# DELD Frequently Asked Questions (Viva Practical)-Sppu Engineering

## FREQUENTLY ASKED QUESTIONS

## DEL LAB GROUP A

## (COMBINATIONAL CIRCUITS)

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*1. Difference between combinational & Sequential Circuit? With Examples.*

**Combinational circuits**are defined as the time independent circuits which do not depends upon previous inputs to generate any output are termed as combinational circuits.

**Sequential circuits**are those which are dependent on clock cycles and depends on present as well as past inputs to generate any output.

**Combinational Circuit –**

- In this output depends only upon present input.
- Speed is fast.
- It is designed easy.
- There is no feedback between input and output.
- This is time independent.
- Elementary building blocks: Logic gates
- Used for arithmetic as well as boolean operations.
- Combinational circuits don’t have capability to store any state.
- As combinational circuits don’t have clock, they don’t require triggering.
- These circuits do not have any memory element.
- It is easy to use and handle.

**Examples –**Encoder, Decoder, Multiplexer, Demultiplexer

**Sequential Circuit –**

- In this output depends upon present as well as past input.
- Speed is slow.
- It is designed tough as compared to combinational circuits.
- There exists a feedback path between input and output.
- This is time dependent.
- Elementary building blocks: Flip-flops
- Mainly used for storing data.
- Sequential circuits have capability to store any state or to retain earlier state.
- As sequential circuits are clock dependent they need triggering.
- These circuits have memory element.
- It is not easy to use and handle.

**Examples –**Flip-flops, Counters

**Block Diagram –**

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*2.What is Minterm & Maxterm, SOP & POS forms?*

*A*

**minterm**is a Boolean expression resulting in 1 for the output of a single cell, and 0s for all other cells in a Karnaugh map, or truth table.*A*

**maxterm**is a sum (OR) of all the variables in the function, in direct or complemented form. A**maxterm**has the property that it is equal to 0 on exactly one row of the truth table.

**Minterm: A minterm is a product term in boolean function in which every element is present is either in normal or in complemented form.***
*

For example if F(a,b,c) is a boolean function then the possible minterms would be

**abc, abc’, ab’c, ab’c’, a’bc, ab,c, a’b’c, a’b’c’ . that is for n varibale boolean function there would be 2^n possible minterms.**
There are used for sum of product(

**SOP**) canonical forms, which is also called disjunctive normal form(DNF). The value correspond to 1 or**true**is selected as minterm.**Maxterm: A maxterm is a sum term in boolean function in which every element is present is either in normal or in complemented form.**

For example if F(a,b,c) is a boolean function then the possible maxterms would be (

**a+b+c), (a+b+c’), (a+b’+c), ( a+b’+c’), (a’+b+c), ( a+b’+c), ( a’+b’+c), (a’+b’+c’) . that is for n varibale boolean function there would be 2^n possible maxterms.**
There are used for product of sum(

**POS**) canonical forms, which is also called conjunctive normal form(CNF). The value correspond to 0 or false is selected as maxterm.##
__BCD ADDER__

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*1. What is difference between BCD and Binary number system?*

**Binary Codes:-**

A binary code is basically used for representation of the binary bits 0 and 1 which are used as group of binary digits and are

*used for processing computer instructions and data,text and string representation,pulse code modulation in communicating across the channels,short and long distance over telephones.*
A binary code can be generally classified as numeric code and alphanumeric code.Alphanumeric code contains alphabets and decimal numbers as a sequence of 0 and 1.Numeric codes only represent information in the form of 0 and 1.

*For example 1:-*
00010110110 is a representation of a binary number.

**BCD codes:-**

BCD stands for binary coded decimal in which the digits of a decimal number are encoded and are grouped into four binary digits one at a time.Its mainly used for accurate representation of decimal number and it's conversion to and fro with other number system.

*But the disadvantage is that arithmetic circuits to design it is complex and storage capacity is less.**The BCD can be categorised as either weighted and non weighted code.Weighted code is again further classified as positive or negative weighted code*.

The positive weighted codes are 8421,2421,5211,4311 and negative weighted codes are 642–3, 631–1, 74–2–1.

To represent BCD code in the form 0 to 9 specific rules are applied.For example in a particular addition of BCD of 2 number if the result is larger than 9 it is treated as illegal code and extra 6(0110) is added to it to correct the result.

For example 8 + 5 in binary is 13. So it is a illegal code and extra 6 needs to be added to it.

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*2. What are the rules of BCD Addition?*

*In BCD addition of two numbers involve following rules:-*

*i*. Maximum value of the sum for two digits = 9 (max digit 1) + 9 (max digit 2) + 1 (previous addition carry) = 19

ii. If sum of two BCD digits is less than or equal to 9 (1001) without carry then the result is a correct BCD number.

iii.If sum of two BCD digits is greater than or equal to 10 (1010) the result is in-correct BCD number. Perform steps 4 for correct BCD sum.

iv.Add 6 (0110) to the result.

