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Department of Information Technology

CS 2202- DIGITAL PRINCIPLES AND SYSTEM DESIGNPart-A Questions and Answers

UNIT-I

BOOLEAN ALGEBRA AND LOGIC GATES

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 havingonly two distinct values: 1 and 0.There are three basic logic operations: AND, OR,

and NOT.

2. What are the basic digital logic gates?

The three basic logic gates are AND gate ,OR gate ,NOT gate

3. 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 ofbinary inputs in order to

perform a particular logical function.

4. 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 usedto perform any type of logic application.

5.What do you mean by canonical form?The representation of Boolean function in SOP or POS form, i.e. each term in the

expression must contain all the literals of the function either in normal form orcomplement form.

6.What is an Alpha numeric code?

As most of the computers and their peripherals process both alphabetic andnumeric information several coding techniques have been invented that represent

this alphanumeric information as a series of 1s and 0s. Such codes are called

Alpha numeric code.

7.What are error detecting codes?

Digital systems should be accurate to the digit. So, detecting errors are veryimportant. The simplest technique for detecting errors is that of adding an extra bit,

known as parity bit. These codes are called error detecting codes.


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8.What is sequential code?

A code is said to be sequential when each succeeding code is one binary numbergreater than its preceding code. Ex. 8421 BCD, EXCESS 3.

9.What are prime implicants?Prime implicants are building blocks of minimum SOPexpression.

10.What is K- MAP?

An arrangement of cells representing the combination of literals or variables in theBoolean expression and used for a systematic simplificationof the expression.

11.What are the advantages of K-map?1) Kmap is a more orderly process with well definedsteps as compared with the

trial and error process sometimes used in algebraic simplification.

2) Requires fewer steps especially for expressions containing many terms, and italways produces a minimum expression.

12.What are dont cares?

Dont care term is a minterm for which the combinational logic may output either1 or 0.

13.Explain about Tabulation method.It is a specific step by step procedure that is guaranteed to produce a simplified

standard form expression for a function. It has two parts. The first is to find the

terms for inclusion in the simplified function. These terms are called prime implicants. The second is to choose among the prime implicants that give an

expression with the least number of literals.

14.Encode the number (2345)10 in BCD and excess 3.

BCD equivalent : 0010 0011 0100 0101Excess 3 : 0101 0110 0111 1000

15.State Duality principle.The dual of any Boolean function can be obtained bychanging each OR sign to an

AND sign and vice versa and complementing any 0 or 1 appearing in the

expression.


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16.What is a weighted code? Give example.

Weighted binary code are those which obey the positional weighting principle.Example. 8421 BCD code.

17.What is a non-weighted binary code ?Give exampleNon-weighted codes are the codes that are not positionally weighted. That is each

position within the binary number is not assigned a fixed value.Example. Excess 3 code.

18.What is a gray code?

The gray code is a binary code in which each successive number differs in only

one bit position. It is a non-weighted code and not an arithmetic code.

19.What is signed binary number?

In binary system, when the sign is represented by including an additional bit withthe magnitude bits, usually a zero represents positive sign and a1 representsnegative sign. These binary numbers are called as signed binary numbers.

20.What is a decimal number system?A decimal number system has a radix (Base r=10) anduses symbols

0,1,2,3,4,5,6,7,8,9. Eg. (237) 10

UNIT-II

COMBINATIONAL LOGIC

1.Define Positive Logic.When high voltage or more positive voltage level isassociated with binary 1 and

while the low orlesspositive level is associated with binary 0 then the systemadhere to this is called positive logic.

2.Define Negative Logic.

When high voltage level is associated with binary 0 and while the low level isassociated with binary 1 then the system adhering to this is called negative

logic

3.List the characteristics of digital Ics

i)propagation delay

ii)power dissipationiii)Fan-in

iv)Fan-outv)Noise margin


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4. What is propagation delay?

It is the average transition delay time for the signal to propagate from input tooutput when the signals change in value.

5. What is Noise margin?It is the limit of a noise voltage, which may bepresent with out impairing the

proper operation ofthe circuit.

