More about Logic Gates

More about Logic Gates 

1.Consider a 4-bit adder with inputs A={A3 A2 A1 A0} and B = {B3 B2 B1 B0} . For which of the following input condition no carry-out would be generated.

a.       A={1011}   B={0101}

b.      A={1011}   B={0001}

c.       A={1011}   B={0100}

d.      A={1001}   B={0101}

Ans . a

         

2. XNOR (A,B)  can be written as

           XNOR = AB+A’B’ ,

      

3.      When 2 8-bit numbers A₇….A₀ and B₇….B₀ in 2’s complement representation are added using ripple-carry adder. The sum bits obtained are s₇…s₀ and the carry bits c₇…c₀.  An overflow is said to have occurred if  

1.      4. Half Adder of input A & B,

5.      Noise Margin High  =  V_OH - V_IH    

          Noise Margin Low  = V_IL - V_OL

          For better output Noise margin high & Noise margin high should be very high ,

          V_OH > V_IH     &  V_IL > V_OL

         Therefore , the correct relationship is V_OH > V_IH >V_IL >  V_OL

_{6 .} 

 7.      Number of NAND gate to implement full adder

       Sum = A⊕ B⊕ C

       Carry = AB + C(A⊕ B)

8.   Half Subtractor

Difference = x ⊕  y

Borrow = x’y  

9. If the output of the digital circuit makes a momentary transition to logic ‘1’ , while otherwise the output is logic ‘0’ , is called as Static-0 hazard.

10 . Full adder using half adder , 2 HA and 1 OR gate

11.(AB+C)’ à  Upper part is for inverse and

·         A & B series , so AND  gate

·         C is in parallel with AB , So OR gate

12. In parallel binary adder/subtractor,  ‘X’ = 1, if there is an overflow during either addition or subtraction.

13. The product term to be included to remove possible static hazard for the function

WX+W’Y’ = xy’

Minterms of WX+W’Y’ = ∑(0,2,6,7)

14. Properties of CMOS logic gates are

·                 Low static power consumption

·                 High packing density

·                 High Noise Margin

      So, High switching speed is not the property.

15. The number “00101110” when converted to BCD is “01000110”

Unit 2 MCQ

Unit-2 MCQ

     1)      A device which converts BCD to seven segments is called

       (a )  Encoder

       (b) Decoder

       (c) Multiplexer 

       (d) None of the above

      

     2)      Most demultiplexers facilitate which type of conversion

(a)    Decimal to hexadecimal

(b)   Single input, multiple outputs  

(c)     AC to DC

(d)    odd parity to even parity

     3)      A 8:1 multiplexer has how many number of select lines?

(a)    2 

(b)    3  

(c)     4 

(d)    5

      4)      How many number of AND gates are required for 8:1 multiplexer?

(a)    4  

(b)    8 

(c)     2  

(d)    6

       5)      The simplified expression for full adder carry is

(a)    AB+BC+CA 

(b)   AB+BC

(c)    ABC

(d)   A+B+C

       6)      A carry look ahead adder is frequently used for addition because

(a)    Costs less

(b)   it is faster

(c)    It is more accurate

(d)   uses fewer gates

      7)      Let the input of a subtractor is A and B, then what will be the output if A=B?

(a)    0   

(b)    1  

(c)     A 

(d)    B

      8)      In a combinational circuit, the output at any time depends only on the _______ at that time.

(a)    Voltage 

(b)    intermediate values   

(c)    input values  

(d)    clock pulses

      9)      In a multiplexer, the selection of a particular input line is controlled by

(a)    Data controller

(b)   selection lines

(c)     logic gates  

(d)    Both data controller and selected lines

      10)   How many NOT gates are required for the construction of a 4:1 multiplexer?

(a)    1

(b)    4 

(c)     2 

(d)    3

      11)   If we record any music in any recorder such type of process is called ________

(a)    Multiplexing 

(b)   De-multiplexing  

(c)     Encoding   

(d)    Decoding

      12)   For an 8-bit input encoder how many combinations are possible?

(a)    8   

(b)   4 

(c)    2^8 

(d)    2^4 

      13)   If two inputs are active on a priority encoder, which will be coded on the output?

(a)     The higher  value  

(b)    the lower value  

(c)     Neither of the inputs 

(d)    Both the inputs

      14)   One that is not the output of magnitude comparator is________

(a)    A>B

(b)   A = B

(c)    A-B

(d)   A<B

     15)   Which one is a basic comparator?

