Questions & Answers On Combinational Circuits
Questions & Answers On Combinational Circuits
Questions & Answers On Combinational Circuits
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Combinational Circuits”.
a) a
b) b
c) c
d) d
View Answer
Answer: d
Explanation: Here, the diagram of option d contains the OR gate followed by the AND gates, so
it is in SOP form.
2. Which of the following logic expressions represents the logic diagram shown?
a) X=AB’+A’B
b) X=(AB)’+AB
c) X=(AB)’+A’B’
d) X=A’B’+AB
View Answer
Answer: d
Explanation: 1st output of AND gate is = A’B’
2nd AND gate’s output is = AB and,
OR gate’s output is = (A’B’)+(AB) = AB + A’B’.
a) Comparator
b) Multiplexer
c) Inverter
d) Demultiplexer
View Answer
Answer: d
Explanation: The given diagram is demultiplexer, because it takes single input & gives many
outputs.
4. What type of logic circuit is represented by the figure shown below?
a) XOR
b) XNOR
c) AND
d) XAND
View Answer
Answer: b
Explanation: After solving the circuit we get (AB)’+AB as output, which is XNOR operation.
5. For a two-input XNOR gate, with the input waveforms as shown below, which output
waveform is correct?
a) d
b) a
c) c
d) b
View Answer
Answer: a
Explanation: When both inputs are same then the o/p is high for a XNOR gate.
i.e., A B O/P
001
010
100
1 1 1.
6. Which of the following combinations of logic gates can decode binary 1101?
a) One 4-input AND gate
b) One 4-input AND gate, one inverter
c) One 4-input AND gate, one OR gate
d) One 4-input NAND gate, one inverter
View Answer
Answer: b
Explanation: For decoding any number output must be high for that code and this is possible in
D option only.
Answer: b
Explanation: Short to ground in the output of a driving gate indicates of a signal loss to all load
gates.
8. For the device shown here, assume the D input is LOW, both S inputs are LOW and the input
is LOW. What is the status of the Y’ outputs?
Answer: d
Explanation: In the given diagram, S0 and S1 are selection bits. So,
I/P S0 S1 O/P
D = 0 0 0 Y0
D = 0 0 1 Y1
D = 0 1 0 Y2
D = 0 1 1 Y3
Hence, inputs are S0 and S1 are Low means 0, so output is Y0.
Answer: b
Explanation: This happens in parallel adders (where we try to add numbers in parallel via more
than one adders). A carry propagation occurs when carry from one adder needs to be forwarded
to other adder and that second adder is holding the computation (addition) because carry from
first adder has not come yet. So, there is a slight delay for second adder and this is known as
carry propagation.
Answer: d
Explanation: Three bits full adder requires 2^3 = 8 combinational circuits.
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Multiplexers (Data Selectors) – 1”.
1. What is a multiplexer?
a) It is a type of decoder which decodes several inputs and gives one output
b) A multiplexer is a device which converts many signals into one
c) It takes one input and results into many output
d) None of the Mentioned
View Answer
Answer: b
Explanation: A multiplexer (or MUX) is a device that selects one of several analog or digital
input signals and forwards the selected input into a single line.
2. Which combinational circuit is renowned for selecting a single input from multiple inputs &
directing the binary information to output line?
a) Data Selector
b) Data distributor
c) Both data selector and data distributor
d) None of the Mentioned
View Answer
Answer: a
Explanation: Data Selector is another name of Multiplexer.
3. It is possible for an enable or strobe input to undergo an expansion of two or more MUX ICs
to the digital multiplexer with the proficiency of large number of
a) Inputs
b) Outputs
c) Selection lines
d) All of the Mentioned
View Answer
Answer: a
Explanation: It is possible for an enable or strobe input to undergo an expansion of two or more
MUX ICs to the digital multiplexer with the proficiency of large number of inputs.
Answer: c
Explanation: the major functioning responsibility of the multiplexing combinational circuit is
generation of selected path between multiple sources and a single destination because it makes
the circuit too flexible.
Answer: c
Explanation: Enable input is used to active the chip, when enable is high the chip works
(ACTIVE), when enable is low the chip does not work (MEMORY).
