Top 50 Objective Questions on Combinational Building Blocks

General Concepts of Combinational Circuits

1.       What is a combinational circuit?

·         A circuit with outputs that depend only on current inputs.

·         Explanation: Outputs are determined solely by the present inputs, not by any previous inputs or states.

2.       Which of the following is not a characteristic of a combinational circuit?

·         Uses memory to store previous states.

·         Explanation: Combinational circuits do not use memory elements.

3.       How many outputs can a combinational circuit have if it has n inputs?

·         Any number m.

·         Explanation: The number of outputs is independent of the number of inputs.

4.       In a combinational circuit, does the output depend on the previous state of the inputs?

·         No.

·         Explanation: Only the current inputs determine the output.

Half Adder

5.       How many inputs does a half adder have?

·         Two.

·         Explanation: A half adder adds two single-bit binary numbers.

6.       What are the outputs of a half adder?

·         Sum and Carry.

·         Explanation: It provides a sum bit and a carry bit as outputs.

7.       What is the sum output when both inputs of a half adder are 1?

·         0.

·         Explanation: 1 + 1 in binary is 10, where 0 is the sum.

8.       What is the carry output when both inputs of a half adder are 1?

·         1.

·         Explanation: 1 + 1 in binary is 10, where 1 is the carry.

Full Adder

9.       How many inputs does a full adder have?

·         Three.

·         Explanation: It adds two bits plus a carry input.

10.   What are the outputs of a full adder?

·         Sum and Carry.

·         Explanation: It produces a sum bit and a carry bit.

11.   How does a full adder differ from a half adder?

·         A full adder can handle a carry input.

·         Explanation: A full adder adds three inputs, while a half adder adds two.

12.   What is the sum output of a full adder when A = 1, B = 1, and Carry in = 1?

·         1.

·         Explanation: 1 + 1 + 1 in binary is 11, where 1 is the sum.

N-Bit Parallel Adder

13.   What is required to add two n-bit binary numbers?

·         An n-bit parallel adder.

·         Explanation: Multiple full adders are used in parallel.

14.   How is the carry output of one full adder connected in an n-bit parallel adder?

·         To the carry input of the next full adder.

·         Explanation: This allows carry propagation across bits.

15.   What does the LSB in a 4-bit parallel adder represent?

·         The least significant bit.

·         Explanation: The lowest order bit in binary addition.

N-Bit Parallel Subtractor

16.   How is subtraction performed in a binary system?

·         By adding the 2’s complement of the subtrahend to the minuend.

·         Explanation: This simplifies subtraction using addition hardware.

17.   What does the carry output indicate in a 4-bit parallel subtractor if A > B?

·         0.

·         Explanation: No borrow is needed if A is greater than B.

18.   What does a result in 2’s complement form indicate in a 4-bit parallel subtractor?

·         A < B.

·         Explanation: It shows a negative result.

Half Subtractor

19.   How many outputs does a half subtractor have?

·         Two.

·         Explanation: It provides a difference and a borrow bit.

20.   What are the outputs of a half subtractor?

·         Difference and Borrow.

·         Explanation: These represent the result of subtraction and the need for borrowing.

21.   What is the difference output when both inputs of a half subtractor are 1?

·         0.

·         Explanation: 1 - 1 equals 0.

22.   What is the borrow output when A = 0 and B = 1 in a half subtractor?

·         1.

·         Explanation: Borrow is required when subtracting a larger number from a smaller one.

Full Subtractor

23.   How many inputs does a full subtractor have?

·         Three.

·         Explanation: It subtracts two bits plus a borrow input.

24.   What are the outputs of a full subtractor?

·         Difference and Borrow.

·         Explanation: It provides the difference and the borrow bit.

25.   What is the difference output of a full subtractor when A = 1, B = 1, and Borrow in = 1?

·         1.

·         Explanation: The difference calculation with an incoming borrow.

26.   What does the borrow output indicate in a full subtractor?

·         The need to borrow from the next higher bit.