6. What is power dissipation?It is the power consumed by the gate, which mustbe available from the power

supply.

7. Why parity checker is needed?

Parity checker is required at the receiver side to check whether the expected parity

is equal tothe calculated parity or not. If they are not equal then it is found that thereceived data has error.

8. What is meant by parity bit?

Parity bit is an extra bit included with a binary message to make the number of 1seither odd or even. The message, including the parity bit is transmitted and then

checked at the receiving and for errors..

9. What is IC?

An integrated circuit is a small silicon semiconductor crystal called a chip

containing electrical components such as transistors, diodes, resistors and capacitors. The various components are interconnected inside the chip to form

an electronic circuit.

10. What are the needs for binary codes?

a.Code is used to represent letters, numbers and punctuation marks.

b.Coding is required for maximum efficiency in singletransmission.c.Binary codes are the major components in the synthesis (artificial generation) of

speech and video signals.

d.By using error detecting codes, errors generated in signal transmission can bedetected.

e.Codes are used for data compression by which large amounts of data are

transmitted in very short duration of time.


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11. Mention the different type of binary codes?

The various types of binary codes are,f.BCD code (Binary Coded decimal).

g.Self-complementing code.

h.The excess-3 (Xs-3) code.i.Gray code.

j.Binary weighted code.k.Alphanumeric code.

l.The ASCII code.m.Extended binary-coded decimal interchange code (EBCDIC).

n.Error-detecting and error-correcting code.

o.Hamming code.

12. List the advantages and disadvantages of BCD code?

The advantages of BCD code are a.Any large decimal number can be easilyconverted into corresponding binary numberb.A person needs to remember only the binary equivalents of decimal

number from 0 to 9.

c.Conversion from BCD into decimal is also very easy.The disadvantages of BCDcode are a.The code is least efficient. It requires several symbols to represent even

small numbers.

b.Binary addition and subtraction can lead to wrong answer.c.Special codes are required for arithmetic operations.

d.This is not a self-complementing code.

e.conversion into other coding schemes requires special methods.

13. What is meant by self-complementing code?A self-complementing code is the one in which the members of the number system

complement on themselves. This requires the following two conditions to be

satisfied.

a.The complement of the number should be obtained from that number byreplacing 1s with 0s and 0s with 1s.

b.The sum of the number and its complement should be equal to decimal 9.

Example of a self-complementing code isi.2-4-2-1 code.

ii.Excess-3 code.


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14. Mention the advantages of ASCII code?

The following are the advantages of ASCII codea.There are 27 =128 possible combinations. Hence, a large number of symbols,

alphabets etc.., can be easily represented.

b.There is a definite order in which the alphabets, etc.., are assigned to each codeword.

c.The parity bits can be added for error-detection and correction.

15. What are the disadvantages of ASCII code?The disadvantages of ASCII code are

a.The length of the code is larger and hence more bandwidth is required for

transmission.b.With more characters and symbols to represent, thisis not completely sufficient.

16. What is the truth table?A truth table lists all possible combinations of inputs and the correspondingoutputs

17. Define figure of merit?Figure of merits is defined as the product of speedand power. The speed is

specified in terms of propagation delay time expressed in nano seconds Figure of

merits=Propagation delay time (ns) * Power (mw) It is specified in pico joules(ns*mw=PJ).

18. Define Combinational circuit.A combinational circuit consist of logic gates whose outputs at anytime are

determined directly from the present combination of inputs without regard toprevious inputs.

19. Define sequential circuits.

Their outputs are a function of the inputs and thestate of memory elements. Thestate of memory elements, in turn, is a function of previous inputs

.

20. What is Binary parallel adder?A binary parallel adder is a digital function thatproduces the arithmetic sum of two

binary numbers in parallel.


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UNIT-III

DESIGN WITH MSI DEVICES1.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.

2.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.

3.Define Encoder?

An encoder has 2n input lines and n output lines. In encoder the output linesgenerate the binary code corresponding to the input value.

4.What is priority Encoder?A priority encoder is an encoder circuit that includes the priority function. Inpriority encoder, if 2 or more inputs are equal to 1 at the same time, the in

put having the highest priority will take precedence.