6. One multiplexer can take the place of
a) Several SSI logic gates
b) Combinational logic circuits
c) Several Ex-NOR gates
d) Several SSI logic gates or combinational logic circuits
View Answer
Answer: d
Explanation: Since, many operational behaviour can be performed by using a multiplexer.
Whereas, a combinational circuit is a combination of many logic gates which makes the circuit
more complex.
Answer: a
Explanation: A digital multiplexer is a combinational circuit that selects one digital information
from several sources and transmits the selected information on a single output line. That is why it
is also known as data selector.
Answer: b
Explanation: The selection of a particular input line is controlled by a set of selected lines in a
multiplexer, which helps to select a particular input from several sources.
9. If the number of n selected input lines is equal to 2^m then it requires _____ select lines.
a) 2
b) m
c) n
d) None of the Mentioned
View Answer
Answer: b
Explanation: If the number of n selected input lines is equal to 2^m then it requires m select lines
to select one of m select lines.
10. How many select lines would be required for an 8-line-to-1-line multiplexer?
a) 2
b) 4
c) 8
d) 3
View Answer
Answer: d
Explanation: 2^n input lines, n control lines and 1 output line available for MUX. Here, 8 input
lines mean 2^3 inputs. So, 3 control lines are possible.
Answer: c
Explanation: A basic multiplexer principle can be demonstrated through the use of a rotary
switch. Since, its behaviour is similar to the multiplexer.
12. How many NOT gates are required for the construction of a 4-to-1 multiplexer?
a) 3
b) 4
c) 2
d) 5
View Answer
Answer: c
Explanation: There are two NOT gates required for the construction of 4-to-1 multiplexer. The
diagram of a 4-to-1 multiplexer is shown below:
13. In the given 4-to-1 multiplexer, if c1 = 0 and c0 = 1 then the output M is
a) X0
b) X1
c) X2
d) X3
View Answer
Answer: b
Explanation: The output will be X1, because c1 = 0 is converted into 1 and it is multiplied with
c0 = 1 which results as X1. And rest of the AND gates gives output as 0.
Answer: c
Explanation: The enable input is also known as strobe which is used to cascade two or more
multiplexer ICs to construct a multiplexer with larger number of inputs.
Explanation: 4 to 1 multiplexer would have 4 inputs. It can be observed from this diagram:
Answer: a
Explanation: The two input multiplexer would have 1 select lines.
Answer: d
Explanation: A combinational circuit that selects one from many inputs is known as Multiplexer.
Answer: a
Explanation: 4 to 1 MUX would have 1 outputs.
Answer: a
Explanation: In multiplexer, different inputs are inserted parallely and then it gives one output
which is in serial form.
Answer: a
Explanation: A combinational circuit is one in which the output depends on the input
combination at the time.
Answer: d
Explanation: A 2^n:1 MUX can implement all logic functions of (n+1) variables without any
additional circuitry. Thus 8:1 MUX can implement all logic functions of (3+1) variables, for 4
variables there are 16 possible combinations. So to use 8:1 MUX use 3 inputs as select lines of
MUX and the 4th input as input of MUX.
Answer: c
Explanation: A basic multiplexer principle can be demonstrated through the use of a rotary
switch. Because rotary switch gives one output corresponding to their inputs.
Answer: d
Explanation: One multiplexer can take the place of several SSI logic gates or combinational logic
circuits because it has a lot of functions to perform different operations.
Answer: a
Explanation: One multiplexer can be used as demultiplexer. Hence, it is called bidirectional.
Answer: b
Explanation: If enable input is high then the multiplexer is disabled because enable input is in
inverted mode always (i.e. E’).
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Demultiplexers (Data Distributors) – 1”.
Answer: d
Explanation: The word demultiplex means “one into many” and distributor. It is clear from the
diagram:
Answer: a
Explanation: For one input, the demultiplexer gives several outputs. That is why, it is called a
data distributor.
3. Most demultiplexers facilitate which type of conversion?
a) Decimal-to-hexadecimal
b) Single input, multiple outputs
c) AC to DC
d) Odd parity to even parity
View Answer
Answer: b
Explanation: Demultiplexer converts single input into multiple outputs.
Answer: a
Explanation: The formula for total no. of outputs is given by: 2^n, where n is the no. of select
lines.
Answer: b
Explanation: As the select input changes, the output of the multiplexer varies according to that
input.
