DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING
SUBJECT CODE: EC1201
DIGITAL ELECTRONICS
(FOR THIRD SEMESTER ECE)
TWO MARK QUESTIONS-ANSWERS
PREPARED BY
Y.IRANEOUS ANNA REJANI (L/ECE)
C.P.SREE BALA LEKSHMI (L/ECE)
Noorul Islam College of Engineering, Kumaracoil
Department of Electronics & Communication
Third semester
EC1201 Digital Electronics
Part - A
Unit – 1
1) Define binary logic?
Binary logic consists of binary variables and logical operations. The variables are
designated by the alphabets such as A, B, C, x, y, z, etc., with each variable having only two
distinct values: 1 and 0. There are three basic logic operations: AND, OR, and NOT.
2) Convert (634) 8 to binary
6 3 4
110 011 100
Ans = 110011100
3) Convert (9B2 - 1A) H to its decimal equivalent.
N = 9 x 16 2 + B x 16 1 + 2 x 16 0 + 1 x 16 -1 + A (10) x 16 -2
= 2304 + 176 + 2 + 0.0625 + 0.039
= 2482.1 10
4) State the different classification of binary codes?
1. Weighted codes
2. Non - weighted codes
3. Reflective codes
4. Sequential codes
5. Alphanumeric codes
6. Error Detecting and correcting codes.
5) Convert 0.640625 decimal number to its octal equivalent.
0.640625 x 8 = 5.125
0.125 x 8 = 1.0
= 0.640 625 10 = (0.51) 8
6) Convert 0.1289062 decimal number to its hex equivalent
0.1289062 x 16 = 2.0625
0.0625 x 16 = 1.0
= 0.21 16
7) Convert 22.64 to hexadecimal number.
16 22 -6
16 1 -1
0
0.64 x 16 = 10.24
0.24 x 16 = 3.84
0.84 x 16 = 13.44
.44 x 16 = 7.04
Ans = (16. A 3 D 7) 16
8) State the steps involved in Gray to binary conversion?
The MSB of the binary number is the same as the MSB of the gray code number. So
write it down. To obtain the next binary digit, perform an exclusive OR operation between
the bit just written down and the next gray code bit. Write down the result.
9) Convert gray code 101011 into its binary equivalent.
Gray Code: 1 0 1 0 1 1
Binary Code: 1 1 0 0 1 0
10) Substract (0 1 0 1) 2 from (1 0 1 1) 2
1 0 1 0
0 1 0 1
Answer = 0 1 1 0
11) Add (1 0 1 0) 2 and (0 0 1 1) 2
1 0 1 0
0 0 1 1
Answer = (1 1 0 1) 2
12) Using 10’s complement subtract 72532 - 3250
M = 72532
10’s complement of N = + 96750
-----------
Sum = 169282
Discard end carry
Answer = 69282
13) Find 2’S complement of (1 0 1 0 0 0 1 1) 2
0 1 0 1 1 1 0 0 1 - 1’s Complement
+ 0 0 0 0 0 0 0 1
0 1 0 1 1 1 0 1 0 - 2’s complement.
14) Substract 1 1 1 0 0 1 2 from 1 0 1 0 1 1 2 using 2’s complement method
1 0 1 0 1 1
+ 0 0 0 1 1 1 - 2’s comp. of 1 1 1 0 0 1
1 1 0 0 1 0 in 2’s complement form
Answer (0 0 1 1 1 0 )2
15) Find the excess -3 code and 9’s complement of the number 403 10
4 0 3
0 1 0 0 0 0 0 0 0 0 1 1
0 0 1 1 0 0 1 1 0 0 1 1 +
0 1 1 1 0 0 1 1 0 1 1 0 ----- excess 3 code
9’s complement 1 0 0 0 1 1 0 0 1 0 0 1
16) What is meant by bit?
A binary digit is called bit
17) Define byte?
Group of 8 bits.
18) List the different number systems?
i) Decimal Number system
ii) Binary Number system
iii) Octal Number system
iv) Hexadecimal Number system
19) State the abbreviations of ASCII and EBCDIC code?
ASCII-American Standard Code for Information Interchange.
EBCDIC-Extended Binary Coded Decimal Information Code.
20) What are the different types of number complements?
i) r’s Complement
ii) (r-1)’s Complement.
21) Given the two binary numbers X = 1010100 and Y = 1000011, perform the subtraction
(a) X -Y and (b) Y - X using 2's complements.
a) X = 1010100
2's complement of Y = 0111101
--------------
Sum = 10010001
Discard end carry
Answer: X - Y = 0010001
b) Y = 1000011
2's complement of X = + 0101100
---------------
Sum = 1101111
There is no end carry, The MSB BIT IS 1.
Answer is Y-X = -(2's complement of 1101111) = - 0010001
22) Given the two binary numbers X = 1010100 and Y = 1000011, perform the subtraction
(a) X -Y and (b) Y - X using 1's complements.
a) X - Y = 1010100 - 1000011
X = 1010100
1's complement of Y = + 0111100
--------------
Sum = 10010000
End -around carry = + 1
--------------
Answer: X - Y = 0010001
b) Y - X = 1000011 - 1010100
Y = 1000011
1's complement of X = + 0101011
-----------
Sum = + 1101110
There is no end carry.
Therefore the answer is Y - X = -(1's complement of 1101110) = -0010001
23) Write the names of basic logical operators.
1. NOT / INVERT
2. AND
3. OR
24) What are basic properties of Boolean algebra?
The basic properties of Boolean algebra are commutative property, associative
property and distributive property.
25) State the associative property of boolean algebra.
The associative property of Boolean algebra states that the OR ing of several variables
results in the same regardless of the grouping of the variables. The associative property is
stated as follows:
A+ (B+C) = (A+B) +C
26) State the commutative property of Boolean algebra.
The commutative property states that the order in which the variables are OR ed
makes no difference. The commutative property is:
A+B=B+A
27) State the distributive property of Boolean algebra.
The distributive property states that AND ing several variables and OR ing the result
with a single variable is equivalent to OR ing the single variable with each of the the several
variables and then AND ing the sums. The distributive property is:
A+BC= (A+B) (A+C)
28) State the absorption law of Boolean algebra.
The absorption law of Boolean algebra is given by X+XY=X, X(X+Y) =X.
29) Simplify the following using De Morgan's theorem [((AB)'C)'' D]'
[((AB)'C)'' D]' = ((AB)'C)'' + D' [(AB)' = A' + B']
= (AB)' C + D'
= (A' + B' )C + D'
30) State De Morgan's theorem.
De Morgan suggested two theorems that form important part of Boolean algebra.
They are,
1) The complement of a product is equal to the sum of the complements.
(AB)' = A' + B'
2) The complement of a sum term is equal to the product of the complements.
(A + B)' = A'B'
31) Reduce A.A'C
A.A'C = 0.C [A.A' = 1]
= 0
31) Reduce A(A + B)
A(A + B) = AA + AB
= A(1 + B) [1 + B = 1]
= A.
32) Reduce A'B'C' + A'BC' + A'BC
A'B'C' + A'BC' + A'BC = A'C'(B' + B) + A'B'C
= A'C' + A'BC [A + A' = 1]
= A'(C' + BC)
= A'(C' + B) [A + A'B = A + B]
33) Reduce AB + (AC)' + AB'C(AB + C)
AB + (AC)' + AB'C(AB + C) = AB + (AC)' + AAB'BC + AB'CC
= AB + (AC)' + AB'CC [A.A' = 0]
= AB + (AC)' + AB'C [A.A = 1]
= AB + A' + C' =AB'C [(AB)' = A' + B']
= A' + B + C' + AB'C [A + AB' = A + B]
= A' + B'C + B + C' [A + A'B = A + B]
= A' + B + C' + B'C
=A' + B + C' + B'
=A' + C' + 1
= 1 [A + 1 =1]
34) Simplify the following expression Y = (A + B)(A + C' )(B' + C' )
Y = (A + B)(A + C' )(B' + C' )
= (AA' + AC +A'B +BC )(B' + C') [A.A' = 0]
= (AC + A'B + BC)(B' + C' )
= AB'C + ACC' + A'BB' + A'BC' + BB'C + BCC'
= AB'C + A'BC'
35) Show that (X + Y' + XY)( X + Y')(X'Y) = 0
(X + Y' + XY)( X + Y')(X'Y) = (X + Y' + X)(X + Y' )(X' + Y) [A + A'B = A + B]
= (X + Y' )(X + Y' )(X'Y) [A + A = 1]
= (X + Y' )(X'Y) [A.A = 1]
= X.X' + Y'.X'.Y
= 0 [A.A' = 0]
36) Prove that ABC + ABC' + AB'C + A'BC = AB + AC + BC
ABC + ABC' + AB'C + A'BC=AB(C + C') + AB'C + A'BC
=AB + AB'C + A'BC
=A(B + B'C) + A'BC
=A(B + C) + A'BC
=AB + AC + A'BC
=B(A + C) + AC
=AB + BC + AC
=AB + AC +BC ...Proved
37) Convert the given expression in canonical SOP form Y = AC + AB + BC
Y = AC + AB + BC
=AC(B + B' ) + AB(C + C' ) + (A + A')BC
=ABC + ABC' + AB'C + AB'C' + ABC + ABC' + ABC
=ABC + ABC' +AB'C + AB'C' [A + A =1]
38) Define duality property.
Duality property states that every algebraic expression deducible from the postulates
of Boolean algebra remains valid if the operators and identity elements are interchanged. If
the dual of an algebraic expression is desired, we simply interchange OR and AND operators
and replace 1's by 0's and 0's by 1's.
39) Find the complement of the functions F1 = x'yz' + x'y'z and F2 = x(y'z' + yz). By
applying De-Morgan's theorem.
F1' = (x'yz' + x'y'z)' = (x'yz')'(x'y'z)' = (x + y' + z)(x + y +z')
F2' = [x(y'z' + yz)]' = x' + (y'z' + yz)'
= x' + (y'z')'(yz)'
= x' + (y + z)(y' + z')
40) Simplify the following expression
Y = (A + B) (A = C) (B + C)
= (A A + A C + A B + B C) (B + C)
= (A C + A B + B C) (B + C)
= A B C + A C C + A B B + A B C + B B C + B C C
= A B C
41) What are the methods adopted to reduce Boolean function?
i) Karnaug map
ii) Tabular method or Quine Mc-Cluskey method
iii) Variable entered map technique.
42) State the limitations of karnaugh map.
i) Generally it is limited to six variable map (i.e) more then six variable
involving expression are not reduced.
ii) The map method is restricted in its capability since they are useful for
simplifying only Boolean expression represented in standard form.
43) What is a karnaugh map?
A karnaugh map or k map is a pictorial form of truth table, in which the map diagram
is made up of squares, with each squares representing one minterm of the function.
44) Find the minterms of the logical expression Y = A'B'C' + A'B'C + A'BC + ABC'
Y = A'B'C' + A'B'C + A'BC + ABC'
=m0 + m1 +m3 +m6
= m(0, 1, 3, 6)
45) Write the maxterms corresponding to the logical expression
Y = (A + B + C' )(A + B' + C')(A' + B' + C)
= (A + B + C' )(A + B' + C')(A' + B' + C)
=M1.M3.M6
= M(1,3,6)
46) What are called don’t care conditions?
In some logic circuits certain input conditions never occur, therefore the
corresponding output never appears. In such cases the output level is not defined, it can be
either high or low. These output levels are indicated by ‘X’ or‘d’ in the truth tables and are
called don’t care conditions or incompletely specified functions.
