimal to BCD encoders. List different decoder ICs and their features. List different encoder ICs and their features. Priority encoder Describe the working of simple priority encoder. With the help of logic symbol, logic circuit and truth table explain the working of Decimal to BCD priority encoder. Differentiate between encoder and priority encoder Understand 05 Application 05 Remember Understand 05 05 Remember 05 Understand 10 Understand 10 Remember 05 Understand 10 Remember Remember 05 05 Remember 05 Understand 10 Understand 05 1.1. Introduction 1 Define combinational logic circuits with examples. 5M Answer: Combinational logic circuits are those whose output is pure function of present inputs only. Examples: 1. Logic gates 2. Adders 3. Subtractors 4. Comparators 5. Encoders 6. Decoders 7. Multiplexers 8. De-multiplexers. 2 List any five combinational logic circuits and their functions. 5M Answer: 1. Logic gates: Enables/disables the circuit based on particular condition specified by its Boolean expression. 2. Adders: Performs binary addition. 3. Subtractors: Performs binary subtraction. 4. Comparators: Compares the magnitudes of binary numbers. 5. Encoders: Converts familiar symbols or numbers into a coded format. 6. Decoders: Converts coded information into familiar symbols or numbers. 7. Multiplexers: Accepts several digital data inputs and selec ts only one of them at any time to pass on to the output. SANDUR POLYTECHNIC 15EC32T Digital Electronics 2

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1.2. Multiplexers: definition, expression, truth-table, realization of simple (2:1) multiplexer using gates, and applications. 3 Define multiplexer and list its applications. 5M Answer:  Multiplex means many into one.  Multiplexer is a combinational logic circuit with many inputs and only one output. It accepts several digital data inputs and selects only one of them at any time to pass on to the output. And selection of data routing is done by ‘control’ or ‘select’ signals.  Applications of multiplexer are: 1. Data selection 2. Data routing 3. Parallel to serial conversion 4. Operation sequencing 5. Logic function generator 6. Communication systems 4 Describe the multiplexing process with suitable digital multiplexer circuit. 5M Answer:  A digital multiplexer is a combinational logic circuit with many inputs but only one output. It accepts several digital data inputs and selects only one of them at any given time to pass onto the output. The routing of the desired digital data input to the output is controlled by SELECT (control) signals.  Multiplexer is also called as data selector circuit.  Figure illustrates the general idea of multiplexers. Figure: General idea of multiplexers  The circuit has n input signals, m select (control) signals and only one output signal. Note that m control signals can select at most 2m input signals thus n≤2m. For example, A 16-to-1 multiplexer has sixteen input signals, four SELECT (control) signals and one output signal. Calculate how many control lines required for 2:1 mux, 4:1 mux and 8:1 mux and sketch their logic symbols. Answer: Let, n be the no. of data inputs m be the no. of control or select lines. Then relation between m and n can be stated as m=log2 n 1. For 2:1 multiplexer, no. of control lines required is m=log2 (2)=1. 2. For 4:1 multiplexer, no. of control lines required is m=log2 (4)=2 3. For 8:1 multiplexer, no. of control lines required is m=log2 (8)=3. 5 SANDUR POLYTECHNIC 15EC32T Digital Electronics 5M 3