Answer: b
Answer: d
Explanation: The output y3 = C1.C0.X.
Answer: b
Explanation: The formula for total no. of outputs is given by: 2^n, where n is the no. of select
lines. In this case n = 3.
Answer: c
Explanation: The number of AND gates required will be equal to the number of outputs in a
demultiplexer.
a) Q2.(Q1)’.Q0.I
b) Q2.Q1.(Q0)’.I
c) Q2.(Q1)’.(Q0)’.I
d) None of the Mentioned
View Answer
Answer: c
Explanation: The output Y4 = Q2.(Q1)’.(Q0)’.I.
11. Which IC is used for the implementation of 1-to-16 DEMUX?
a) IC 74154
b) IC 74155
c) IC 74139
d) IC 74138
View Answer
Answer: a
Explanation: IC 74154 is used for the implementation of 1-to-16 DEMUX, whose output is
inverted input.
This set of Digital Electronic/Circuits Multiple Choice Questions & Answers focuses on
“Demultiplexers(Data Distributors) – 2”.
Answer: a
Explanation: For one input, the demultiplexer gives several outputs. That is why, it is called a
data distributor.
Answer: b
Explanation: Demultiplexer converts single input into multiple outputs.
Answer: b
Explanation: As the select input changes, the output of the multiplexer varies according to that
input.
Answer: d
Explanation: The output y3 = C1.C0.X.
Answer: b
Explanation: The formula for total no. of outputs is given by: 2^n, where n is the no. of select
lines. In this case n = 3.
Answer: c
Explanation: The number of AND gates required will be equal to the number of outputs in a
demultiplexer.
8. Which IC is used for the implementation of 1-to-16 DEMUX?
a) IC 74154
b) IC 74155
c) IC 74139
d) IC 74138
View Answer
Answer: a
Explanation: IC 74154 is used for the implementation of 1-to-16 DEMUX, whose output is
inverted input.
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Liquid Crystal Displays”.
Answer: a
Explanation: The full form of LCD is “LIquid Crystal Display”.
Answer: c
Explanation: The optical properties of liquid crystals depend on the direction of light travels
through a layer of the material.
3. By which properties, the orientation of molecules in a layer of liquid crystals can be changed?
a) Magnetic field
b) Electric field
c) Electromagnetic field
d) None of the Mentioned
View Answer
Answer: b
Explanation: In LCD, the electric field is induced by a small electric voltage applied across it;
Due to which the orientation of molecules in a layer of liquid crystals can be changed.
Answer: b
Explanation: An electric field (induced by a small electric voltage) can change the orientation of
molecules in a layer of liquid crystal and thus affect its optical properties. Such a process is
termed an electro-optical effect, and it forms the basis for LCDs.
Answer: a
Explanation: For LCDs, the change in optical properties results from orienting the molecular
axes either along or perpendicular to the applied electric field, the preferred direction being
determined by the details of the molecule’s chemical structure.
Answer: c
Explanation: The first LCDs became commercially available in the late 1960s and were based on
a light-scattering effect known as the dynamic scattering mode.
Answer: c
Explanation: LCDs operate from a frequency ranges from 30Hz to 60Hz.
Answer: b
Explanation: There are 7 LEDs used in a 7 segment display.
Answer: a
Explanation: The ac voltage applied between segment and a common element is called the
backplane(bp). In which each segment is driven by an EX-OR gate.
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Encoders”.
Answer: c
Explanation: Decimal-to-bcd converter decimal values are inputs which range from 0-9. So, total
ten inputs a decimal-to-BCD encoder has.
Answer: a
Explanation: BCD means it should be of 4 bits. So, It has 4 outputs.
Answer: a
Explanation: An encoder different from a decoder because of the output of an encoder is a binary
code for 1-of-N input.
Answer: b
Explanation: If we record any music in any recorder, it means that we are giving data to a
recorder. So, such process is called encoding.
Answer: d
Answer: a
Explanation: There are 3 OR gates are required causing it has 3 outputs.
Answer: b
Explanation: There are 2^8 combinations are possible for a 8-bit input encoder but out of which
only 8 are used using 3 output lines. It is a disadvantage of encoder.