47) What is a prime implicant?
A prime implicant is a product term obtained by combining the maximum possible
number of adjacent squares in the map.
48) What is an essential implicant?
If a min term is covered by only one prime implicant, the prime implicant is
said to be essential
Unit – II
49. What is a Logic gate?
Logic gates are the basic elements that make up a digital system. The electronic gate
is a circuit that is able to operate on a number of binary inputs in order to perform a
particular logical function.
50.Give the classification of logic families
Bipolar Unipolar
Saturated Non Saturated PMOS
NMOS
CMOS
RTL Schottky TTL
ECL DTL
I I C
TTL
51. What are the basic digital logic gates?
The three basic logic gates are
AND gate
OR gate
NOT gate
52. Which gates are called as the universal gates? What are its advantages?
The NAND and NOR gates are called as the universal gates. These gates are used to
perform any type of logic application.
53.Classify the logic family by operation?
The Bipolar logic family is classified into
Saturated logic
Unsaturated logic.
The RTL, DTL, TTL, I2L, HTL logic comes under the saturated logic family.
The Schottky TTL, and ECL logic comes under the unsaturated logic family.
54.State the classifications of FET devices.
FET is classified as
1. Junction Field Effect Transistor (JFET)
2. Metal oxide semiconductor family (MOS).
55.Mention the classification of saturated bipolar logic families.
The bipolar logic family is classified as follows:
RTL- Resistor Transistor Logic
DTL- Diode Transistor logic
I2L- Integrated Injection Logic
TTL- Transistor Transistor Logic
ECL- Emitter Coupled Logic
56.Mention the different IC packages?
DIP- Dual in line package
LCC- Leadless Chip Carrier
PLCC- Plastic Leaded Chip carrier
PQFP- Plastic Quad Flat Pack
PGA- Pin Grid Array
57. Mention the important characteristics of digital IC’s?
Fan out
Power dissipation
Propagation Delay
Noise Margin
Fan In
Operating temperature
Power supply requirements
58. Define Fan-out?
Fan out specifies the number of standard loads that the output of the gate can drive
with out impairment of its normal operation.
59. Define power dissipation?
Power dissipation is measure of power consumed by the gate when fully driven by all
its inputs.
60. What is propagation delay?
Propagation delay is the average transition delay time for the signal to propagate from
input to output when the signals change in value. It is expressed in ns.
61. Define noise margin?
It is the maximum noise voltage added to an input signal of a digital circuit that does
not cause an undesirable change in the circuit output. It is expressed in volts.
62. Define fan in?
Fan in is the number of inputs connected to the gate without any degradation in the
voltage level.
63. What is Operating temperature?
All the gates or semiconductor devices are temperature sensitive in nature. The
temperature in which the performance of the IC is effective is called as operating
temperature. Operating temperature of the IC vary from 00 C to 700 c.
64.What is High Threshold Logic?
Some digital circuits operate in environments, which produce very high noise signals.
For operation in such surroundings there is available a type of DTL gate which possesses a
high threshold to noise immunity. This type of gate is called HTL logic or High Threshold
Logic.
65. What are the types of TTL logic?
1. Open collector output
2. Totem-Pole Output
3. Tri-state output.
66. What is depletion mode operation MOS?
If the channel is initially doped lightly with p-type impurity a conducting channel
exists at zero gate voltage and the device is said to operate in depletion mode.
67. What is enhancement mode operation of MOS?
If the region beneath the gate is left initially uncharged the gate field must induce a
channel before current can flow. Thus the gate voltage enhances the channel current and such
a device is said to operate in the enhancement mode.
68. Mention the characteristics of MOS transistor?
1. The n- channel MOS conducts when its gate- to- source voltage is positive.
2. The p- channel MOS conducts when its gate- to- source voltage is negative
73. When does the noise margin allow digital circuits to function properly.
When noise voltages are within the limits of VNA(High State Noise Margin) and VNK
for a particular logic family.
74. What happens to output when a tristate circuit is selected for high impedance.
Output is disconnected from rest of the circuits by internal circuitry.
75. What is 14000 series.
It is the oldest and standard CMOS family. The devices are not pin compatible or
electrically compatible with any TTL Series.
76. Implement the Boolean Expression for EX – OR gate using NAND Gates.
77. Define combinational logic
When logic gates are connected together to produce a specified output for certain
specified combinations of input variables, with no storage involved, the resulting circuit is
called combinational logic.
78. Explain the design procedure for combinational circuits
The problem definition
Determine the number of available input variables & required O/P variables.
Assigning letter symbols to I/O variables
Obtain simplified Boolean expression for each O/P.
Obtain the logic diagram.
79. Define Half adder and full adder
The logic circuit that performs the addition of two bits is a half adder. The circuit that
performs the addition of three bits is a full adder.
90. Draw the logic Symbol and construct the truth table for the two input EX – OR gate.
81. Define Decoder?
A decoder is a multiple - input multiple output logic circuit that converts coded
inputs into coded outputs where the input and output codes are different.
82. What is binary decoder?
A decoder is a combinational circuit that converts binary information from n input
lines to a maximum of 2n out puts lines.
83. Define Encoder?
An encoder has 2n input lines and n output lines. In encoder the output lines generate
the binary code corresponding to the input value.
84. What is priority Encoder?
A priority encoder is an encoder circuit that includes the priority function. In priority
encoder, if 2 or more inputs are equal to 1 at the same time, the input having the highest
priority will take precedence.
85. Define multiplexer?
Multiplexer is a digital switch. If allows digital information from several sources to be
routed onto a single output line.
86. What do you mean by comparator
A comparator is a special combinational circuit designed primarily to compare the
relative magnitude of two binary numbers.
87. List basic types of programmable logic devices.
. Read only memory
. Programmable logic Array
. Programmable Array Logic
88. Define ROM
Read only memory is a device that includes both the decoder and the OR gates
within a single IC package.
89. Define address and word:
In a ROM, each bit combination of the input variable is called on address. Each bit
combination that comes out of the output lines is called a word.
90. State the types of ROM
. Masked ROM.
. Programmable Read only Memory
. Erasable Programmable Read only memory.
. Electrically Erasable Programmable Read only Memory.
91. What is programmable logic array? How it differs from ROM?
In some cases the number of don’t care conditions is excessive, it is more economical
to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept;
however it does not provide full decoding of the variables and does not generates all the
minterms as in the ROM.
92. Which gate is equal to AND-invert Gate?
NAND gate.
93. Which gate is equal to OR-invert Gate?
NOR gate.
94. Bubbled OR gate is equal to--------------
NAND gate
95. Bubbled AND gate is equal to--------------
NOR gate
Unit 3
96. What are the classification of sequential circuits?
The sequential circuits are classified on the basis of timing of their signals into two
types. They are,
1)Synchronous sequential circuit.
2)Asynchronous sequential circuit.
97. Define Flip flop.
The basic unit for storage is flip flop. A flip-flop maintains its output state either at 1
or 0 until directed by an input signal to change its state.
98.What are the different types of flip-flop?
There are various types of flip flops. Some of them are mentioned below they are,
RS flip-flop
SR flip-flop
D flip-flop
JK flip-flop
T flip-flop
99.What is the operation of RS flip-flop?
• When R input is low and S input is high the Q output of flip-flop is
set.
• When R input is high and S input is low the Q output of flip-flop is
reset.
• When both the inputs R and S are low the output does not change
• When both the inputs R and S are high the output is unpredictable.
100.What is the operation of SR flip-flop?
• When R input is low and S input is high the Q output of flip-flop is
set.
• When R input is high and S input is low the Q output of flip-flop is
reset.
• When both the inputs R and S are low the output does not change.
• When both the inputs R and S are high the output is unpredictable.
101.What is the operation of D flip-flop?
In D flip-flop during the occurrence of clock pulse if D=1, the output Q is set and if
D=0, the output is reset.
102. What is the operation of JK flip-flop?
• When K input is low and J input is high the Q output of flip-flop is
set.
• When K input is high and J input is low the Q output of flip-flop is
reset.
• When both the inputs K and J are low the output does not change
• When both the inputs K and J are high it is possible to set or reset the
flip-flop (ie) the output toggle on the next positive clock edge.
103. What is the operation of T flip-flop?
T flip-flop is also known as Toggle flip-flop.
• When T=0 there is no change in the output.
• When T=1 the output switch to the complement state (ie) the output
toggles.
104. Define race around condition.
In JK flip-flop output is fed back to the input. Therefore change in the output results
change in the input. Due to this in the positive half of the clock pulse if both J and K are high
then output toggles continuously. This condition is called ‘race around condition’.
105. What is edge-triggered flip-flop?
The problem of race around condition can solved by edge triggering flip flop. The
term edge triggering means that the flip-flop changes state either at the positive edge or
negative edge of the clock pulse and it is sensitive to its inputs only at this transition of the
clock.
106. What is a master-slave flip-flop?
A master-slave flip-flop consists of two flip-flops where one circuit serves as a master
and the other as a slave.
107.Define rise time.
The time required to change the voltage level from 10% to 90% is known as rise
time(tr).
108.Define fall time.
The time required to change the voltage level from 90% to 10% is known as fall
time(tf).
109.Define skew and clock skew.
The phase shift between the rectangular clock waveforms is referred to as skew and
the time delay between the two clock pulses is called clock skew.
110.Define setup time.
The setup time is the minimum time required to maintain a constant voltage levels at
the excitation inputs of the flip-flop device prior to the triggering edge of the clock pulse in
order for the levels to be reliably clocked into the flip flop. It is denoted as tsetup.
111. Define hold time.
The hold time is the minimum time for which the voltage levels at the excitation
inputs must remain constant after the triggering edge of the clock pulse in order for the levels
to be reliably clocked into the flip flop. It is denoted as thold .
112. Define propagation delay.
A propagation delay is the time required to change the output after the application of
the input.
113.Define registers.
A register is a group of flip-flops flip-flop can store one bit information. So an n-bit register
has a group of n flip-flops and is capable of storing any binary information/number
containing n-bits.
114.Define shift registers.
The binary information in a register can be moved from stage to stage within the
register or into or out of the register upon application of clock pulses. This type of bit
movement or shifting is essential for certain arithmetic and logic operations used in
microprocessors. This gives rise to group of registers called shift registers.
115.What are the different types of shift type?
There are five types. They are,
Serial In Serial Out Shift Register
Serial In Parallel Out Shift Register
Parallel In Serial Out Shift Register
Parallel In Parallel Out Shift Register
Bidirectional Shift Register
116.Explain the flip-flop excitation tables for RS FF.
RS flip-flop
In RS flip-flop there are four possible transitions from the present state to the
next state. They are,
0 0 transition: This can happen either when R=S=0 or when R=1 and
S=0.
0 1 transition: This can happen only when S=1 and R=0.
1 0 transition: This can happen only when S=0 and R=1.
1 1 transition: This can happen either when S=1 and R=0 or S=0 and
R=0.
117.Explain the flip-flop excitation tables for JK flip-flop
In JK flip-flop also there are four possible transitions from present state to next state.
They are,
0 0 transition: This can happen when J=0 and K=1 or K=0.
0 1 transition: This can happen either when J=1 and K=0 or when
J=K=1.
1 0 transition: This can happen either when J=0 and K=1 or when
J=K=1.
1 1 transition: This can happen when K=0 and J=0 or J=1.
