DIGITAL ELECTRONICS QUESTION AND ANSWER

Q.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. 

Q. 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. 

Q. 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. 

Q. 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. 

Q. 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. 

Q. 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.

Q. Define multiplexer
 Multiplexer is a digital switch. If allows digital information from several sources to be 
routed onto a single output line. 

Q. What do you mean by comparator 
 A comparator is a special combinational circuit designed primarily to compare the 
relative magnitude of two binary numbers. 

Q. 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. 

Q. 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. 

Q.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 

Q.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. 

Q. 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. 

Q. 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. 

Q. 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 iscalled ‘race around condition’. 

Q. 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. 

Q. 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. 

Q.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. 

Q.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. 

Q.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  

 Q.Define synchronous sequential circuit 
 In synchronous sequential circuits, signals can affect the memory elements only at 
discrete instant of time. 

Q. Define Asynchronous sequential circuit? 
 In  asynchronous  sequential  circuits  change  in  input  signals  can  affect  memory 
element at any instant of time.