9. The discrepancy of 0 output due to all inputs being 0 or D0, being 0 is resolved by using
additional input known as
a) Enable
b) Disable
c) Strobe
d) None of the Mentioned
View Answer
Answer: a
Explanation: Such problems are resolved by using enable input, which behaves as active if it gets
0 as input.
Answer: b
Explanation: A multiplexer or MUX is a combination circuit that contains more than one input
line, one output line and more than one selection line. Whereas, an encoder is also considered a
type of multiplexer but without a single output line.
11. If two inputs are active on a priority encoder, which will be coded on the output?
a) The higher value
b) The lower value
c) Neither of the inputs
d) Both of the inputs
View Answer
Answer: a
Explanation: If two inputs are active on a priority encoder, the input of higher value will be
coded in the output.
Parity Generators/Checkers - Digital Circuits Questions and Answers -
Sanfoundry
by Manish
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Parity Generators/Checkers – 1”.
Answer: d
Explanation: A BCD to Decimal decoder has 10 number of outputs because the decimal digit’s
range is from 0 to 9.
2. Which digital system translates coded characters into a more useful form?
a) Encoder
b) Display
c) Counter
d) Decoder
View Answer
Answer: d
Explanation: Decoder converts the coded characters into our required data form.
Answer: b
Explanation: A LOW on all gate enable inputs is necessary to operate a 1-line-to-16 line decoder
because it is in inverted form always.
5. A BCD decoder will have how many rows in its truth table?
a) 3
b) 9
c) 8
d) 10
View Answer
Answer: a
Explanation: Because of it’s input ranges from 0 to 9.
6. How many possible outputs would a decoder have with a 6-bit binary input?
a) 32
b) 64
c) 128
d) 16
View Answer
Answer: c
Explanation: The possible outputs would be: 2^6=64.
7. One way to convert BCD to binary using the hardware approach is:
a) By using MSI IC circuits
b) By using a keyboard encoder
c) By using an ALU
d) By using UART
View Answer
Answer: a
Explanation: One way to convert BCD to binary using the hardware approach is MSI (medium
scale integration) IC circuits.
Answer: d
Explanation: ‘d’ is correct because 1-of-10 stands for BCD to decimal decoder.
9. A truth table with output columns numbered 0–15 may be for which type of decoder IC?
a) Hexadecimal 1-of-16
b) Dual octal outputs
c) Binary-to-hexadecimal
d) Hexadecimal-to-binary
View Answer
Answer: a
Explanation: A truth table with output columns numbered 0–15 may be for Hexadecimal 1-of-16.
Because, hexadecimal occupies less space in a system.
10. How can the active condition (HIGH or LOW) or the decoder output be determined from the
logic symbol?
a) A bubble indicates active-HIGH
b) A bubble indicates active-LOW
c) A square indicates active-HIGH
d) A square indicates active-LOW
View Answer
Answer: b
Explanation: A bubble indicates active-LOW in a decoder always.
This set of Digital Electronic/Circuits online test focuses on “Parity Generators/Checkers – 2”.
Answer: d
Explanation: A checksum can be generated simply by adding bits. Hence, one’s complement
arithmetic uses checksum.
2. Which error detection method consists of just one redundant bit per data unit?
a) Simple parity check
b) Two-dimensional parity check
c) CRC
d) Checksum
View Answer
Answer: a
Explanation: Simple parity check method consists of just one redundant bit per data unit.
3. How many types of parity bits are found?
a) 2
b) 3
c) 4
d) 1
View Answer
Answer: a
Explanation: There are two types of parity bits, namely even parity and odd parity.
Answer: a
Explanation: A simple form of error detection is achieved by adding an extra bit to the
transmitted word. The additional bit is known as parity bits.
Answer: a
Explanation: The given BCD number 101011 has even priority because it has even number of
1’s (i.e. 4).
Answer: b
Explanation: Simple parity check involves parity check polynomials.
Answer: b
Explanation: The odd parity output of decimal number 9 is 1 because the BCD number for 9 is
1001 and it has even number of 1’s.
8. If odd parity is used for ASCII error detection, the number of 0s per 8-bit symbol is _______
a) Indeterminate
b) 42
c) Even
d) Odd
View Answer
Answer: a
Explanation: If odd parity is used for ASCII error detection, the number of 0s per 8-bit symbol is
indeterminate because it is applicable only for 6-bit symbol.
