Computer Awareness - SPLessons
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Logical Gates

Logical Gates

shape Introduction

Digital systems are designed by using Logical Gates. These gates are the AND, OR, NOT, NAND, NOR, Ex-OR and Ex-NOR gates. The basic operations are described below with the help of truth tables.

shape Concept

Input Output
A B AND NAND OR NOR Ex-OR Ex-NOR
0 0 0 1 0 1 0 1
0 1 0 1 1 0 1 0
1 0 0 1 1 0 1 0
1 1 1 0 1 0 0 1


AND: The AND gate is an electronic circuit that gives a high output (1) only if all its inputs are high.  A dot (.) is used to show the AND operation i.e. A.B
OR: The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs are high.  A plus (+) is used to show the OR operation.
NOT: The NOT gate is an electronic circuit that produces an inverted version of the input at its output. If the input variable is 1, the inverted output is 0.
NAND: This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate.  The outputs of all NAND gates are high if any of the inputs are low.
NOR: This is a NOT-OR gate which is equal to an OR gate followed by a NOT gate.  The outputs of all NOR gates are low if any of the inputs are high.
Ex-OR: The 'Exclusive-OR' gate which will give a high output if either, but not both, of its two inputs are high.
Ex-NOR: The 'Exclusive-NOR' gate circuit does the opposite to the EOR gate. It will give a low output if either, but not both, of its two inputs are high.

shape Quiz

1. The output of an AND is HIGH when the three inputs, A, B, and C is _______.
    A. A = 1, B = 1, C = 0 B. A = 0, B = 0, C = 0 C. A = 1, B = 1, C = 1 D. A = 1, B = 0, C = 1

Answer: Option C
2. When _____ circuits used with an IC, what does the term “QUAD” indicate?
    A. 2 B. 4 C. 6 D. 8

Answer: Option B
3. The output of an OR gate with three inputs, A, B, and C, is LOW when ________.
    A. A = 0, B = 0, C = 0 B.A = 0, B = 0, C = 1 C. A = 0, B = 1, C = 1 D. A = 1, B = 0, C = 1

Answer: Option A
4. In which case the output s LOW when one or more inputs are zero ?
    A. OR gate B.NOT gate C. AND gate D. NOR gate

Answer: Option C
5. 4049 IC contains how many pins?
    A. 14 B. 16 C. 18 D. 20

Answer: Option B
6. When NOR gate output is HIGH?
    A. All inputs are high B. any inputs is high C. any input is low D. all inputs are low

Answer: Option D
7. The Boolean expression for a 3-input AND gate is ________.
    A. X = AB B. X = ABC X = A+B+C X = AB+C

Answer: Option B
8. The output of a NOT gate is HIGH when ________.
    A. input is low B. input is high C. power is applied to the gates IC D. None of the above

Answer: Option A
9. If the input to a NOT gate is A and the output is X, then ________.
    A. X = A B. X = Bar on A C. X = 0 D. none of the above

Answer: Option B
10. What is the Boolean expression for a three input AND gate?
    A. X = A+B+C B. X + A . B . C C. A – B – C D. A $ B $ C

Answer: Option B
11. The output of an Exclusive-OR is High if ______.
    A. All inputs are low B. all inputs are high C. input are unequal D. none of the above

Answer: Option C
12. A high voltage in digital electronics is represented as __________.
    A. 1 B. 0 C. 2 D. 3

Answer: Option A
13. Branch of the mathematics which deals with relationships of logic variables is ______
    A. Algebra B. Geometry C. Boolean algebra D. Logarithms

Answer: Option C
14. In positive logic, 1 represents _______.
    A. ON B. OFF C. a low voltage D. a high voltage

Answer: Option A
15. Boolean equation for OR gate is?
    A. A + B = C B. A – B = C C. A * B = C D. A / B = C

Answer: Option A
16. Logical algebra was developed by _______.
    A. Pascal B. Pythagoras C. George Boole D. Poisson

Answer: Option C
17. The gate which is called an inventor is _________.
    A. NOR B. NOT C. AND D. NAND

Answer: Option B
18. Which is called universal gate _____.
    A. OR gate B. AND gate C. NOR gate D. NAND gate

Answer: Option C
19. The time required for a gate to change its state called______.
    A. Rise time B. Propagation time C. Charging time D. Decay time

Answer: Option B
20. The exclusive NOR gate is equivalent to which gate followed by an inverter ?
    A. OR B. AND C. NAND D. XOR

Answer: Option D

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