Digital logic Short Answer

1st Semester

Year 2017

  1. Express FOM in ASCII code.

=100 0110 100 1111 100 1101 

  1. Why gray codes are used in k-map instead of binary codes? (2012)

Gray codes actually follow Adjacency property i.e. between two successive gray codes there is only one-bit change and in the binary code, there will be multiple value changes. Because of Adjacency property when one plots a K-MAP, literals can be grouped keeping other literal constant.

  1. What is the maximum number of half adder that we can use in n bit parallel adder & why?

A FA has both a carry in and a carry out. A HA doesn’t have a carry in. A half adder can be used to add the least significant digit of a binary number or two binary digits alone. FAs can be chained together to add a multi-bit number:
//  an N- bit parallel adder, there must be N number of full adder circuits. 

  1. Identify & state the similarity between latch & flip-flop.
  • Latch is level sensitive whereas flip flop is edge sensitive.
  • Latch has no clock signal but flip flop has.
  • Latch is asynchronous sequential circuit but flip flop is synchronous.
  • FLIP flop IS SYNCHRONOUS VERSION OF latch. Latch is made from gates but Flip flop is made up of Latches.
  • Latch is a temporary buffer but Flip flop is a 1 bit storage element.
  1. What is the use of cascaded counters?

used to divide a high-frequency clock signal to obtain the highly accurate pulse frequencies.
//The cascade is the larger counters that can be built by combining of
smaller counters together

  1. Which shift register constructed using n flip flops can count 2n states & why?

An n-stage Johnson counter yields a count sequence of length 2n, so it may be considered to be a  mod 2n counter.  They can be implemented easily using D- or JK-type flip-flops.

  1. Difference between grained & coarse grained FPGA.
  • In terms of architecture, FPGAs are considered as ‘fine-grain’ devices while CPLDs are ‘coarse-grain’.
  • FPGAs are great for more complex applications while CPLDs are better for simpler ones.
  • FPGAs are made up of tiny logic blocks while CPLDs are made of larger blocks.
  • FPGA is a RAM-based digital logic chip while CPLD is EEPROM-based.

     // FPGA is short for Field-Programmable Gate Array, is a type of a programmable logic chip

  1. Define Noise immunity.

Noise immunity is the ability of a device or component to operate in the presence of noise disturbance   or . Noise Immunity. the ability of an apparatus or system to perform its functions when interference (noise) is present. Noise immunity is rated according to the noise intensity at which the disruption of the equipment’s functions is still within permissible limits. 

  1. Difference between truth table & characteristics table & excitation table.(2015,2014)

An excitation table shows the minimum inputs that are necessary to generate a particular next state when the current state is known.
           Truth table lists possible inputs and the corresponding output. Or  Truth Table is a table which represents all the possible values of logical variables/statements along with all the possible results of the given combinations of values.
A  state table is a truth table in which at least one of the inputs is the current state, and at least one of the outputs is the next state. It is also known as a state transition table

 Year 2016

  1. Mention advantage of parity method of error detection
  • Errors on a noisy line can be caught quickly
  • only the errant word has to be re-transmitted
  1. Is it possible to design a combinational circuit that adds two 3-bit numbers using only half adder?
  • It is not possible to design a combination circuit that add two 3 bit number using only half adder .For addition of two bit binary number there is a need of two full adder & 1 half adder because there exist carry while adding the bits.
  1. What is the advantage of flip flop over latch?
  • flip-flops are controllable latch.
  • flip plop is an edge-triggered sensitive device but latch is a level sensitive device.
  • flip-flop outputs are free from the state transitions that occur with transparent latches as data inputs change.


  1. Why BCD counter is defined as truncated counter?

BCD counter is defined as truncated counter because it doesn’t count the full sequence i.e. BCD counter counts a 4-bit counter from 0000 state to 1001 state. 

  1. What is the strength of Johnson counter over Ring counter?(2011)

The main advantage of the Johnson counter is that it only needs half the number of flip-flops compared to the standard ring counter for the same MOD.         

  1. What is the importance of refreshing circuit in DRAM?

In a DRAM chip, each bit of memory data is stored as the presence or absence of an electric charge on a small capacitor on the chip. As time passes, the charges in the memory cells leak away, so without being refreshed the stored data would eventually be lost.

  1. Difference between CPLD & SPLD.

//SPLD – simple programmable logic device
//CPLD – complex programmable logic device
//FPGA- field programmable gate array


  • In terms of architecture, FPGAs are considered as ‘fine-grain’ devices while CPLDs are ‘coarse-grain’.
  • FPGAs are great for more complex applications while CPLDs are better for simpler ones.
  • FPGA is a RAM-based digital logic chip while CPLD is EEPROM-based.
  • FPGAs are more expensive while CPLDs are much cheaper.


Year 2015

  1. Is gray code weight or un-weighted code?

It is the non-weighted code and it is not arithmetic codes. That means there are no specific weights assigned to the bit position. It has a very special feature that, only one bit will change each time the decimal number is incremented

  1. Suppose you are adding 1101 & 1001 using binary adder. What is final carry out bit?

=10110 so carry out bit is 1 

  1. Define power dissipation (consumption) of flip flop.

It is the total power required to operate the device & functioning properly. 