Answer: b
Explanation: A single-bit error can be detected by using two-dimensional parity check method.
Since, it converts the 4-bit number into 8-bit and count the number of one’s.
10. Which gates are ideal for checking the parity bits?
a) AND
b) NAND
c) EX-OR
d) EX-NOR
View Answer
Answer: c
Explanation: Exclusive-OR gates are ideal for checking the parity of a binary number because
they produce an output when the input has an odd number of 1’s. Therefore, an even-parity input
to an EX-OR gate produces a low output, while an odd parity input produces a high output.
Answer: c
Explanation: A code converter is a logic circuit that changes data presented in one type of binary
code to another type of binary code.
2. Use the weighting factors to convert the following BCD numbers to binary:
0101 0011 & 0010 0110 1000
a) 01010011 001001101000
b) 11010100 100001100000
c) 110101 100001100
d) 101011 001100001
View Answer
Answer: c
Explanation: Firstly, convert every 4 sets of binary to decimal from the given: 0101=5, 0011=3.
Then convert 53 to decimal, which will give 110101. Again, do the same with the next 4 set of
binary digits.
Answer: a
Explanation: Gray code is useful because only one bit changes at a time, which is implemented
easily in Coded representation of a shaft’s mechanical position.
Answer: a
Explanation: One way to convert BCD to binary using the hardware approach is MSI IC (i.e.
medium scale integration) circuits.
6. Why is the Gray code more practical to use when coding the position of a rotating shaft?
a) All digits change between counts
b) Two digits change between counts
c) Only one digit changes between counts
d) None of the Mentioned
View Answer
Answer: c
Explanation: The Gray code is more practical to use when coding the position of a rotating shaft
because only one digit changes between counts that is reflected to the next count.
Answer: d
Explanation: The reflected binary code is also known as gray code because one digit reflected to
the next bit.
Answer: c
Explanation: Reflected binary codes were applied to mathematical puzzles before they became
known to engineers.
Answer: a
Explanation: The given BCD number 00101001 has three 1s. So, it can be rewritten as 0000001-
1, 0001000-8, 0010100-20 and after addition, we get 0011101 as output.
Answer: c
Explanation: Conversion of binary number 100101 into gray code takes place in this way:
Firstly, write the 1st digit as it is from left; now add the two numbers sequentially.
This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses
on “Magnitude Comparator”.
1. All the comparisons made by comparator is done using
a) 1 circuit
b) 2 circuits
c) 3 circuits
d) 4 circuits
View Answer
Answer: a
Explanation: Because, all of the output is compared with each other and it is possible only by
using 1 circuit.
Answer: c
Explanation: In a digital comparator, only 3 outputs are possible (i.e. A = B, A> B, A < B). So, a
< b is incorrect option.
Answer: a
Explanation: In a digital comparator, only 3 outputs are possible (i.e. A = B, A >B, A < B). Other
than this, the output will be 0.
Answer: a
Explanation: There are two inputs required for a digital comparator (i.e. A & B).
Answer: a
Explanation: If A > B, it means that it satisfies one of the condition among three. Hence the
output will be 1.
Answer: a
Explanation: A XNOR gate is a basic comparator, because its output is “1” only if its two input
bits are equal.
Answer: b
Explanation: Comparators are used in central processing unit (CPUs). Because, all the arithmetic
and logical operations are performed in the CPU.
8. A circuit that compares two numbers and determine their magnitude is called
a) Height comparator
b) Size comparator
c) Comparator
d) Magnitude comparator
View Answer
Answer: d
Explanation: A circuit that compares two numbers and determine their magnitude is called
magnitude comparator.
Answer: a
Explanation: A procedure that specifies finite set of steps is called algorithm.
Answer: b
Explanation: There are two main types of Digital Comparator available and these are: Identity
Comparator & Magnitude Comparator.
Answer: a
Explanation: An Identity Comparator is a digital comparator that has only one output terminal
for when A = B either “HIGH” A = B = 1 or “LOW” A = B = 0.
Answer: c
Explanation: A Magnitude Comparator is a digital comparator which has three output terminals,
one each for equality, A = B greater than, A > B and less than A < B.
Answer: b
Explanation: TTL 74LS85 is a 4-bit magnitude comparator.