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*3. What is Binary Adder?*

**Full adders**are devices used to add binary numbers. They are used in computers. They take two binary numbers and put them together to get a sum. In its most basic form, it uses two XOR gates, two AND gates, and an OR gate. These gates figure out what the numbers being sent in mean and if the digit needs to be turned on or not.

When solving this example: 1001 + 1100 (9 + 12 in decimal form), it would need five single bits to give an answer. The first digits in each number would go to one adder, 1 and 0. It would calculate that 1 + 0 = 1, so that first digit would be 1. The second digits from each are 0 and 0, and 0 + 0 = 0, so the second digit is 0. the third digit from each are 1 and 0, and 1 + 0 = 1, so the third digit is 1. The fourth digit in each are 1 and 1, and 1 + 1 = 10, but because this is a 2 digit number, it holds the first digit (0) and send the second digit to the next adder. The next adder sees 1, and 1 = 1, so the fifth digit is 1. If we take the sum digits in order, they come out as 10101 (21), so 1001 + 1100 = 10101 (9 + 12 = 21).

4. What are the rules of Binary Addition?

5. Which are Basic Gates, Universal Gates and Derived Gates?

###
**Code Converter **

1. What is Gray Code?

2. What are the properties of Gray Code?

3. How to convert binary code to Gray code and Gray to Binary?

4. How to Obtain Excess3 Code?

5. Properties of Excess 3 Code?

6. What is Combinational Circuit?

## Multiplexer

1. What is Mux, Demux, Decoder?

2. Application of MUX, Demux

3. Mux:=Ratio of Select lines with number of input lines

4. Demux:= Ratio of select lines with number of Output Lines

5. Working of MUX, DEMUX & Decoder

## Comparator

1. What is Comparator?

2. Real time application of Comparator

3. Which IC is used as 4 bit comparator?

4. How Comparator IC works?

## Parity Generator

1. What is parity Bit?

2. Types of Parity? Some data will be given you have to identify the parity of
data.

3. What is Parity Generator & Parity Detector?

4. Applications of Parity Bit

## GROUP B (SEQUENTIAL CIRCUITS)

### Flip Flop Conversion ----

1. What is Flip Flop?

2. Types of Flip Flop

3. Truth Table and Excitation table of All Flip Flops

4. What is truth table?

5. What is excitation table?

6. Full Form of J-K, S-R, D & T FF

7. Drawback of S-R FF, Race around Condition, Master Slave J-K FF

8. Process of Conversion of one FF to another.

9. What is Preset and Clear input of FF?

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__Asynchronous Up/Down Counter--__

1. What is Counter?

2. What is difference between Synchronous and Asynchronous Counter?

3. What is Ripple Counter?

4. What is Register?

5. What is up counter & Down Counter?

6. How to design Combined UP and Down Counter?

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__MOD N Counter __

1. What is IC 7490? How it works?

2. What is the output of mod n counter?

3. How may IC require to design Mod 100 Counter?

4. How many FF are used in IC 7490.

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__Ring and Johnson Counter __

1. Which FF’s are used to design Ring and Johnson Counter?

2. What is the real time application of Ring and Johnson Counter?

3. Is it a Synchronous circuit or Asynchronous?

4. Types of Registers

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__Sequence Generator __

1. What is Sequence Generator?

2. How many Flip flops are used to design a given sequence (ex: 2-3-6-8-2 etc.)

3. Which FFs are used to design sequence generator?

4. What is the difference between Sequence Generator & Counter?

5. Difference between Mealy and Moore circuit

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__ASM & MUX Controller Method __

1. What is ASM Chart? How it is different from Flow Chart?

2. What is MUX Controller Method?

3. What is State Diagram?

4. What is State Table?

5. What is State Transition Table?

### GROUP C (VHDL)

1. What is VHDL?

2. Full Form of VHDL

3. Different Modelling Style of VHDL

4. What is Entity?

5. What is component?

6. What is port and their types?

7. What is the difference between Data Flow, Behavioural & Structural
Architecture?

8. What is Library, Package etc.

9. What Is Signal in VHDL?

10.What is Statement?

11.What is Process?

12.What is sensitivity List?

13.What is Simulation?

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__Full Adder__

1. What is HA, FA ?

2. Truth table of half Adder, Full Adder, Half Substractor, Full Substractor

3. Equations of half Adder, Full Adder, Half Substractor, Full Substractor

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__MUX __

1. What is MUX?

2. What is STD_Logic_Vector & STD_Logic?

3. Syntax for Variable Definition.

4. What is Concurrent and Sequential Statements?

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__D FF__

1. What is D FF

2. What is Behavioural Architecture?

3. TT of D FF.

4. What is CLKÃ‰vent? Rising Edge Falling Edge?

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__Counter __

1. What is UP/Down Counter?

2. What are Triggering methods?

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