  1. If the initial state of 4–bit ring counter is 1001, what is its state after 7th clock pulse?

= 0001 

  1. List the advantage of PLDs over fixed function ICs.
  • Reduction in board space requirement.
  • Reduction in power requirement.
  • Design security.
  • Compact circuitry.
  • Higher switching speed.
  1. Compare between TTl & ECL. (2012)

TTL- Transistor Transistor Logic
ECL- Emitter Coupled Logic
CMOS – Complementary Metal Oxide Semiconductor
Following are the typical feature of CMOS

  1. lowest power dissipation
  2. excellent noise immunting
  3. high parking density
  4. wide range of power supply
  5. highest fan-out among all logic families

 Following are characteristics of ECL logic family

  1. Propagation rate is about 1 to 2ns
  2. Noise immunity and power dissipation is worst of all the logic families.
  3. High Level is 0.8V and Low Level 1.8V
  4. it has differential input amplifier, internal temperature and voltage compensated bias network; either follower output


Year 2014

  1. Difference periodic & non periodic wave.

A wave which repeats pattern over a period is called periodic wave. It can be represented by a mathematical equation. They are deterministic signals.
                Random signals, can’t represented by any mathematical equation. 

  1. Decimal numbers are weighted numbers. Justify

Yes, because each digit position is being assigned with a specific weight. 

  1. What is the number of selection line if multiplexer have 1234 input lines?
  • 11
  1. How many flip flops are required to divide a frequency by 16?

=24  = 16 ,so  4 flip flops are required
    // freq divide by 2N  N=3  => divide by 8 (8 counters required 3 flip flop)

  1. Is it possible to implement counting function using data flip flop? support your answer

Yes, synchronous & Asynchronous counteris   used for counting  

  1. If state of 4-bit Johnson counter is 1100, what is its state after 4th clock pulse?

1st clock pulse=0000
2nd clock pulse =0000
3rd clock pulse=1000
4th clock pulse=1100

  1. How sizes of data register & address register are calculated in computer system?

Capacity(c) =2M*N
 Where ,
2M=Distnict location
M=size of address
N=Size of  data

  1. Write advantage of PLA over ROM.
  • In PLA both AND and OR arrays are configurable unlike in ROM, whereas only the OR gates array is configurable.
  • PLA has the capability to take ‘don’t care terms’ (Boolean overlaps) into account in which ROMs are incapable.
  • . ROM has all the combinations of product terms, and therefore, considered as the most general purpose combinational logic device in contrast to PLA, which doesn’t have all the combinations.


  1. Explain any two operational characteristics of an IC.

Fan out specifies the number of standard loads that the output of the gate can drive without impairment of its normal operation
power dissipation:
Power dissipation is measure of power consumed by the gate when fully driven by all its inputs.
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.
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

Fan in:
Fan in is the number of inputs connected to the gate without any degradation in the voltage level.
Year 2013

  1. On what basis flip flop is categorized?
  1. On the basis of data movement
  • Right shift register
  • Left shift register
  • Universal shift register


  1. On the basis of input and output
  • SISO (serial in serial out) , SIPO (serial in parallel out) ,PIPO (parallel in parallel out), PISO (parallel in serial out)


  1. Why the output of last flip flop is connected to the input of first flip flop in case of Ring counter?

A ring counter is literally a ring of flip-flops with the output of one flip-flop connected to the input of the next. By connecting the output of the last flip-flop to the input of the first, you’ve created a ring. This allows data to circulate around the ring with each clock pulse.

  1. Difference between PAL & PLA.

Drawbacks of PLA

  • PLA were hard to be implemented
  • PLA reduced the speed performance of circuits.

Advantage of PAL

  • Simple to manufacturers
  • Less expensive
  • Better performance

Disadvantage of PAL

  • Not flexible as compared PLA, because OR plane is fixed.


  1. In case of memory read operation what is the function of data register?

Data register  holds the data that has been retrieved from memory (in case of read) or to be stored into memory (in case of write).

  1. Compare between LCD & LED on the basis of power consumption, viewing angle & picture quality. (2012,2010)

Viewing Angles

Very good. Wider viewing angle than LCD TVs. Good to very good: Image fades slightly from extreme angles at sides, above, or below.
Power consumption consume less power around 70% Requires less power to operate

picture Quality

Excellent. Very good


Year 2012

  1. Which logic gates have output same when two units are same?

AND logic gate 

  1. How many flip flops are required to construct a mod-90 counter

2^7 = 128 so 7 flip flop required 

  1. List two advantage of analog signal. (2011,2009)

Analog – No quantization errors, requires less bandwidth, low cost and can be easily constructed because of less pre-processing requirements.
Digital – Fast processing, easier for storage, strong immunity to noise, parallel processing possibility, error correction possibilities, easy portability

  1. Show the necessary steps to convert analog to digital signal.

c.code word generation


Year 2011

  1. Write advantage of BCD code.

it is a fast and efficient system to convert the decimal numbers into binary numbers as compared to the pure binary system. 