118.Explain the flip-flop excitation tables for D flip-flop
In D flip-flop the next state is always equal to the D input and it is independent of the
present state. Therefore D must be 0 if Qn+1 has to 0,and if Qn+1 has to be 1 regardless the
value of Qn.
119. Explain the flip-flop excitation tables for T flip-flop
When input T=1 the state of the flip-flop is complemented; when T=0,the state of the
flip-flop remains unchanged. Therefore, for 0 0 and 1 1 transitions T must be 0 and for
0 1 and 1 0 transitions must be 1.
120. Define sequential circuit?
In sequential circuits the output variables dependent not only on the present input
variables but they also depend up on the past history of these input variables.
121.Give the comparison between combinational circuits and sequential circuits.
Combinational circuits Sequential circuits
Memory unit is not required Memory unity is required
Parallel adder is a combinational circuit Serial adder is a sequential circuit
122. What do you mean by present state?
The information stored in the memory elements at any given time define.s the present
state of the sequential circuit.
123. What do you mean by next state?
The present state and the external inputs determine the outputs and the next state of
the sequential circuit.
124. State the types of sequential circuits?
1. Synchronous sequential circuits
2. Asynchronous sequential circuits
125. Define synchronous sequential circuit
In synchronous sequential circuits, signals can affect the memory elements only at
discrete instant of time.
126. Define Asynchronous sequential circuit?
In asynchronous sequential circuits change in input signals can affect memory
element at any instant of time.
127.Give the comparison between synchronous & Asynchronous sequential circuits?
Synchronous sequential circuits Asynchronous sequential circuits.
Memory elements are clocked flip-flops Memory elements are either unlocked flip -
flops or time delay elements.
Easier to design More difficult to design
128.Define flip-flop
Flip - flop is a sequential device that normally samples its inputs and changes its
outputs only at times determined by clocking signal.
129. Draw the logic diagram for SR latch using two NOR gates.
130. The following wave forms are applied to the inputs of SR latch. Determine the Q
waveform Assume initially Q = 1
Here the latch input has to be pulsed momentarily to cause a change in the latch
output state, and the output will remain in that new state even after the input pulse is over.
131.What is race around condition?
In the JK latch, the output is feedback to the input, and therefore changes in the output
results change in the input. Due to this in the positive half of the clock pulse if J and K are
both high then output toggles continuously. This condition is known as race around
condition.
132.What are the types of shift register?
1. Serial in serial out shift register?
2. Serial in parallel out shift register
3. Parallel in serial out shift register
4. Parallel in parallel out shift register
5. Bidirectional shift register shift register
133.State the types of counter?
1. Synchronous counter
2. Asynchronous Counter
134.Give the comparison between synchronous & Asynchronous counters.
Asynchronous counters Synchronous counters
In this type of counter flip-flops are
connected in such a way that output of 1st
flip-flop drives the clock for the next flipflop.
In this type there is no connection between
output of first flip-flop and clock input of
the next flip - flop
All the flip-flops are Not clocked
simultaneously
All the flip-flops are clocked
simultaneously
135.The t pd for each flip-flop is 50 ns. Determine the maximum operating frequency for
MOD - 32 ripple counter
f max (ripple) = 5 x 50 ns = 4 MHZ
Unit 4
136. What are secondary variables?
-present state variables in asynchronous sequential circuits
137.What are excitation variables?
-next state variables in asynchronous sequential circuits
138. What is fundamental mode sequential circuit?
-input variables changes if the circuit is stable
-inputs are leve45.What are the different techniques used in state assignment?
-shared row state assignment
-one hot state assignment
146.What are the steps for the design of asynchronous sequential circuit?
-construction of primitive flow table
-reduction of flow table
-state assignment is made
-realization of primitive flow table
147.What is hazard?
-unwanted switching transients
148.What is static 1 hazard?
-output goes momentarily 0 when it should remain at 1
149.What is static 0 hazard?
-output goes momentarily 1 when it should remain at 0
150. What is dynamic hazard?
-output changes 3 or more times when it changes from 1 to 0 or 0 to 1
151.What is the cause for essential hazards?
-unequal delays along 2 or more path from same input
151.What is flow table?
-state table of an synchronous sequential network
152.What is SM chart?
-describes the behavior of a state machine
-used in hardware design of digital systems
152.What are the advantages of SM chart?
-easy to understand the operation
-east to convert to several equivalent forms
153. What is primitive flow chart?
-one stable state per row
154.What is combinational circuit?
Output depends on the given input. It has no storage element.
155.What is state equivalence theorem ?
Two states SA and SB, are equivalent if and only if for every possible input X
sequence, the outputs are the same and the next states are equivalent
i.e., if SA (t + 1) = SB (t + 1) and ZA = ZB then SA = SB.
156.What do you mean by distinguishing sequences?
Two states, SA and SB of sequential machine are distinguishable if and only if their
exists at least one finite input sequence. Which, when applied to sequential machine causes
different output sequences depending on whether SA or SB is the initial state.
157. Prove that the equivalence partition is unique
Consider that there are two equivalence partitions exists : PA and PB, and PA ) PB.
This states that, there exist 2 states Si & Sj which are in the same block of one partition and
not in the same block of the other. If Si & Sj are in different blocks of say PB, there exists at
least on input sequence which distinguishes Si & Sj and therefore, they cannot be in the same
block of PA.
158.Define compatibility
States Si and Sj said to be compatible states, if and only if for every input sequence
that affects the two states, the same output sequence, occurs whenever both outputs are
specified and regardless of whether Si on Sj is the initial state.
159.Define merger graph.
The merger graph is defined as follows. It contains the same number of vertices as the
state table contains states. A line drawn between the two state vertices indicates each
compatible state pair. It two states are incompatible no connecting line is drawn.
160.Define incompatibility
The states are said to be incompatible if no line is drawn in between them. If implied
states are incompatible, they are crossed & the corresponding line is ignored.
161.Explain the procedure for state minimization.
1. Partition the states into subsets such that all states in the same subsets are 1 -
equivalent.
2. Partition the states into subsets such that all states in the same subsets are 2 -
equivalent.
3. Partition the states into subsets such that all states in the same subsets are 3 -
equivalent.
162.Define closed covering
A Set of compatibles is said to be closed if, for every compatible contained in the set,
all its implied compatibles are also contained in the set. A closed set of compatibles, which
contains all the states of M, is called a closed covering.
163.Define machine equivalence
Two machines, M1 and M2 are said to be equivalent if and only if, for every state in
M1, there is a corresponding equivalent state in M2 & vice versa.
164.Define state table.
For the design of sequential counters we have to relate present states and next states.
The table, which represents the relationship between present states and next states, is called
state table.
165. Define total state
The combination of level signals that appear at the inputs and the outputs of the
delays define what is called the total state of the circuit.
166.What are the steps for the design of asynchronous sequential circuit?
1. Construction of a primitive flow table from the problem statement.
2. Primitive flow table is reduced by eliminating redundant states using the state
reduction
3. State assignment is made
4. The primitive flow table is realized using appropriate logic elements.
167. Define primitive flow table :
It is defined as a flow table which has exactly one stable state for each row in the
table. The design process begins with the construction of primitive flow table.
168.What are the types of asynchronous circuits ?
1. Fundamental mode circuits
2. Pulse mode circuits
169.Give the comparison between state Assignment Synchronous circuit and state assignment
asynchronous circuit.
In synchronous circuit, the state assignments are made with the objective of circuit
reduction. In asynchronous circuits, the objective of state assignment is to avoid critical races.
170.What are races?
When 2 or more binary state variables change their value in response to a change in
an input variable, race condition occurs in an asynchronous sequential circuit. In case of
unequal delays, a race condition may cause the state variables to change in an unpredictable
manner.
171.Define non critical race.
If the final stable state that the circuit reaches does not depend on the order in which
the state variable changes, the race condition is not harmful and it is called a non critical race.
172.Define critical race?
If the final stable state depends on the order in which the state variable changes, the
race condition is harmful and it is called a critical race.
173What is a cycle?
A cycle occurs when an asynchronous circuit makes a transition through a series of
unstable states. If a cycle does not contain a stable state, the circuit will go from one unstable
to stable to another, until the inputs are changed.
174.List the different techniques used for state assignment.
1. Shared row state assignment
2. One hot state assignment.
175.Write a short note on fundamental mode asynchronous circuit.
Fundamental mode circuit assumes that. The input variables change only when the
circuit is stable. Only one input variable can change at a given time and inputs are levels and
not pulses.
176. Write a short note on pulse mode circuit.
Pulse mode circuit assumes that the input variables are pulses instead of level. The
width of the pulses is long enough for the circuit to respond to the input and the pulse width
must not be so long that it is still present after the new state is reached.
177.Define secondary variables
The delay elements provide a short term memory for the sequential circuit. The
present state and next state variables in asynchronous sequential circuits are called secondary
variables.
178. Define flow table in asynchronous sequential circuit.
In asynchronous sequential circuit state table is known as flow table because of the
behaviour of the asynchronous sequential circuit. The stage changes occur in independent of
a clock, based on the logic propagation delay, and cause the states to .flow. from one to
another.
179. A pulse mode asynchronous machine has two inputs. If produces an output whenever
two consecutive pulses occur on one input line only. The output remains at 1 until a pulse has
occurred on the other input line. Write down the state table for the machine.
180.What is fundamental mode.
A transition from one stable state to another occurs only in response to a change in the
input state. After a change in one input has occurred, no other change in any input occurs
until the circuit enters a stable state. Such a mode of operation is referred to as a fundamental
mode.
181. Write short note on shared row state assignment.
Races can be avoided by making a proper binary assignment to the state variables.
Here, the state variables are assigned with binary numbers in such a way that only one state
variable can change at any one state variable can change at any one time when a state
transition occurs. To accomplish this, it is necessary that states between which transitions
occur be given adjacent assignments. Two binary are said to be adjacent if they differ in only
one variable.
182. Write short note on one hot state assignment.
The one hot state assignment is another method for finding a race free state
assignment. In this method, only one variable is active or hot for each row in the original
flow table, ie, it requires one state variable for each row of the flow table. Additional row are
introduced to provide single variable changes between internal state transitions.
Unit 5
183. Explain ROM
A read only memory(ROM) is a device that includes both the decoder and the OR
gates within a single IC package. It consists of n input lines and m output lines. Each bit
combination of the input variables is called an address. Each bit combination that comes out
of the output lines is called a word. The number of distinct addresses possible with n input
variables is 2n.
184. What are the types of ROM?
1.PROM
2.EPROM
3.EEPROM
185. Explain PROM.
PROM (Programmable Read Only Memory)
It allows user to store data or program. PROMs use the fuses with material
like nichrome and polycrystalline. The user can blow these fuses by passing
around 20 to 50 mA of current for the period 5 to 20μs.The blowing of fuses is
called programming of ROM. The PROMs are one time programmable. Once
programmed, the information is stored permanent.
186. Explain EPROM.
EPROM(Erasable Programmable Read Only Memory)
EPROM use MOS circuitry. They store 1’s and 0’s as a packet of charge in a
buried layer of the IC chip. We can erase the stored data in the EPROMs by
exposing the chip to ultraviolet light via its quartz window for 15 to 20
minutes. It is not possible to erase selective information. The chip can be
reprogrammed.
187. Explain EEPROM.
EEPROM(Electrically Erasable Programmable Read Only Memory)
EEPROM also use MOS circuitry. Data is stored as charge or no charge on an
insulated layer or an insulated floating gate in the device. EEPROM allows
selective erasing at the register level rather than erasing all the information
since the information can be changed by using electrical signals.