·         Explanation: Indicates if borrowing is necessary for the next bit.

Multiplexers

27.   What does a multiplexer do?

·         Selects one of the many inputs and routes it to a single output.

·         Explanation: It acts like a switch, selecting one input.

28.   How many select inputs are required for an 8:1 multiplexer?

·         3.

·         Explanation: 2^3 = 8, so three select inputs are needed.

29.   What is the purpose of the enable input in a multiplexer?

·         To allow or disable the operation of the multiplexer.

·         Explanation: Controls whether the multiplexer is active.

30.   How many data inputs does a 16:1 multiplexer have?

·         16.

·         Explanation: Sixteen possible inputs.

Demultiplexers

31.   What is the function of a demultiplexer?

·         Distributes a single input to multiple outputs.

·         Explanation: It channels the input to one of many outputs.

32.   How many outputs does a 1:4 demultiplexer have?

·         Four.

·         Explanation: One input can be routed to one of four outputs.

33.   How are the output lines selected in a demultiplexer?

·         By the select lines.

·         Explanation: Select lines determine which output line receives the input.

34.   Which combinational circuit is equivalent to a single pole multiple way switch?

·         Demultiplexer.

·         Explanation: It directs a single input to multiple outputs like a switch.

Decoders

35.   What is a decoder used for in digital circuits?

·         Converts binary information from n inputs to a maximum of 2^n unique outputs.

·         Explanation: Decodes input into one of many outputs.

36.   How many outputs does a 3 to 8 line decoder have?

·         Eight.

·         Explanation: Three inputs can generate eight unique outputs.

37.   What is the difference between a decoder and a demultiplexer?

·         A decoder does not have a data input; it only decodes the inputs into outputs.

·         Explanation: Decoders convert input codes to outputs, while demultiplexers distribute a single input.

38.   What is the output of a 2 to 4 line decoder for input 01?

·         2nd output line is high.

·         Explanation: Each input combination activates a specific output.

Encoders

39.   What is the primary function of an encoder?

·         Converts 2^n input lines to n output lines.

·         Explanation: Encodes inputs into a smaller number of outputs.

40.   How many output lines does an 8 to 3 encoder have?

·         Three.

·         Explanation: It encodes eight inputs into three outputs.

41.   What is a priority encoder?

·         An encoder that assigns priority to inputs if multiple inputs are high simultaneously.

·         Explanation: It prioritizes the highest order input when multiple inputs are active.

42.   What output does a priority encoder produce if the highest priority input is active?

·         The binary code corresponding to the highest priority input.

·         Explanation: The output reflects the highest active input.

Practical Applications and Examples

43.   What type of combinational circuit can be used to perform binary addition?

·         Parallel Adder.

·         Explanation: Multiple adders combine to add multi-bit numbers.

44.   What combinational circuit is used in digital watches to decode BCD to seven-segment display?

·         Decoder.

·         Explanation: Converts BCD to a format suitable for display.

45.   Which circuit is used to switch between multiple data sources?

·         Multiplexer.

·         Explanation: It selects one of many data inputs.

46.   How can binary subtraction be performed using adders?

·         By using a parallel adder with the 2’s complement of the subtrahend.

·         Explanation: Subtraction is performed by adding the complement.

47.   What is the use of inverters in a 4-bit parallel subtractor?

·         To generate the 1’s complement of the subtrahend.

·         Explanation: Inverters are used to create the complement for subtraction.

48.   In which type of circuits do we use a priority encoder?

·         Interrupt systems in microprocessors.

·         Explanation: Priority encoders handle multiple interrupt requests.

49.   What happens if all inputs to a priority encoder are low?

·         The output will typically be zero or a specific code indicating no input is active.

·         Explanation: Indicates no active inputs.

50.   Which combinational circuit can be used for data distribution to multiple lines in communication systems?

·         Demultiplexer.

·         Explanation: It channels data from one input to multiple outputs.