  1. What is the output of frequency to the Mod-8 counter is 800 Hz?

Output frequency = 800 HZ/2^4 = 50 HZ 

  1. What is the weight of 1 in binary number 0.0000100?

=0.03125 (5th position)
            //   0.5 0.25 ,0.125,0.0625  

  1. List the level of integration.

From list complexity to most complex
1.Small scale Integration (SSI)

  1. Medium Scale Integration (MSI)

3.Large Scale Integration (LSI)
4.Very Large Scale Integration(VLSI)
5.Ultra large Scale Integration (ULSI)

Year 2009

  1. Write one advantage of alphanumeric code from other codes.

Using these codes, we can interface input-output devices such as keyboards, monitors, printers. with computer.
// alphanumeric codes are the codes that represent numbers and alphabetic characters. 

  1. Draw a circuit diagram to count 6 different numbers using Johnson Counter.

= 6 ( D- flip flop) 

  1. Write one application area of ECL & CMOS each.
  • TTL circuits utilize BJTs while CMOS circuits utilize FETs.
  • CMOS allows a much higher density of logic functions in a single chip compared to TTL.
  • TTL circuits consumes more power compared to CMOS circuits at rest.
  • CMOS chips are a lot more susceptible to static discharge compared to TTL chips.
  • There are CMOS chips that have TTL logic and are meant as replacements for TTL chips
  1. Why PLA is better than PAL?
Programmable Logic Array Programmable Array Logic
PLA has both programmable AND and OR planes PAL has only programmable AND planes and OR plane is fixed
PLA has more flexibility in the logic circuit function implementation PAL has less flexibility in the logic circuit function implementation
PAL is quite difficult to manufacture PAL is simpler to manufacture
PLA has  reduced speed performance PLA has  good speed performance
PLA devices are manufactured in a larger size. PAL devices are manufactured in a smaller size.
  1. How many flip flops are required to generate a 7Hz pulse from 56 Hz pulse?

= 2^3 required for 7Hz ,so 3 flip flop & additional 2^3 so total flip flop  2^6 .so we need now only 3 flip flop.  

Year 2010

  1. Define a Clock Pulse?

clock pulse is a signal used to synchronize the operations of an electronic system. Clock pulses are continuous, precisely spaced changes in voltage. See clock speed. 

  1. What are error detection codes? Give an example.

Error detection is the process of detecting the errors that are present in the data transmitted from transmitter to receiver, in a communication system. We use some redundancy codes to detect these errors, by adding to the data while it is transmitted from source (transmitter). These codes are called “Error detecting codes”.

  1. What is the weight of 0 in binary number 10111?

= 8 

  1. How many Full Adders and Half Adders are required if you want to add two 4 bit words?

= 3 full hader & 1 half adder 

  1. Differentiate Combinational circuit from Sequential Circuit.
Combinational Logic Circuits Sequential Logic Circuits
Output is a function of the present inputs (Time Independent Logic). Output is a function of clock, present inputs and the previous states of the system.
Do not have the ability to store data (state). Have memory to store the present states that is sent as control input (enable) for the next operation.
Used mainly for Arithmetic and Boolean operations. Used for storing data (and hence used in RAM).
Logic gates are the elementary building blocks. Flip flops (binary storage device) are the elementary building unit.
Independent of clock and hence does not require triggering to operate. Clocked (Triggered for operation with electronic pulses).
Example: Adder [1+0=1; Dependency only on present inputs i.e., 1 and 0]. Example: Counter [Previous O/P +1=Current O/P; Dependency on present input as well as previous state].


  1. List the properties of TTL.
  • Higher power Consumption
  • Large Propagation delay
  • Less packing density


Year 2006

  1. A De-multiplexor is having 5 select lines. How many input lines does it have?

= 32 

  1. How many flip flops are required to construct a mod-65 counter?


  1. How many clock pulses are required to take all the output bits from a 5-bit serial-in parallel-out shift register if the shift register is already loaded?

= 1

  1. What are the two most important advantages of LCD displays compared to the conventional types of displays?
  • no radiation emission from screen
  • lighter in weight with respect to the screen size
  • less power and generate less heat


Year 2005

  1. Why NAND & NOR gates are called universal gates?’

The NAND and NOR gates are known as universal gates, since any logic function can be implemented using NAND or NOR gates.
Circuits using NAND and NOR are popular as they are easier to design and therefore cheaper. Functions of other gates can easily be implemented using NAND and NOR gates. For this reason they are called universal gates

  1. State De-Morgan’s theorem mathematically.

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’

  • What are the number of 2-input NAND gates to construct 3-input AND gate?


  1. If n are the input select lines of a MUX, how many data input lines are there in the MUX?

=2n inout input lines are required

  1. . How many JK flip-flops are needed to construct a mod-80 counter?

=7 flip flops are needed.

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