189. What is RAM?
Random Access Memory. Read and write operations can be carried out.
190. Define ROM
A read only memory is a device that includes both the decoder and the OR gates
within a single IC package.
191. Define address and word:
In a ROM, each bit combination of the input variable is called on address. Each bit
combination that comes out of the output lines is called a word.
192. What are the types of ROM.
1. Masked ROM.
2. Programmable Read only Memory
3. Erasable Programmable Read only memory.
4. Electrically Erasable Programmable Read only Memory.
193. What is programmable logic array? How it differs from ROM?
In some cases the number of don’t care conditions is excessive, it is more economical
to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept;
however it does not provide full decoding of the variables and does not generates all the
minterms as in the ROM.
194.What is mask - programmable?
With a mask programmable PLA, the user must submit a PLA program table to the
manufacturer.
195. What is field programmable logic array?
The second type of PLA is called a field programmable logic array. The user by
means of certain recommended procedures can program the EPLA.
196. List the major differences between PLA and PAL
PLA:
Both AND and OR arrays are programmable and Complex
Costlier than PAL
PAL
AND arrays are programmable OR arrays are fixed
Cheaper and Simpler
197. Define PLD.
Programmable Logic Devices consist of a large array of AND gates and OR gates that
can be programmed to achieve specific logic functions.
198. Give the classification of PLDs.
PLDs are classified as PROM(Programmable Read Only Memory), Programmable
Logic Array(PLA), Programmable Array Logic (PAL), and Generic Array Logic(GAL)
199. Define PROM.
PROM is Programmable Read Only Memory. It consists of a set of fixed AND gates
connected to a decoder and a programmable OR array.
200. Define PLA
PLA is Programmable Logic Array(PLA). The PLA is a PLD that consists of a
programmable AND array and a programmable OR array.
201. Define PAL
PAL is Programmable Array Logic. PAL consists of a programmable AND array and
a fixed OR array with output logic.
202. Why was PAL developed ?
It is a PLD that was developed to overcome certain disadvantages of PLA, such as
longer delays due to additional fusible links that result from using two programmable arrays
and more circuit complexity.
203. Define GAL
GAL is Generic Array Logic. GAL consists of a programmable AND array and a
fixed OR array with output logic.
204. Why the input variables to a PAL are buffered
The input variables to a PAL are buffered to prevent loading by the large number of
AND gate inputs to which available or its complement can be connected.
205. What does PAL 10L8 specify ?
PAL - Programmable Logic Array
10 - Ten inputs
L - Active LOW Ouput
8 - Eight Outputs
206. What is CPLD ?
CPLDs are Complex Programmable Logic Devices. They are larger versions of PLDs
with a centralized internal interconnect matrix used to connect the device macro cells
together.
207.Define bit, byte and word.
The smallest unit of binary data is bit. Data are handled in a 8 bit unit called byte. ted address in any sequence.
214. List the two categories of RAMs.
The two categories of RAMs are static RAM(SRAM) and dynamic RAM (DRAM).
215. Define Static RAM and dynamic RAM
Static RAM use flip flops as storage elements and therefore store data indefinitely as
long as dc power is applied.
Dynamic RAMs use capacitors as storage elements and cannot retain data very long
without capacitors being recharged by a process called refreshing.
216. List the two types of SRAM
Asynchronous SRAMs and Synhronous Burst SRAMs
217.List the basic types of DRAMs
Fast Page Mode DRAM,Extended Data Out DRAM(EDO DRAM),Burst EDO
DRAM and Synchronous DRAM.
218. Define a bus
A bus is a set of conductive paths that serve to interconnect two or more functional
components of a system or several diverse systems.
219. Define Cache memory
It is a relatively small, high-speed memory that can store the most recently used
instructions or data from larger but slower main memory.
220. What is the technique adopted by DRAMs.
DRAMs use a technique called address multiplexing to reduce the number of address
lines.
221.Give the feature of UV EPROM
UV EPROM is electrically programmable by the user, but the store data must be
erased by exposure to ultra violet light over a period of several minutes.
222.Give the feature of flash memory.
The ideal memory has high storage capacity, non-volatility; in-system read and write
capability, comparatively fast operation. The traditional memory technologies such as ROM,
PROM, EEPROM individually exhibits one of these characteristics, but no single technology
has all of them except the flash memory.
223.What are Flash memories ?
They are high density read/write memories that are non-volatile, which means data
can be stored indefinitely with out power.
224. List the three major operations in a flash memory.
Programming, Read and Erase operation
225. What is a FIFO memory ?
The term FIFO refers to the basic operation of this type of memory in which the first
data bit written into the memory is to first to be read out.
226.List basic types of programmable logic devices.
1. Read only memory
2. Programmable logic Array
3. Programmable Array Logic
227. Define ROM
A read only memory is a device that includes both the decoder and the OR gates
within a single IC package.
228.Define address and word:
In a ROM, each bit combination of the input variable is called on address. Each bit
combination that comes out of the output lines is called a word.
229. What are the types of ROM ?
1. Masked ROM.
2. Programmable Read only Memory
3. Erasable Programmable Read only memory.
4. Electrically Erasable Programmable Read only Memory.
300. What is programmable logic array? How it differs from ROM?
In some cases the number of don’t care conditions is excessive, it is more economical
to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept;
however it does not provide full decoding of the variables and does not generates all the
minterms as in the ROM.
301.What is mask - programmable?
With a mask programmable PLA, the user must submit a PLA PLA program table to
the manufacturer.
302.Give the comparison between PROM and PLA.
PROM PLA
1. And array is fixed and OR Both AND and OR arrays are
array is programmable. Programmable.
2. Cheaper and simple to use. Costliest and complex than
PROMS.
Part – B
Unit-I
1) Simplify the boolean function using tabulation method.
F = (0,1,2,8,10,11,14,15)
List all the min terms
Arrange them as per the number of ones based on binary equivalent
Compare one group with another for difference in one and replace the bit with dash.
Continue this until no further grouping possible.
The unchecked terms represent the prime implicants.
F = W'X'Y' + X'Z' + WY
2) Determine the prime implicants of the function
F (W,X,Y,Z) = (1,4,6,7,8,9,10,11,15)
List all the min terms
Arrange them as per the number of ones based on binary equivalent
Compare one group with another for difference in one and replace the bit with dash.
Continue this until no further grouping possible.
The unchecked terms represent the prime implicants.
F = X'Y'Z + W'XZ' + W'XY + XYZ + WYZ + WX'
Minimum Set of prime implicants F = X'Y'Z + W'XZ' + XYZ + WX'
3) Simplify the Boolean function using K-map.
F(A,B,C,D,E) = (0,2,4,6,9,13,21,23,25,29,31)
Five variables hence two variable k maps one for A = 0 and the other for A = 1.
F = A'B'E' + BD'E + ACE
4) Obtain the canonical sum of products of the function Y = AB + ACD
Y = AB (C + C')(D + D') + ACD (B + B')
Y = ABCD + ABCD' + ABC'D + ABC'D' + AB'CD
5) State the postulates and theorems of Boolean algebra.
X + 0 = X X · 1 = X
X + X' = 1 X · X' = 0
X + X = X X · X = X
X + 1 = 1 X · 0 = 0
(X')' = X
X + Y = Y + X XY = YX
X + (Y + Z) = (X + Y) + Z X(YZ) = (XY)Z
X(Y + Z) = XY + XZ X + YX = (X + Y) (X + Z)
(X + Y)' = X'Y' (XY)' = X' + Y'
X + XY = X X(X + Y) = X
Unit-II
6. Explain with neat diagrams TTL.
Disadvantages of other families
Diagram of TTL
Theory
Working principle
7. Discuss all the characteristics of digital IC’s.
Fan out
Power dissipation
Propagation Delay
Noise Margin
Fan In
Operating temperature
Power supply requirements
8. Explain with neat diagram how an open collector TTL operates.
Disadvantages of other families
Diagram of open collector gate TTL
Theory
Working principle
9. Explain the different applications of open collector TTL.
Wired logic
Common bus system
Drive a lamp or relay
10. Explain in detail about schottky TTL.
Disadvantages of other families
Diagram of schottky TTL
Theory
Working principle
Advantages
11.Explain in detail about three state gate.
Disadvantages of other families
Explanation about three state gate
Theory
Working principle
12.Explain with necessary diagrams MOS & CMOS.
PMOS
NMOS
CMOS
Diagrams
13.Design a 4-bit binary adder/subtractor circuit.
Basic equations
Comparison of equations
Design using twos complement
Circuit diagram
Unit-III
14.Explain the working of BCD Ripple Counter with the help of state diagram and logic
diagram.
BCD Ripple Counter Count sequence
Truth Table
State diagram representing the Truth Table
Truth Table for the J-K Flip Flop
Logic Diagram
15.Design a logic circuit to convert the BCD code to Excess – 3 code.
Truth Table for BCD to Excess – 3 conversion.
K-map simplification
Logic circuit implementing the Boolean Expression
16.Design and explain a comparator to compare two identical words.
Two numbers represented by A = A3A2A1A0 & B = B3B2B1B0
If two numbers equal P = Ai ⊕Bi
Obtain the logic Expression .
Obtain the logic diagram.
17.Design a sequential detector which produces an output 1 every time the input sequence
1011 is detected.
Construct state diagram
Obtain the flow table
Obtain the flow table & output table
Transition table
Select flip flop
Excitation table
Logic diagram
18.. Explain in detail about serial in serial out shift register.
Block diagram
Theoretical explanation
Logic diagram
Working
Unit-IV
19.Explain with neat diagram the different hazards and the way to eliminate them.
Classification of hazards
Static hazard & Dynamic hazard definitions
K map for selected functions
Method of elimination
Essential hazards
20.State with a neat example the method for the minimization of primitive flow table.
Consider a state diagram
Obtain the flow table
Using implication table reduce the flow table
Using merger graph obtain maximal compatibles
Verify closed & covered conditions
Plot the reduced flow table
21.Design a asynchronous sequential circuit with 2 inputs T and C. The output attains a value
of 1 when T = 1 & c moves from 1 to 0. Otherwise the output is 0.
Obtain the state diagram
Obtain the flow table
Using implication table reduce the flow table
Using merger graph obtain maximal compatibles
Verify closed & covered conditions
Plot the reduced flow table
Obtain transition table
Excitation table
Logic diagram
22.. Explain in detail about Races.
Basics of races
Problem created due to races
Classification of races
Remedy for races
cycles
23.. Explain the different methods of state assignment
Three row state assignment
Shared row state assignment
Four row flow table
Multiple row state assignment
Prevention of races.
Unit-V
24Explain in detail about PLA with a specific example.
Explanation about ROM
Classifications of ROM
Architecture of ROM
Specification of PLA
Specific Example
Related Diagram
Related Table.
25.Implement the following using a mux. F(a,b,c,d) = (0,1,3,4,8,9,15)
Obtain the truth table
From the truth table realize the expressions for the outputs and inputs
Realize the logic diagram.
26.Explain with neat diagrams a RAM architecture.
Different Memories
Classification of memories
RAM architecture diagram
Timing waveforms
Coincident Decoding
Read write operations
27.Explain in detail about PLA and PAL.
Basic ROM
Classification of PROM
Logic difference between Prom & PLA
Logic diagram implementing a function
Logic difference between Prom & PAL
Logic diagram implementing a function
28.Explain with neat diagrams a ROM architecture.
