# digital electronics question paper

B.E./B.Tech. DEGREE EXAMINATION,

Third Semester

Mechatronics Engineering

EC 154 — DIGITAL ELECTRONICS

Time : Three hours Maximum : 100 marks

Answer ALL questions.

PART A — (10 ? 2 = 20 marks)

1. Prove that .

2. Realize the function using only two input NAND gates.

3. Distinguish between EPROM and EEPROM.

4. Implement the function using gates.

5. Draw the symbol and Truth Table for J–K flip–flop.

6. Define modulus of a counter.

7. What are secondary states (internal states)?

8. How the desired delay is generated in the level mode and pulse mode asynchronous sequential circuits?

9. List the advantages of ASM chart over state graph.

10. What is an SM block?

PART B — (5 ? 16 = 80 marks)

11. Draw the ASM chart for a serial sequential adder and explain how the next state and output functions can be synthesized from the chart.

12. (a) Simplify the following function using tabular minimization method.

.

Or

(b) (i) Explain how subtraction can be performed using 2’s complement

arithmetic with the help of suitable examples. (8)

(ii) Prove the following :

. (5)

(iii) Express the above logic expression in product of sum (POS) form using the duality principle. (3)

13. (a) (i) Give the Truth table for the full adder and then write the logic

expression for SUM and carry, and draw the full adder circuit using

gates. Also explain how the full adder circuit can be obtained using

two half adders and OR gate. (8)

(ii) Draw the general structure of programmable logic array (PLA) in block schematic form and explain how it can be used to realize combinational functions. (8)

Or

(b) (i) A single pole double through switch is to be realized using gates. Call the Two inputs of the switch as A and B, a third input C receives the control information in the form of a code 0 or 1. It is desired that C = 0 sets the switch to A and C = 1 sets the switch at B as indicated in the following diagram. Obtain the logic expression for the switch and draw the gate realization. (8)

Diagram for Question No. 13 (b) (i).

(ii) Design a binary to Gray code converter (use 3 bits). (8)

14. (a) (i) Draw the circuit of a BCD ripple counter and explain its working

with the help of Timing diagram. (8)

(ii) Draw the circuit of a shift register with the following features :

Parallel load facility, serial left and serial right shift facility and explain its function. (8)

Or

(b) (i) List the advantages of synchronous counter over that of ripple

counter. (4)

(ii) Explain how the J–K flip–flop can be made to function as a D flip–flop and as a T flip–flop. (4)

(iii) Draw the logic diagram of a RAM cell and explain with suitable diagram how the READ and WRITE operation is performed. (8)

15. (a) Minimize the Moore machine shown below and obtain the logic diagram for the machine.

Next state, Y

Present State, y Input, x = 0 x = 1 Output, 2

A A B 0

B C B 0

C C D 1

D E D 1

E E F 0

F G F 0

G G D 1

Table for question 15 (a)

Or

(b) Design a Toggle flip–flop whose function table and Input Output waveforms are given below :

Diagram for Question 15 (b)

T Qt Qt+

0 0 0

0 1 1

1 0 1

1 1 0

Truth Table of T F–F

Diagram for Question No. 15 (b)