5.Define multiplexer?

Multiplexer is a digital switch. If allows digital information from several sources to

be routed onto a single output line.

6.What do you mean by comparator

A comparator is a special combinational circuit designed primarily to compare therelative magnitude of two binary numbers.

7.List basic types of programmable logic devices.

. Read only memory

. Programmable logic Array

. Programmable Array Logic

8.Explain ROM

A read only memory(ROM) is a device that includes both the decoder and the ORgates within a single IC package. It consists of n input lines andm 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 ofdistinct addresses possible with n input variables is 2n.


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9.Define address and word:

In a ROM, each bit combination of the input variable is called on address. Each bitcombination that comes out of the output lines is called a word.

10.State the types of ROM. Masked ROM.

. Programmable Read only Memory

. Erasable Programmable Read only memory.

. Electrically Erasable Programmable Read only Memory.

11.What is programmable logic array? How it differs from ROM?

In some cases the number of dont care conditions is excessive, it is moreeconomical to use a second type of LSI component called a PLA. A PLA is simila

r to a ROM in concept; however it does not provide full decoding of the variables

and does notgenerates all the minterms as in the ROM.

12.What is RAM?

Random Access Memory. Read and write operations canbe carried out.

13.. What is programmable logic array? How it differs from ROM?

In some cases the number of dont care conditions is excessive, it is more

economical to use a second type of LSI component called a PLA. A PLA is similarto 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.

14.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

15.Define PLD.

Programmable Logic Devices consist of a large arrayof AND gates and OR gates

that can be programmed to achieve specific logic functions.

16.Give the classification of PLDs.

PLDs are classified as PROM(Programmable Read Only Memory), ProgrammableLogic Array(PLA), Programmable Array Logic (PAL), and Generic Array

Logic(GAL)


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17.

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.18.

Define PLAPLA is Programmable Logic Array(PLA). The PLA is a

PLD that consists of a programmable ANDarray and a programmable OR array.

19.

Define PALPAL is Programmable Array Logic. PAL consists of a

programmable AND array and a fixed OR

array with output logic.20.Give the comparison between PROM and PLA.

PROM PLA

1. And array is fixed and OR array isprogrammable.

2. Cheaper and simple to use.

1.Both AND and OR arrays are

Programmable.

2.Costliest and complex than PROMS.

UNITIVSYNCHRONOUS SEQUENTIAL LOGIC

1.

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.

2.Define Flip flop.

The basic unit for storage is flip flop. A flip-flop maintains its output state either at 1


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or 0 until directed by an input signal to change it

s state.3.

.What are the different types of flip-flop?

There are various types of flip flops. Some of themare mentioned below they are,

RS flip-flop

SR flip-flop

D flip-flop

JK flip-flop

T flip-flop4.

.What is the operation of D flip-flop?

In D flip-flop during the occurrence of clock pulseif D=1, the output Q is set and if D=0, the output

is reset.

5.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 outpu

t 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.

6.What is the operation of T flip-flop?

T flip-flop is also known as Toggle flip-flop.


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When T=0 there is no change in the output.

When T=1 the output switch to the complement state

(ie) the output toggles.

7.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 toggle

s

continuously. This condition is called race aroundcondition.

8.

What is edge-triggered flip-flop?The problem of race around condition can solved byedge triggering flip flop. The term edge

triggering means that the flip-flop changes state e

ither at the positive edge or negative edge of theclock pulse and it is sensitive to its inputs only

at this transition of the clock.

9.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 otheras a slave.

10..Define rise time.

The time required to change the voltage level from

10% to 90% is known as rise time(t

r).

11.

Define fall time.The time required to change the voltage level from

90% to 10% is known as fall time(t

f).

12.Define registers.


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A register is a group of flip-flops flip-flop can s

tore one bit information. So an n-bit register hasa

group of n flip-flops and is capable of storing any

binary information/number containing n-bits.13.