Different Memories
Classification of memories
ROM architecture diagram
Timing waveforms
Coincident Decoding
Read write operations
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ENGINEERING
SUBJECT CODE: EC1201
DIGITAL ELECTRONICS
(FOR THIRD SEMESTER ECE)
TWO MARK QUESTIONS-ANSWERS
PREPARED BY
Y.IRANEOUS ANNA REJANI (L/ECE)
C.P.SREE BALA LEKSHMI (L/ECE)
Noorul Islam College of Engineering, Kumaracoil
Department of Electronics & Communication
Third semester
EC1201 Digital Electronics
Part - A
Unit – 1
1) Define binary logic?
Binary logic consists of binary variables and logical operations. The variables are
designated by the alphabets such as A, B, C, x, y, z, etc., with each variable having only two
distinct values: 1 and 0. There are three basic logic operations: AND, OR, and NOT.
2) Convert (634) 8 to binary
6 3 4
110 011 100
Ans = 110011100
3) Convert (9B2 - 1A) H to its decimal equivalent.
N = 9 x 16 2 + B x 16 1 + 2 x 16 0 + 1 x 16 -1 + A (10) x 16 -2
= 2304 + 176 + 2 + 0.0625 + 0.039
= 2482.1 10
4) State the different classification of binary codes?
1. Weighted codes
2. Non - weighted codes
3. Reflective codes
4. Sequential codes
5. Alphanumeric codes
6. Error Detecting and correcting codes.
5) Convert 0.640625 decimal number to its octal equivalent.
0.640625 x 8 = 5.125
0.125 x 8 = 1.0
= 0.640 625 10 = (0.51) 8
6) Convert 0.1289062 decimal number to its hex equivalent
0.1289062 x 16 = 2.0625
0.0625 x 16 = 1.0
= 0.21 16
7) Convert 22.64 to hexadecimal number.
16 22 -6
16 1 -1
0
0.64 x 16 = 10.24
0.24 x 16 = 3.84
0.84 x 16 = 13.44
.44 x 16 = 7.04
Ans = (16. A 3 D 7) 16
8) State the steps involved in Gray to binary conversion?
The MSB of the binary number is the same as the MSB of the gray code number. So
write it down. To obtain the next binary digit, perform an exclusive OR operation between
the bit just written down and the next gray code bit. Write down the result.
9) Convert gray code 101011 into its binary equivalent.
Gray Code: 1 0 1 0 1 1
Binary Code: 1 1 0 0 1 0
10) Substract (0 1 0 1) 2 from (1 0 1 1) 2
1 0 1 0
0 1 0 1
Answer = 0 1 1 0
11) Add (1 0 1 0) 2 and (0 0 1 1) 2
1 0 1 0
0 0 1 1
Answer = (1 1 0 1) 2
12) Using 10’s complement subtract 72532 - 3250
M = 72532
10’s complement of N = + 96750
-----------
Sum = 169282
Discard end carry
Answer = 69282
13) Find 2’S complement of (1 0 1 0 0 0 1 1) 2
0 1 0 1 1 1 0 0 1 - 1’s Complement
+ 0 0 0 0 0 0 0 1
0 1 0 1 1 1 0 1 0 - 2’s complement.
14) Substract 1 1 1 0 0 1 2 from 1 0 1 0 1 1 2 using 2’s complement method
1 0 1 0 1 1
+ 0 0 0 1 1 1 - 2’s comp. of 1 1 1 0 0 1
1 1 0 0 1 0 in 2’s complement form
Answer (0 0 1 1 1 0 )2
15) Find the excess -3 code and 9’s complement of the number 403 10
4 0 3
0 1 0 0 0 0 0 0 0 0 1 1
0 0 1 1 0 0 1 1 0 0 1 1 +
0 1 1 1 0 0 1 1 0 1 1 0 ----- excess 3 code
9’s complement 1 0 0 0 1 1 0 0 1 0 0 1
16) What is meant by bit?
A binary digit is called bit
17) Define byte?
Group of 8 bits.
18) List the different number systems?
i) Decimal Number system
ii) Binary Number system
iii) Octal Number system
iv) Hexadecimal Number system
19) State the abbreviations of ASCII and EBCDIC code?
ASCII-American Standard Code for Information Interchange.
EBCDIC-Extended Binary Coded Decimal Information Code.
20) What are the different types of number complements?
i) r’s Complement
ii) (r-1)’s Complement.
21) Given the two binary numbers X = 1010100 and Y = 1000011, perform the subtraction
(a) X -Y and (b) Y - X using 2's complements.
a) X = 1010100
2's complement of Y = 0111101
--------------
Sum = 10010001
Discard end carry
Answer: X - Y = 0010001
b) Y = 1000011
2's complement of X = + 0101100
---------------
Sum = 1101111
There is no end carry, The MSB BIT IS 1.
Answer is Y-X = -(2's complement of 1101111) = - 0010001
22) Given the two binary numbers X = 1010100 and Y = 1000011, perform the subtraction
(a) X -Y and (b) Y - X using 1's complements.
a) X - Y = 1010100 - 1000011
X = 1010100
1's complement of Y = + 0111100
--------------
Sum = 10010000
End -around carry = + 1
--------------
Answer: X - Y = 0010001
b) Y - X = 1000011 - 1010100
Y = 1000011
1's complement of X = + 0101011
-----------
Sum = + 1101110
There is no end carry.
Therefore the answer is Y - X = -(1's complement of 1101110) = -0010001
23) Write the names of basic logical operators.
1. NOT / INVERT
2. AND
3. OR
24) What are basic properties of Boolean algebra?
The basic properties of Boolean algebra are commutative property, associative
property and distributive property.
25) State the associative property of boolean algebra.
The associative property of Boolean algebra states that the OR ing of several variables
results in the same regardless of the grouping of the variables. The associative property is
stated as follows:
A+ (B+C) = (A+B) +C
26) State the commutative property of Boolean algebra.
The commutative property states that the order in which the variables are OR ed
makes no difference. The commutative property is:
A+B=B+A
27) State the distributive property of Boolean algebra.
The distributive property states that AND ing several variables and OR ing the result
with a single variable is equivalent to OR ing the single variable with each of the the several
variables and then AND ing the sums. The distributive property is:
A+BC= (A+B) (A+C)
28) State the absorption law of Boolean algebra.
The absorption law of Boolean algebra is given by X+XY=X, X(X+Y) =X.
29) Simplify the following using De Morgan's theorem [((AB)'C)'' D]'
[((AB)'C)'' D]' = ((AB)'C)'' + D' [(AB)' = A' + B']
= (AB)' C + D'
= (A' + B' )C + D'
30) State De Morgan's theorem.
De Morgan suggested two theorems that form important part of Boolean algebra.
They are,
1) The complement of a product is equal to the sum of the complements.
(AB)' = A' + B'
2) The complement of a sum term is equal to the product of the complements.
(A + B)' = A'B'
31) Reduce A.A'C
A.A'C = 0.C [A.A' = 1]
= 0
31) Reduce A(A + B)
A(A + B) = AA + AB
= A(1 + B) [1 + B = 1]
= A.
32) Reduce A'B'C' + A'BC' + A'BC
A'B'C' + A'BC' + A'BC = A'C'(B' + B) + A'B'C
= A'C' + A'BC [A + A' = 1]
= A'(C' + BC)
= A'(C' + B) [A + A'B = A + B]
33) Reduce AB + (AC)' + AB'C(AB + C)
AB + (AC)' + AB'C(AB + C) = AB + (AC)' + AAB'BC + AB'CC
= AB + (AC)' + AB'CC [A.A' = 0]
= AB + (AC)' + AB'C [A.A = 1]
= AB + A' + C' =AB'C [(AB)' = A' + B']
= A' + B + C' + AB'C [A + AB' = A + B]
= A' + B'C + B + C' [A + A'B = A + B]
= A' + B + C' + B'C
=A' + B + C' + B'
=A' + C' + 1
= 1 [A + 1 =1]
34) Simplify the following expression Y = (A + B)(A + C' )(B' + C' )
Y = (A + B)(A + C' )(B' + C' )
= (AA' + AC +A'B +BC )(B' + C') [A.A' = 0]
= (AC + A'B + BC)(B' + C' )
= AB'C + ACC' + A'BB' + A'BC' + BB'C + BCC'
= AB'C + A'BC'
35) Show that (X + Y' + XY)( X + Y')(X'Y) = 0
(X + Y' + XY)( X + Y')(X'Y) = (X + Y' + X)(X + Y' )(X' + Y) [A + A'B = A + B]
= (X + Y' )(X + Y' )(X'Y) [A + A = 1]
= (X + Y' )(X'Y) [A.A = 1]
= X.X' + Y'.X'.Y
= 0 [A.A' = 0]
36) Prove that ABC + ABC' + AB'C + A'BC = AB + AC + BC
ABC + ABC' + AB'C + A'BC=AB(C + C') + AB'C + A'BC
=AB + AB'C + A'BC
=A(B + B'C) + A'BC
=A(B + C) + A'BC
=AB + AC + A'BC
=B(A + C) + AC
=AB + BC + AC
=AB + AC +BC ...Proved
37) Convert the given expression in canonical SOP form Y = AC + AB + BC
Y = AC + AB + BC
=AC(B + B' ) + AB(C + C' ) + (A + A')BC
=ABC + ABC' + AB'C + AB'C' + ABC + ABC' + ABC
=ABC + ABC' +AB'C + AB'C' [A + A =1]
38) Define duality property.
Duality property states that every algebraic expression deducible from the postulates
of Boolean algebra remains valid if the operators and identity elements are interchanged. If
the dual of an algebraic expression is desired, we simply interchange OR and AND operators
and replace 1's by 0's and 0's by 1's.
39) Find the complement of the functions F1 = x'yz' + x'y'z and F2 = x(y'z' + yz). By
applying De-Morgan's theorem.
F1' = (x'yz' + x'y'z)' = (x'yz')'(x'y'z)' = (x + y' + z)(x + y +z')
F2' = [x(y'z' + yz)]' = x' + (y'z' + yz)'
= x' + (y'z')'(yz)'
= x' + (y + z)(y' + z')
40) Simplify the following expression
Y = (A + B) (A = C) (B + C)
= (A A + A C + A B + B C) (B + C)
= (A C + A B + B C) (B + C)
= A B C + A C C + A B B + A B C + B B C + B C C
= A B C
41) What are the methods adopted to reduce Boolean function?
i) Karnaug map
ii) Tabular method or Quine Mc-Cluskey method
iii) Variable entered map technique.
42) State the limitations of karnaugh map.
i) Generally it is limited to six variable map (i.e) more then six variable
involving expression are not reduced.
ii) The map method is restricted in its capability since they are useful for
simplifying only Boolean expression represented in standard form.
43) What is a karnaugh map?
A karnaugh map or k map is a pictorial form of truth table, in which the map diagram
is made up of squares, with each squares representing one minterm of the function.
44) Find the minterms of the logical expression Y = A'B'C' + A'B'C + A'BC + ABC'
Y = A'B'C' + A'B'C + A'BC + ABC'
=m0 + m1 +m3 +m6
= m(0, 1, 3, 6)
45) Write the maxterms corresponding to the logical expression
Y = (A + B + C' )(A + B' + C')(A' + B' + C)
= (A + B + C' )(A + B' + C')(A' + B' + C)
=M1.M3.M6
= M(1,3,6)
46) What are called don’t care conditions?
In some logic circuits certain input conditions never occur, therefore the
corresponding output never appears. In such cases the output level is not defined, it can be
either high or low. These output levels are indicated by ‘X’ or‘d’ in the truth tables and are
called don’t care conditions or incompletely specified functions.