(a)    XOR

(b)    XNOR

(c)     AND 

(d)    NAND

  

     16)   Full adder can be changed into full subtractor by introducing

(a)    AND gate 

(b)   OR gate 

(c)    NOT gate 

(d)   EX-OR gate

     17)   One application of a digital multiplexer is to facilitate

(a)    Data generation

(b)   Serial-to-parallel conversion

(c)    Parity checking

(d)   Data selector

     18)   It directs input from input to a selected output line.

(a)    Demultiplexer

(b)   Multiplexer

(c)    Coder

(d)   Both a  and b

     19)   The number of half and full adders required to add 66 bit numbers is

            (a) 8 half-adders and 8 full-adders

            (b) 1 half-adder and 15 full-adders

            (c) 16 half adders and 1 full adder

            (d) 4 half adders and 12 full adders.

     

      20)   A carry propagation can be represented as

(a)    C=A.B

(b)   C=A+B

(c)    Both a and b

(d)   Neither a and b

Unit I MCQ

Unit-1 MCQ

     1)      The universal gate is

(a)    AND gate

(b)   OR gate

(c)     NAND gate

(d)    None of the above

    2)      The inputs of a NAND gate are connected together. The resulting circuit is

(a)    AND gate

(b)    NOT gate

(c)     OR gate

(d)    None of the above

     3)      Two’s complement of the binary number 0101 is

(a)    1011

(b)   1111

(c)     1101

(d)    None of the above

     4)      In 2’s complement representation the number 11100101 represents the decimal number…

(a)    +37  

(b)    -31

(c)    +27

(d)    -27

     5)      For the gate in the given figure output will be---

(a)    0

(b)    1

(c)     A

(d)    A’

       6)      In the expression A+BC the total number of min terms will be

(a) 2

(b)    3

(c)  4

(d)    5

       7)      Decimal 43 in hexadecimal number system and BCD number system is respectively----- and ----

(a) B2 and 01000011 

(b)    2B and 01000011

(c)  2B and 00110100

(d)     B2 and 01000100

       8)      The circuit of the given figure realizes the function

(a)  Y=(A’+B’)C+ (DE)’

(b)     Y= A’+B’+C’+D’+E’

(c)   Y= AB+C+DE

(d)      Y=  AB+C(D+E)

       9)      Complement of the expression A’B+CD’ is____

(a)    (A’+B)(C’+D) 

(b)    (A+B’)(C’+D)

(c)     (A+B’)(C+D’)

(d)    (A’+B)(C+D’)  

       10)   The canonical SOP for the function Y=A+B is

(a)    AB+A’B

(b)   AB+AB’+A’B

(c)    A+B 

(d)   AB+B+A

        11)   For the truth table of the given figure Y=

(a)    A+B+C

(b)    A’+BC

(c)     A’

(d)    B’

        

         12)   For the min term designation Y=∑m(1,3,5,7)the complete expression is

          (a) ABC+A’B’C’

          (b)A’B’C+AB’C+ABC+A’BC

          (c) A’B’C’+A’B’C+A’BC+AB’C

          (d) A’B’C’+ABC+A’B’C+AB’C

               

         13)   The hexadecimal number (3E8)₁₆ is equal to the decimal number

           (a) 1000

           (b) 982

           (c) 768

           (d) 323

               

         14)   For the K-Map in the figure the simplified expression is

(a)    AC+BD’   

(b)   AC+BD

(c)    AC’+BD

(d)   BC’D+ACD

 

           15)   1’s complement of 11101101 is

(a)    00010010

(b)   00010011

(c)    00011110

(d)   00001010

           16)   All logical operations can be obtained by means of

(a)    AND  and  OR gates

(b)   NOT  and OR gates

(c)    NAND and NOR gates

(d)   AND  and NOT gates

            17)   Reflected binary code is also known as

(a)    BCD code 

(b)    Gray code

(c)    Binary code

(d)   ASCII code

           18)   The binary representation of BCD number 0010 1001(decimal 29) is______

            (a) 0011101

            (b) 0001101

            (c) 0101010

            (d) 0110101

     

            19)   The basic logic gate whose output is the complement of the input is

(a)    OR gate

(b)    AND gate

(c)    NOT gate

(d)    EXOR gate

           20)   The primary use of Gray code is

(a)    Coded representation of a shaft’s mechanical position.

(b)   Turning on/off software switches.

(c)    To convert the angular position of a shaft on rotating machinery into hexadecimal code.

(d)   To represent the correct ASCII code to indicate the angular position of a shaft on rotating machinery.