Define shift registers.The binary information in a register can be moved f

rom stage to stage within the register or into orout of the register upon application of clock pulse

s. This type of bit movement or shifting is essenti

alfor certain arithmetic and logic operations used in

microprocessors. This gives rise to group of

registers called shift registers.14.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

15.

Explain the flip-flop excitation tables for RS FF.

In RS flip-flop there are four possible transitionsfrom the present state to the next state. They are

,

0 to 0 transition: This can happen either when R=S=

0 or when R=1 and S=0.

0 to 1 transition: This can happen only when S=1 an

d R=0.


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1 to 0 transition: This can happen only when S=0 an

d R=1.

1 to 1 transition: This can happen either when S=1

and R=0 or S=0 and R=0.16.

. Define sequential circuit?In sequential circuits the output variables depende

nt not only on the present input variables but theyalso depend up on the past history of these input v

ariables.

17..Give the comparison between combinational circuits

and sequential circuits.

Combinational circuits Sequential circuitsMemory unit is not required Memory unity is required

Parallel adder is a combinational circuit Serial ad

der is a sequential circuit18.

What do you mean by present state?

The information stored in the memory elements at any given time defines the present state of the

sequential circuit.

19.What do you mean by next state?

The present state and the external inputs determinethe outputs and the next state of the sequential

circuit.

20. State the types of sequential circuits?

1. Synchronous sequential circuits2. Asynchronous sequential circuits.

Unit- V

ASYNCHRONOUS SEQUENTIAL LOGIC1.

Define Asynchronous sequential circuit?

In asynchronous sequential circuits change in inputsignals can affect memory element at any instant

of time.2.


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What is race around condition?

In the JK latch, the output is feedback to the input, and therefore changes in the output results chan

ge

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.

3..What are secondary variables?

present state variables in asynchronous sequentialcircuits

4.

What are excitation variables?next state variables in asynchronous sequential cir

cuits.

5.What is fundamental mode sequential circuit?

input variables changes if the circuit is stable

inputs are levels, not pulses

only one input can change at a given time

6.What are pulse mode circuit?

inputs are pulses

width of pulses are long for circuit to respond to

the input

pulse width must not be so long that it is still pr

esent after the new state is reached

7.What is the significance of state assignment?

In synchronous circuits-state assignments are madewith the objective of circuit reduction


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Asynchronous circuits-its objective is to avoid cri

tical races.8.

When do race conditions occur?

two or more binary state variables change their val

ue in response to the change in i/p variable9. What is non critical race?

final stable state does not depend on the order in

which the state variable changes

race condition is not harmful

10. What is critical race?

final stable state depends on the order in which the state variable changes

race condition is harmful11. When does a cycle occur?

asynchronous circuit makes a transition through a series of unstable state

12.What are the different techniques used in

state assignment?

shared row state assignment

one hot state assignment

13. What are the steps for the design of asyn

chronous sequential circuit?

construction of primitive flow table

reduction of flow table

state assignment is made

realization of primitive flow table14. What is hazard? What are its types


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unwanted switching transients

static, dynamic and essential.

15 .What is static 1 hazard and static 0 hazard?

output goes momentarily 0 when it should remain at

1

output goes momentarily 1 when it should remain at

016. What is dynamic hazard?

output changes 3 or more times when it changes from1 to 0 or 0 to 117. 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.

18. Define non critical race.

If the final stable state that the circuit reachesdoes not depend on the order in which the state

variable changes, the race condition is not harmfuland it is called a non critical race.

19. Define critical race?

If the final stable state depends on the order in w

hich the state variable changes, the race conditionis

harmful and it is called a critical race.

20.. Write a short note on fundamental mode asynchrono

us 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 timeand inputs are levels and not pulses.


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21.

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 tothe input and the pulse width must not be so long

that it is still present after the new state is reached.

22.Define secondary variables

The delay elements provide a short term memory for

the sequential circuit. The present state and nextstate variables in asynchronous sequential circuits

are called secondary variables.

23.Define flow table in asynchronous sequential circuit.

In asynchronous sequential circuit state table is k

nown as flow table because of the behaviour of theasynchronous 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.

24.

What is fundamental mode.

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