47) What is a prime implicant?
A prime implicant is a product term obtained by combining the maximum possible
number of adjacent squares in the map.
48) What is an essential implicant?
If a min term is covered by only one prime implicant, the prime implicant is
said to be essential
Unit – II
49. What is a Logic gate?
Logic gates are the basic elements that make up a digital system. The electronic gate
is a circuit that is able to operate on a number of binary inputs in order to perform a
particular logical function.
50.Give the classification of logic families
Bipolar Unipolar
Saturated Non Saturated PMOS
NMOS
CMOS
RTL Schottky TTL
ECL DTL
I I C
TTL
51. What are the basic digital logic gates?
The three basic logic gates are
AND gate
OR gate
NOT gate
52. Which gates are called as the universal gates? What are its advantages?
The NAND and NOR gates are called as the universal gates. These gates are used to
perform any type of logic application.
53.Classify the logic family by operation?
The Bipolar logic family is classified into
Saturated logic
Unsaturated logic.
The RTL, DTL, TTL, I2L, HTL logic comes under the saturated logic family.
The Schottky TTL, and ECL logic comes under the unsaturated logic family.
54.State the classifications of FET devices.
FET is classified as
1. Junction Field Effect Transistor (JFET)
2. Metal oxide semiconductor family (MOS).
55.Mention the classification of saturated bipolar logic families.
The bipolar logic family is classified as follows:
RTL- Resistor Transistor Logic
DTL- Diode Transistor logic
I2L- Integrated Injection Logic
TTL- Transistor Transistor Logic
ECL- Emitter Coupled Logic
56.Mention the different IC packages?
DIP- Dual in line package
LCC- Leadless Chip Carrier
PLCC- Plastic Leaded Chip carrier
PQFP- Plastic Quad Flat Pack
PGA- Pin Grid Array
57. Mention the important characteristics of digital IC’s?
Fan out
Power dissipation
Propagation Delay
Noise Margin
Fan In
Operating temperature
Power supply requirements
58. Define Fan-out?
Fan out specifies the number of standard loads that the output of the gate can drive
with out impairment of its normal operation.
59. Define power dissipation?
Power dissipation is measure of power consumed by the gate when fully driven by all
its inputs.
60. What is propagation delay?
Propagation delay is the average transition delay time for the signal to propagate from
input to output when the signals change in value. It is expressed in ns.
61. Define noise margin?
It is the maximum noise voltage added to an input signal of a digital circuit that does
not cause an undesirable change in the circuit output. It is expressed in volts.
62. Define fan in?
Fan in is the number of inputs connected to the gate without any degradation in the
voltage level.
63. What is Operating temperature?
All the gates or semiconductor devices are temperature sensitive in nature. The
temperature in which the performance of the IC is effective is called as operating
temperature. Operating temperature of the IC vary from 00 C to 700 c.
64.What is High Threshold Logic?
Some digital circuits operate in environments, which produce very high noise signals.
For operation in such surroundings there is available a type of DTL gate which possesses a
high threshold to noise immunity. This type of gate is called HTL logic or High Threshold
Logic.
65. What are the types of TTL logic?
1. Open collector output
2. Totem-Pole Output
3. Tri-state output.
66. What is depletion mode operation MOS?
If the channel is initially doped lightly with p-type impurity a conducting channel
exists at zero gate voltage and the device is said to operate in depletion mode.
67. What is enhancement mode operation of MOS?
If the region beneath the gate is left initially uncharged the gate field must induce a
channel before current can flow. Thus the gate voltage enhances the channel current and such
a device is said to operate in the enhancement mode.
68. Mention the characteristics of MOS transistor?
1. The n- channel MOS conducts when its gate- to- source voltage is positive.
2. The p- channel MOS conducts when its gate- to- source voltage is negative
73. When does the noise margin allow digital circuits to function properly.
When noise voltages are within the limits of VNA(High State Noise Margin) and VNK
for a particular logic family.
74. What happens to output when a tristate circuit is selected for high impedance.
Output is disconnected from rest of the circuits by internal circuitry.
75. What is 14000 series.
It is the oldest and standard CMOS family. The devices are not pin compatible or
electrically compatible with any TTL Series.
76. Implement the Boolean Expression for EX – OR gate using NAND Gates.
77. Define combinational logic
When logic gates are connected together to produce a specified output for certain
specified combinations of input variables, with no storage involved, the resulting circuit is
called combinational logic.
78. Explain the design procedure for combinational circuits
The problem definition
Determine the number of available input variables & required O/P variables.
Assigning letter symbols to I/O variables
Obtain simplified Boolean expression for each O/P.
Obtain the logic diagram.
79. Define Half adder and full adder
The logic circuit that performs the addition of two bits is a half adder. The circuit that
performs the addition of three bits is a full adder.
90. Draw the logic Symbol and construct the truth table for the two input EX – OR gate.
81. Define Decoder?
A decoder is a multiple - input multiple output logic circuit that converts coded
inputs into coded outputs where the input and output codes are different.
82. What is binary decoder?
A decoder is a combinational circuit that converts binary information from n input
lines to a maximum of 2n out puts lines.
83. Define Encoder?
An encoder has 2n input lines and n output lines. In encoder the output lines generate
the binary code corresponding to the input value.
84. What is priority Encoder?
A priority encoder is an encoder circuit that includes the priority function. In priority
encoder, if 2 or more inputs are equal to 1 at the same time, the input having the highest
priority will take precedence.
85. Define multiplexer?
Multiplexer is a digital switch. If allows digital information from several sources to be
routed onto a single output line.
86. What do you mean by comparator
A comparator is a special combinational circuit designed primarily to compare the
relative magnitude of two binary numbers.
87. List basic types of programmable logic devices.
. Read only memory
. Programmable logic Array
. Programmable Array Logic
88. Define ROM
Read only memory is a device that includes both the decoder and the OR gates
within a single IC package.
89. Define address and word:
In a ROM, each bit combination of the input variable is called on address. Each bit
combination that comes out of the output lines is called a word.
90. State the types of ROM
. Masked ROM.
. Programmable Read only Memory
. Erasable Programmable Read only memory.
. Electrically Erasable Programmable Read only Memory.
91. What is programmable logic array? How it differs from ROM?
In some cases the number of don’t care conditions is excessive, it is more economical
to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept;
however it does not provide full decoding of the variables and does not generates all the
minterms as in the ROM.
92. Which gate is equal to AND-invert Gate?
NAND gate.
93. Which gate is equal to OR-invert Gate?
NOR gate.
94. Bubbled OR gate is equal to--------------
NAND gate
95. Bubbled AND gate is equal to--------------
NOR gate
Unit 3
96. What are the classification of sequential circuits?
The sequential circuits are classified on the basis of timing of their signals into two
types. They are,
1)Synchronous sequential circuit.
2)Asynchronous sequential circuit.
97. Define Flip flop.
The basic unit for storage is flip flop. A flip-flop maintains its output state either at 1
or 0 until directed by an input signal to change its state.
98.What are the different types of flip-flop?
There are various types of flip flops. Some of them are mentioned below they are,
RS flip-flop
SR flip-flop
D flip-flop
JK flip-flop
T flip-flop
99.What is the operation of RS flip-flop?
• When R input is low and S input is high the Q output of flip-flop is
set.
• When R input is high and S input is low the Q output of flip-flop is
reset.
• When both the inputs R and S are low the output does not change
• When both the inputs R and S are high the output is unpredictable.
100.What is the operation of SR flip-flop?
• When R input is low and S input is high the Q output of flip-flop is
set.
• When R input is high and S input is low the Q output of flip-flop is
reset.
• When both the inputs R and S are low the output does not change.
• When both the inputs R and S are high the output is unpredictable.
101.What is the operation of D flip-flop?
In D flip-flop during the occurrence of clock pulse if D=1, the output Q is set and if
D=0, the output is reset.
102. What is the operation of JK flip-flop?
• When K input is low and J input is high the Q output of flip-flop is
set.
• When K input is high and J input is low the Q output of flip-flop is
reset.
• When both the inputs K and J are low the output does not change
• When both the inputs K and J are high it is possible to set or reset the
flip-flop (ie) the output toggle on the next positive clock edge.
103. What is the operation of T flip-flop?
T flip-flop is also known as Toggle flip-flop.
• When T=0 there is no change in the output.
• When T=1 the output switch to the complement state (ie) the output
toggles.
104. Define race around condition.
In JK flip-flop output is fed back to the input. Therefore change in the output results
change in the input. Due to this in the positive half of the clock pulse if both J and K are high
then output toggles continuously. This condition is called ‘race around condition’.
105. What is edge-triggered flip-flop?
The problem of race around condition can solved by edge triggering flip flop. The
term edge triggering means that the flip-flop changes state either at the positive edge or
negative edge of the clock pulse and it is sensitive to its inputs only at this transition of the
clock.
106. What is a master-slave flip-flop?
A master-slave flip-flop consists of two flip-flops where one circuit serves as a master
and the other as a slave.
107.Define rise time.
The time required to change the voltage level from 10% to 90% is known as rise
time(tr).
108.Define fall time.
The time required to change the voltage level from 90% to 10% is known as fall
time(tf).
109.Define skew and clock skew.
The phase shift between the rectangular clock waveforms is referred to as skew and
the time delay between the two clock pulses is called clock skew.
110.Define setup time.
The setup time is the minimum time required to maintain a constant voltage levels at
the excitation inputs of the flip-flop device prior to the triggering edge of the clock pulse in
order for the levels to be reliably clocked into the flip flop. It is denoted as tsetup.
111. Define hold time.
The hold time is the minimum time for which the voltage levels at the excitation
inputs must remain constant after the triggering edge of the clock pulse in order for the levels
to be reliably clocked into the flip flop. It is denoted as thold .
112. Define propagation delay.
A propagation delay is the time required to change the output after the application of
the input.
113.Define registers.
A register is a group of flip-flops flip-flop can store one bit information. So an n-bit register
has a group of n flip-flops and is capable of storing any binary information/number
containing n-bits.
114.Define shift registers.
The binary information in a register can be moved from stage to stage within the
register or into or out of the register upon application of clock pulses. This type of bit
movement or shifting is essential for certain arithmetic and logic operations used in
microprocessors. This gives rise to group of registers called shift registers.
115.What are the different types of shift type?
There are five types. They are,
Serial In Serial Out Shift Register
Serial In Parallel Out Shift Register
Parallel In Serial Out Shift Register
Parallel In Parallel Out Shift Register
Bidirectional Shift Register
116.Explain the flip-flop excitation tables for RS FF.
RS flip-flop
In RS flip-flop there are four possible transitions from the present state to the
next state. They are,
0 0 transition: This can happen either when R=S=0 or when R=1 and
S=0.
0 1 transition: This can happen only when S=1 and R=0.
1 0 transition: This can happen only when S=0 and R=1.
1 1 transition: This can happen either when S=1 and R=0 or S=0 and
R=0.
117.Explain the flip-flop excitation tables for JK flip-flop
In JK flip-flop also there are four possible transitions from present state to next state.
They are,
0 0 transition: This can happen when J=0 and K=1 or K=0.
0 1 transition: This can happen either when J=1 and K=0 or when
J=K=1.
1 0 transition: This can happen either when J=0 and K=1 or when
J=K=1.
1 1 transition: This can happen when K=0 and J=0 or J=1.
