# Logic Design Questions and Answers – State Graphs and Tables – Design of a Sequence Detector

This set of Logic Design Multiple Choice Questions & Answers (MCQs) focuses on “State Graphs and Tables – Design of a Sequence Detector”.

1. How can we classify a sequence generator circuit?
a) A clocked Mealy circuit
b) A clocked Moore circuit
c) A general sequential circuit
d) A parity checker

Explanation: A sequence detector detects a specific sequence of bits present in a bit stream. For that the output at every state depends on the input and the present state of flip-flops used that is the main characteristic of a clocked Mealy circuit. Hence the right answer is a clocked Mealy circuit.

2. Let, a sequence detector is used to detect a sequence of 1101. How many times the output of the detector will go high for the input 1011011010011010111101 if overlapping sequence is not allowed?
a) 1
b) 4
c) 2
d) 3

Explanation: We can find the sequence 1101 total 3 times as

`   1011011010011010111101 `

That’s why the total number of high outputs will be 3.
We are not taking 1011011010011010111101 that sequence as a detected one because the first bit of that sequence overlaps the first sequence and here overlapping is not allowed.

3. What is the sequence detected by the following sequence detector?

a) 1000
b) 1001
c) 1010
d) 1011

Explanation: To get the sequence, we have to follow all the next state transitions and ignore the paths where no next state transition occurs and for the last state we have to select the path for which a logic 1 output occurs. Thus we get the transitions. Here the representation goes as (Present State, Input) ⟶ (Next State, Output)
(A, 1) ⟶ (B, 0)
(B, 0) ⟶ (C, 0)
(C, 0) ⟶ (D, 0)
(D, 1) ⟶ (A, 1)
Thus we see that the input sequence 1001 makes all the transitions and gives the final; output 0. Hence, the correct sequence is 1001.

4. How many time does the output of a sequence detector go high?
a) Only once the required sequence is detected
b) As many times as the sequence is detected
c) Remains high after once the sequence is detected
d) Remains low once the sequence is detected

Explanation: The main function of a sequence detector is to detect a specific sequence. So whenever the sequence is detected, the output becomes high. But the output immediately comes to low state just after the sequence is detected and remains low until it is detected again.

5. A sequence detector is set to detect the sequence 1010. How many times the output will go high if a stream of 1010101110010101011 is sent as an input and overlapping sequence is countable?
a) 1
b) 2
c) 3
d) 4

Explanation: As overlapping is allowed, we can find the sequence total 4 times as
1010101110010101011
1010101110010101011
1010101110010101011
1010101110010101011
Hence 4 is the right answer.

6. What is the name of the element of a state diagram which uses right corner boxes to represent the state of any object?
a) Transitions
b) States
c) Iteration maker
d) Input

Explanation: State diagrams are pictorial representations used in computer science and electronics to represent the behavior or systems. The state of any flip flop is displayed inside a right corner box or a circle in any state diagram. Hence, the right answer is states.

7. How many states are there in the state diagram of any sequence detector?
a) Undetermined
b) Equal to the length of the sequence to be detected
c) One more than the length of the desired sequence
d) One less than the length of the desired sequence

Explanation: The state diagram of any sequence detector is build by counting each bit of the sequence and a new state is added every time a bit is detected according to the desired sequence. As an example, if 1011 has to be detected, then we must have 4 states as A(1), B(10), C(101), D(1011) until it comes to state D and the sequence is detected. Hence the correct answer is equal to the length of the sequence to be detected.

8. Which of the following is not needed to build a sequence detector?
a) Multiplexer
b) Clock
c) Flip-Flop
d) Logic gates

Explanation: Clock, Flip-flop and logic gates are necessary for creating a sequence detector circuit. Clock helps in flow of the bit stream to the input terminal. D or JK flip-flops are used to input and output the bit stream and logic gates are necessary to build detection logic. But multiplexers have no function here to build a sequence detector. Hence the correct answer is the multiplexer.

9. For a sequence detector of the sequence 1001, an input is given as 10010101100010011001. What are the serial numbers of clock pulses when the output will be 1?
a) 3, 15, 19
b) 4, 16, 20
c) 0, 0, 20
d) 1, 12, 16

Explanation: A sequence detector detects a specific sequence of bits present in a bit stream. For that the output at every state depends on the input and the present state of flip-flops used. Here we can see that the required sequence 1001 is detected 3 times.

`  1001010110001001 1001 `

As the output of the detector goes high after the sequence is detected hence, we see that the 1001 sequence is detected after 4th, 16th and 20th clock pulse. Hence the answer will be 4, 16, 20.

10. Which of the following is essential for design of a sequence detector?
a) AND gates
b) XOR gates
c) Flip-flops
d) OR gates

Explanation: A sequence detector detects a specific sequence of bits present in a bit stream. For that we need to make a data stream to being shifted and checked. Thus a flip flop is essential for that purpose as we will need to make a shift register.

11. Which one of the following will generate high output 3 times for the sequence 1101?
a) 11011001101001101101
b) 10010011011101000101
c) 10101101101011011010
d) 11011101000110010110

Explanation: A sequence detector detects a specific sequence of bits present in a bit stream. For that we need to make a data stream to being shifted and checked. The sequence 1101100110100110101 has three times the sequence 1101.

`11011001101001011101`

Hence the sequence will be 11011001101001011101.

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