118.Explain the flip-flop excitation tables for D flip-flop
In D flip-flop the next state is always equal to the D input and it is independent of the
present state. Therefore D must be 0 if Qn+1 has to 0,and if Qn+1 has to be 1 regardless the
value of Qn.
119. Explain the flip-flop excitation tables for T flip-flop
When input T=1 the state of the flip-flop is complemented; when T=0,the state of the
flip-flop remains unchanged. Therefore, for 0 0 and 1 1 transitions T must be 0 and for
0 1 and 1 0 transitions must be 1.
120. Define sequential circuit?
In sequential circuits the output variables dependent not only on the present input
variables but they also depend up on the past history of these input variables.
121.Give the comparison between combinational circuits and sequential circuits.
Combinational circuits Sequential circuits
Memory unit is not required Memory unity is required
Parallel adder is a combinational circuit Serial adder is a sequential circuit
122. What do you mean by present state?
The information stored in the memory elements at any given time define.s the present
state of the sequential circuit.
123. What do you mean by next state?
The present state and the external inputs determine the outputs and the next state of
the sequential circuit.
124. State the types of sequential circuits?
1. Synchronous sequential circuits
2. Asynchronous sequential circuits
125. Define synchronous sequential circuit
In synchronous sequential circuits, signals can affect the memory elements only at
discrete instant of time.
126. Define Asynchronous sequential circuit?
In asynchronous sequential circuits change in input signals can affect memory
element at any instant of time.
127.Give the comparison between synchronous & Asynchronous sequential circuits?
Synchronous sequential circuits Asynchronous sequential circuits.
Memory elements are clocked flip-flops Memory elements are either unlocked flip -
flops or time delay elements.
Easier to design More difficult to design
128.Define flip-flop
Flip - flop is a sequential device that normally samples its inputs and changes its
outputs only at times determined by clocking signal.
129. Draw the logic diagram for SR latch using two NOR gates.
130. The following wave forms are applied to the inputs of SR latch. Determine the Q
waveform Assume initially Q = 1
Here the latch input has to be pulsed momentarily to cause a change in the latch
output state, and the output will remain in that new state even after the input pulse is over.
131.What is race around condition?
In the JK latch, the output is feedback to the input, and therefore changes in the output
results change in the input. Due to this in the positive half of the clock pulse if J and K are
both high then output toggles continuously. This condition is known as race around
condition.
132.What are the types of shift register?
1. Serial in serial out shift register?
2. Serial in parallel out shift register
3. Parallel in serial out shift register
4. Parallel in parallel out shift register
5. Bidirectional shift register shift register
133.State the types of counter?
1. Synchronous counter
2. Asynchronous Counter
134.Give the comparison between synchronous & Asynchronous counters.
Asynchronous counters Synchronous counters
In this type of counter flip-flops are
connected in such a way that output of 1st
flip-flop drives the clock for the next flipflop.
In this type there is no connection between
output of first flip-flop and clock input of
the next flip - flop
All the flip-flops are Not clocked
simultaneously
All the flip-flops are clocked
simultaneously
135.The t pd for each flip-flop is 50 ns. Determine the maximum operating frequency for
MOD - 32 ripple counter
f max (ripple) = 5 x 50 ns = 4 MHZ
Unit 4
136. What are secondary variables?
-present state variables in asynchronous sequential circuits
137.What are excitation variables?
-next state variables in asynchronous sequential circuits
138. What is fundamental mode sequential circuit?
-input variables changes if the circuit is stable
-inputs are leve45.What are the different techniques used in state assignment?
-shared row state assignment
-one hot state assignment
146.What are the steps for the design of asynchronous sequential circuit?
-construction of primitive flow table
-reduction of flow table
-state assignment is made
-realization of primitive flow table
147.What is hazard?
-unwanted switching transients
148.What is static 1 hazard?
-output goes momentarily 0 when it should remain at 1
149.What is static 0 hazard?
-output goes momentarily 1 when it should remain at 0
150. What is dynamic hazard?
-output changes 3 or more times when it changes from 1 to 0 or 0 to 1
151.What is the cause for essential hazards?
-unequal delays along 2 or more path from same input
151.What is flow table?
-state table of an synchronous sequential network
152.What is SM chart?
-describes the behavior of a state machine
-used in hardware design of digital systems
152.What are the advantages of SM chart?
-easy to understand the operation
-east to convert to several equivalent forms
153. What is primitive flow chart?
-one stable state per row
154.What is combinational circuit?
Output depends on the given input. It has no storage element.
155.What is state equivalence theorem ?
Two states SA and SB, are equivalent if and only if for every possible input X
sequence, the outputs are the same and the next states are equivalent
i.e., if SA (t + 1) = SB (t + 1) and ZA = ZB then SA = SB.
156.What do you mean by distinguishing sequences?
Two states, SA and SB of sequential machine are distinguishable if and only if their
exists at least one finite input sequence. Which, when applied to sequential machine causes
different output sequences depending on whether SA or SB is the initial state.
157. Prove that the equivalence partition is unique
Consider that there are two equivalence partitions exists : PA and PB, and PA ) PB.
This states that, there exist 2 states Si & Sj which are in the same block of one partition and
not in the same block of the other. If Si & Sj are in different blocks of say PB, there exists at
least on input sequence which distinguishes Si & Sj and therefore, they cannot be in the same
block of PA.
158.Define compatibility
States Si and Sj said to be compatible states, if and only if for every input sequence
that affects the two states, the same output sequence, occurs whenever both outputs are
specified and regardless of whether Si on Sj is the initial state.
159.Define merger graph.
The merger graph is defined as follows. It contains the same number of vertices as the
state table contains states. A line drawn between the two state vertices indicates each
compatible state pair. It two states are incompatible no connecting line is drawn.
160.Define incompatibility
The states are said to be incompatible if no line is drawn in between them. If implied
states are incompatible, they are crossed & the corresponding line is ignored.
161.Explain the procedure for state minimization.
1. Partition the states into subsets such that all states in the same subsets are 1 -
equivalent.
2. Partition the states into subsets such that all states in the same subsets are 2 -
equivalent.
3. Partition the states into subsets such that all states in the same subsets are 3 -
equivalent.
162.Define closed covering
A Set of compatibles is said to be closed if, for every compatible contained in the set,
all its implied compatibles are also contained in the set. A closed set of compatibles, which
contains all the states of M, is called a closed covering.
163.Define machine equivalence
Two machines, M1 and M2 are said to be equivalent if and only if, for every state in
M1, there is a corresponding equivalent state in M2 & vice versa.
164.Define state table.
For the design of sequential counters we have to relate present states and next states.
The table, which represents the relationship between present states and next states, is called
state table.
165. Define total state
The combination of level signals that appear at the inputs and the outputs of the
delays define what is called the total state of the circuit.
166.What are the steps for the design of asynchronous sequential circuit?
1. Construction of a primitive flow table from the problem statement.
2. Primitive flow table is reduced by eliminating redundant states using the state
reduction
3. State assignment is made
4. The primitive flow table is realized using appropriate logic elements.
167. Define primitive flow table :
It is defined as a flow table which has exactly one stable state for each row in the
table. The design process begins with the construction of primitive flow table.
168.What are the types of asynchronous circuits ?
1. Fundamental mode circuits
2. Pulse mode circuits
169.Give the comparison between state Assignment Synchronous circuit and state assignment
asynchronous circuit.
In synchronous circuit, the state assignments are made with the objective of circuit
reduction. In asynchronous circuits, the objective of state assignment is to avoid critical races.
170.What are races?
When 2 or more binary state variables change their value in response to a change in
an input variable, race condition occurs in an asynchronous sequential circuit. In case of
unequal delays, a race condition may cause the state variables to change in an unpredictable
manner.
171.Define non critical race.
If the final stable state that the circuit reaches does not depend on the order in which
the state variable changes, the race condition is not harmful and it is called a non critical race.
172.Define critical race?
If the final stable state depends on the order in which the state variable changes, the
race condition is harmful and it is called a critical race.
173What is a cycle?
A cycle occurs when an asynchronous circuit makes a transition through a series of
unstable states. If a cycle does not contain a stable state, the circuit will go from one unstable
to stable to another, until the inputs are changed.
174.List the different techniques used for state assignment.
1. Shared row state assignment
2. One hot state assignment.
175.Write a short note on fundamental mode asynchronous circuit.
Fundamental mode circuit assumes that. The input variables change only when the
circuit is stable. Only one input variable can change at a given time and inputs are levels and
not pulses.
176. Write a short note on pulse mode circuit.
Pulse mode circuit assumes that the input variables are pulses instead of level. The
width of the pulses is long enough for the circuit to respond to the input and the pulse width
must not be so long that it is still present after the new state is reached.
177.Define secondary variables
The delay elements provide a short term memory for the sequential circuit. The
present state and next state variables in asynchronous sequential circuits are called secondary
variables.
178. Define flow table in asynchronous sequential circuit.
In asynchronous sequential circuit state table is known as flow table because of the
behaviour of the asynchronous sequential circuit. The stage changes occur in independent of
a clock, based on the logic propagation delay, and cause the states to .flow. from one to
another.
179. A pulse mode asynchronous machine has two inputs. If produces an output whenever
two consecutive pulses occur on one input line only. The output remains at 1 until a pulse has
occurred on the other input line. Write down the state table for the machine.
180.What is fundamental mode.
A transition from one stable state to another occurs only in response to a change in the
input state. After a change in one input has occurred, no other change in any input occurs
until the circuit enters a stable state. Such a mode of operation is referred to as a fundamental
mode.
181. Write short note on shared row state assignment.
Races can be avoided by making a proper binary assignment to the state variables.
Here, the state variables are assigned with binary numbers in such a way that only one state
variable can change at any one state variable can change at any one time when a state
transition occurs. To accomplish this, it is necessary that states between which transitions
occur be given adjacent assignments. Two binary are said to be adjacent if they differ in only
one variable.
182. Write short note on one hot state assignment.
The one hot state assignment is another method for finding a race free state
assignment. In this method, only one variable is active or hot for each row in the original
flow table, ie, it requires one state variable for each row of the flow table. Additional row are
introduced to provide single variable changes between internal state transitions.
Unit 5
183. Explain ROM
A read only memory(ROM) is a device that includes both the decoder and the OR
gates within a single IC package. It consists of n input lines and m output lines. Each bit
combination of the input variables is called an address. Each bit combination that comes out
of the output lines is called a word. The number of distinct addresses possible with n input
variables is 2n.
184. What are the types of ROM?
1.PROM
2.EPROM
3.EEPROM
185. Explain PROM.
PROM (Programmable Read Only Memory)
It allows user to store data or program. PROMs use the fuses with material
like nichrome and polycrystalline. The user can blow these fuses by passing
around 20 to 50 mA of current for the period 5 to 20μs.The blowing of fuses is
called programming of ROM. The PROMs are one time programmable. Once
programmed, the information is stored permanent.
186. Explain EPROM.
EPROM(Erasable Programmable Read Only Memory)
EPROM use MOS circuitry. They store 1’s and 0’s as a packet of charge in a
buried layer of the IC chip. We can erase the stored data in the EPROMs by
exposing the chip to ultraviolet light via its quartz window for 15 to 20
minutes. It is not possible to erase selective information. The chip can be
reprogrammed.
187. Explain EEPROM.
EEPROM(Electrically Erasable Programmable Read Only Memory)
EEPROM also use MOS circuitry. Data is stored as charge or no charge on an
insulated layer or an insulated floating gate in the device. EEPROM allows
selective erasing at the register level rather than erasing all the information
since the information can be changed by using electrical signals.
189. What is RAM?
Random Access Memory. Read and write operations can be carried out.
190. Define ROM
A read only memory is a device that includes both the decoder and the OR gates
within a single IC package.
191. Define address and word:
In a ROM, each bit combination of the input variable is called on address. Each bit
combination that comes out of the output lines is called a word.
192. What are the types of ROM.
1. Masked ROM.
2. Programmable Read only Memory
3. Erasable Programmable Read only memory.
4. Electrically Erasable Programmable Read only Memory.
193. What is programmable logic array? How it differs from ROM?
In some cases the number of don’t care conditions is excessive, it is more economical
to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept;
however it does not provide full decoding of the variables and does not generates all the
minterms as in the ROM.
194.What is mask - programmable?
With a mask programmable PLA, the user must submit a PLA program table to the
manufacturer.
195. What is field programmable logic array?
The second type of PLA is called a field programmable logic array. The user by
means of certain recommended procedures can program the EPLA.
196. List the major differences between PLA and PAL
PLA:
Both AND and OR arrays are programmable and Complex
Costlier than PAL
PAL
AND arrays are programmable OR arrays are fixed
Cheaper and Simpler
197. Define PLD.
Programmable Logic Devices consist of a large array of AND gates and OR gates that
can be programmed to achieve specific logic functions.
198. Give the classification of PLDs.
PLDs are classified as PROM(Programmable Read Only Memory), Programmable
Logic Array(PLA), Programmable Array Logic (PAL), and Generic Array Logic(GAL)
199. Define PROM.
PROM is Programmable Read Only Memory. It consists of a set of fixed AND gates
connected to a decoder and a programmable OR array.
200. Define PLA
PLA is Programmable Logic Array(PLA). The PLA is a PLD that consists of a
programmable AND array and a programmable OR array.
201. Define PAL
PAL is Programmable Array Logic. PAL consists of a programmable AND array and
a fixed OR array with output logic.
202. Why was PAL developed ?
It is a PLD that was developed to overcome certain disadvantages of PLA, such as
longer delays due to additional fusible links that result from using two programmable arrays
and more circuit complexity.
203. Define GAL
GAL is Generic Array Logic. GAL consists of a programmable AND array and a
fixed OR array with output logic.
204. Why the input variables to a PAL are buffered
The input variables to a PAL are buffered to prevent loading by the large number of
AND gate inputs to which available or its complement can be connected.
205. What does PAL 10L8 specify ?
PAL - Programmable Logic Array
10 - Ten inputs
L - Active LOW Ouput
8 - Eight Outputs
206. What is CPLD ?
CPLDs are Complex Programmable Logic Devices. They are larger versions of PLDs
with a centralized internal interconnect matrix used to connect the device macro cells
together.
207.Define bit, byte and word.
The smallest unit of binary data is bit. Data are handled in a 8 bit unit called byte. ted address in any sequence.
214. List the two categories of RAMs.
The two categories of RAMs are static RAM(SRAM) and dynamic RAM (DRAM).
215. Define Static RAM and dynamic RAM
Static RAM use flip flops as storage elements and therefore store data indefinitely as
long as dc power is applied.
Dynamic RAMs use capacitors as storage elements and cannot retain data very long
without capacitors being recharged by a process called refreshing.
216. List the two types of SRAM
Asynchronous SRAMs and Synhronous Burst SRAMs
217.List the basic types of DRAMs
Fast Page Mode DRAM,Extended Data Out DRAM(EDO DRAM),Burst EDO
DRAM and Synchronous DRAM.
218. Define a bus
A bus is a set of conductive paths that serve to interconnect two or more functional
components of a system or several diverse systems.
219. Define Cache memory
It is a relatively small, high-speed memory that can store the most recently used
instructions or data from larger but slower main memory.
220. What is the technique adopted by DRAMs.
DRAMs use a technique called address multiplexing to reduce the number of address
lines.
221.Give the feature of UV EPROM
UV EPROM is electrically programmable by the user, but the store data must be
erased by exposure to ultra violet light over a period of several minutes.
222.Give the feature of flash memory.
The ideal memory has high storage capacity, non-volatility; in-system read and write
capability, comparatively fast operation. The traditional memory technologies such as ROM,
PROM, EEPROM individually exhibits one of these characteristics, but no single technology
has all of them except the flash memory.
223.What are Flash memories ?
They are high density read/write memories that are non-volatile, which means data
can be stored indefinitely with out power.
224. List the three major operations in a flash memory.
Programming, Read and Erase operation
225. What is a FIFO memory ?
The term FIFO refers to the basic operation of this type of memory in which the first
data bit written into the memory is to first to be read out.
226.List basic types of programmable logic devices.
1. Read only memory
2. Programmable logic Array
3. Programmable Array Logic
227. Define ROM
A read only memory is a device that includes both the decoder and the OR gates
within a single IC package.
228.Define address and word:
In a ROM, each bit combination of the input variable is called on address. Each bit
combination that comes out of the output lines is called a word.
229. What are the types of ROM ?
1. Masked ROM.
2. Programmable Read only Memory
3. Erasable Programmable Read only memory.
4. Electrically Erasable Programmable Read only Memory.
300. What is programmable logic array? How it differs from ROM?
In some cases the number of don’t care conditions is excessive, it is more economical
to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept;
however it does not provide full decoding of the variables and does not generates all the
minterms as in the ROM.
301.What is mask - programmable?
With a mask programmable PLA, the user must submit a PLA PLA program table to
the manufacturer.
302.Give the comparison between PROM and PLA.
PROM PLA
1. And array is fixed and OR Both AND and OR arrays are
array is programmable. Programmable.
2. Cheaper and simple to use. Costliest and complex than
PROMS.
Part – B
Unit-I
1) Simplify the boolean function using tabulation method.
F = (0,1,2,8,10,11,14,15)
List all the min terms
Arrange them as per the number of ones based on binary equivalent
Compare one group with another for difference in one and replace the bit with dash.
Continue this until no further grouping possible.
The unchecked terms represent the prime implicants.
F = W'X'Y' + X'Z' + WY
2) Determine the prime implicants of the function
F (W,X,Y,Z) = (1,4,6,7,8,9,10,11,15)
List all the min terms
Arrange them as per the number of ones based on binary equivalent
Compare one group with another for difference in one and replace the bit with dash.
Continue this until no further grouping possible.
The unchecked terms represent the prime implicants.
F = X'Y'Z + W'XZ' + W'XY + XYZ + WYZ + WX'
Minimum Set of prime implicants F = X'Y'Z + W'XZ' + XYZ + WX'
3) Simplify the Boolean function using K-map.
F(A,B,C,D,E) = (0,2,4,6,9,13,21,23,25,29,31)
Five variables hence two variable k maps one for A = 0 and the other for A = 1.
F = A'B'E' + BD'E + ACE
4) Obtain the canonical sum of products of the function Y = AB + ACD
Y = AB (C + C')(D + D') + ACD (B + B')
Y = ABCD + ABCD' + ABC'D + ABC'D' + AB'CD
5) State the postulates and theorems of Boolean algebra.
X + 0 = X X · 1 = X
X + X' = 1 X · X' = 0
X + X = X X · X = X
X + 1 = 1 X · 0 = 0
(X')' = X
X + Y = Y + X XY = YX
X + (Y + Z) = (X + Y) + Z X(YZ) = (XY)Z
X(Y + Z) = XY + XZ X + YX = (X + Y) (X + Z)
(X + Y)' = X'Y' (XY)' = X' + Y'
X + XY = X X(X + Y) = X
Unit-II
6. Explain with neat diagrams TTL.
Disadvantages of other families
Diagram of TTL
Theory
Working principle
7. Discuss all the characteristics of digital IC’s.
Fan out
Power dissipation
Propagation Delay
Noise Margin
Fan In
Operating temperature
Power supply requirements
8. Explain with neat diagram how an open collector TTL operates.
Disadvantages of other families
Diagram of open collector gate TTL
Theory
Working principle
9. Explain the different applications of open collector TTL.
Wired logic
Common bus system
Drive a lamp or relay
10. Explain in detail about schottky TTL.
Disadvantages of other families
Diagram of schottky TTL
Theory
Working principle
Advantages
11.Explain in detail about three state gate.
Disadvantages of other families
Explanation about three state gate
Theory
Working principle
12.Explain with necessary diagrams MOS & CMOS.
PMOS
NMOS
CMOS
Diagrams
13.Design a 4-bit binary adder/subtractor circuit.
Basic equations
Comparison of equations
Design using twos complement
Circuit diagram
Unit-III
14.Explain the working of BCD Ripple Counter with the help of state diagram and logic
diagram.
BCD Ripple Counter Count sequence
Truth Table
State diagram representing the Truth Table
Truth Table for the J-K Flip Flop
Logic Diagram
15.Design a logic circuit to convert the BCD code to Excess – 3 code.
Truth Table for BCD to Excess – 3 conversion.
K-map simplification
Logic circuit implementing the Boolean Expression
16.Design and explain a comparator to compare two identical words.
Two numbers represented by A = A3A2A1A0 & B = B3B2B1B0
If two numbers equal P = Ai ⊕Bi
Obtain the logic Expression .
Obtain the logic diagram.
17.Design a sequential detector which produces an output 1 every time the input sequence
1011 is detected.
Construct state diagram
Obtain the flow table
Obtain the flow table & output table
Transition table
Select flip flop
Excitation table
Logic diagram
18.. Explain in detail about serial in serial out shift register.
Block diagram
Theoretical explanation
Logic diagram
Working
Unit-IV
19.Explain with neat diagram the different hazards and the way to eliminate them.
Classification of hazards
Static hazard & Dynamic hazard definitions
K map for selected functions
Method of elimination
Essential hazards
20.State with a neat example the method for the minimization of primitive flow table.
Consider a state diagram
Obtain the flow table
Using implication table reduce the flow table
Using merger graph obtain maximal compatibles
Verify closed & covered conditions
Plot the reduced flow table
21.Design a asynchronous sequential circuit with 2 inputs T and C. The output attains a value
of 1 when T = 1 & c moves from 1 to 0. Otherwise the output is 0.
Obtain the state diagram
Obtain the flow table
Using implication table reduce the flow table
Using merger graph obtain maximal compatibles
Verify closed & covered conditions
Plot the reduced flow table
Obtain transition table
Excitation table
Logic diagram
22.. Explain in detail about Races.
Basics of races
Problem created due to races
Classification of races
Remedy for races
cycles
23.. Explain the different methods of state assignment
Three row state assignment
Shared row state assignment
Four row flow table
Multiple row state assignment
Prevention of races.
Unit-V
24Explain in detail about PLA with a specific example.
Explanation about ROM
Classifications of ROM
Architecture of ROM
Specification of PLA
Specific Example
Related Diagram
Related Table.
25.Implement the following using a mux. F(a,b,c,d) = (0,1,3,4,8,9,15)
Obtain the truth table
From the truth table realize the expressions for the outputs and inputs
Realize the logic diagram.
26.Explain with neat diagrams a RAM architecture.
Different Memories
Classification of memories
RAM architecture diagram
Timing waveforms
Coincident Decoding
Read write operations
27.Explain in detail about PLA and PAL.
Basic ROM
Classification of PROM
Logic difference between Prom & PLA
Logic diagram implementing a function
Logic difference between Prom & PAL
Logic diagram implementing a function
28.Explain with neat diagrams a ROM architecture.
Different Memories
Classification of memories
ROM architecture diagram
Timing waveforms
Coincident Decoding
Read write operations
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