Logic Gates and Networks

If we use an AND gate to inhibit a signal from passing one of the inputs must be ___________ a) LOW b) HIGH c) Inverted d) Floating

Answer: a Explanation: AND gate means A*B and OR gate means A+B and to inhibit means to get low signal, one of the input must be low. It means (0*1=0 or 1*0=0) we will get low output signal. Thus, AND gate outputs 1 only when all inputs are at logic level 1 else it outputs 0.

The NOR gate output will be high if the two inputs are __________ a) 00 b) 01 c) 10 d) 11

Answer: a Explanation: In 01, 10 or 11 output is low if any of the I/P is high. So, the correct option will be 00.

The AND function can be used to ___________ and the OR function can be used to _____________ a) Enable, disable b) Disable, enable c) Synchronize, energize d) Detect, invert

Answer: a Explanation: The AND gate and OR gate are used for enabling and disabling respectively because of their multiplicity and additivity property. The AND gate outputs 1 when all inputs are at logic 1, whereas the OR gate outputs 0 when all inputs are at logic 0.

Which of the following are known as universal gates? a) NAND & NOR b) AND & OR c) XOR & OR d) EX-NOR & XOR

Answer: a Explanation: The NAND & NOR gates are known as universal gates because any digital circuit can be realized completely by using either of these two gates, and also they can generate the 3 basic gates AND, OR and NOT.

The dependency notation ">=1" inside a block stands for which operation? a) OR b) XOR c) AND d) XNOR

Answer: a Explanation: The dependency notation ">=1" inside a block stands for OR operation.

How many two input AND gates and two input OR gates are required to realize Y = BD + CE + AB? a) 3, 2 b) 4, 2 c) 1, 1 d) 2, 3

Answer: a Explanation: There are three product terms. So, three AND gates of two inputs are required. As only two input OR gates are available, so two OR gates are required to get the logical sum of three product terms.

How many two-input AND and OR gates are required to realize Y = CD+EF+G? a) 2, 2 b) 2, 3 c) 3, 3 d) 3, 2

Answer: a Explanation: Y = CD + EF + G The number of two input AND gate = 2 The number of two input OR gate = 2.

Logic gate circuits contain predictable gate functions that open theirs ____________ a) Outputs b) Inputs c) Pre-state d) Impedance state

Answer: b Explanation: Logic gate circuits contain predictable gate functions that open their inputs because we are free to give any types of inputs.

The following switching functions are to be implemented using a decoder: f1 = ∑m(1, 2, 4, 8, 10, 14) f2 = ∑m(2, 5, 9, 11) f3 = ∑m(2, 4, 5, 6, 7) The minimum configuration of decoder will be __________ a) 2 to 4 line b) 3 to 8 line c) 4 to 16 line d) 5 to 32 line

Answer: c Explanation: 4 to 16 line decoder as the minterms are ranging from 1 to 14.

How many NAND circuits are contained in a 7400 NAND IC? a) 1 b) 2 c) 4 d) 8

Answer: c Explanation: 7400 IC's pin has total 14 pin. Pin no 7 use for GND and pin no 14 used for +vcc and remaining pins used for connections. For a NAND gate two inputs are required and one output is obtained means for NAND gate 3 pin connections are required. Thus, a 7400IC contains 4 NAND gates with each having 3 pins. Therefore, total 12 pins dedicated for the NAND operation. Rest 2 pins for power supply.

A single transistor can be used to build which of the following digital logic gates? a) AND gates b) OR gates c) NOT gates d) NAND gates

Answer: c Explanation: A transistor can be used as a switch. That is when base is low collector is high (input zero, output one) and base is high collector is low (input 1, output 0).

Exclusive-OR (XOR) logic gates can be constructed from what other logic gates? a) OR gates only b) AND gates and NOT gates c) AND gates, OR gates, and NOT gates d) OR gates and NOT gates

Answer: c Explanation: Expression for XOR is: A.(B')+(A').B So in the above expression, the following logic gates are used: AND, OR, NOT. Thus, 2 AND gates with two-inputs and 1 OR gate with two-inputs will be required for constructing a XOR gate.

Which input values will cause an AND logic gate to produce a HIGH output? a) At least one input is HIGH b) At least one input is LOW c) All inputs are HIGH d) All inputs are LOW

Answer: c Explanation: For AND gate, the output is high only when both inputs are high. That's why the high output in AND will occurs only when all the inputs are high. However, in case of OR gate, if atleast one input is high, the output will be high.

The basic logic gate whose output is the complement of the input is the ___________ a) OR gate b) AND gate c) INVERTER gate d) XOR gate

Answer: c Explanation: It is also called NOT gate and it simply inverts the input, such that 1 becomes 0 and 0 becomes 1.

The gates required to build a half adder are __________ a) EX-OR gate and NOR gate b) EX-OR gate and OR gate c) EX-OR gate and AND gate d) EX-NOR gate and AND gate

Answer: c Explanation: The gates required to build a half adder are EX-OR gate and AND gate. EX-OR outputs the SUM of the two input bits whereas AND outputs the CARRY of the two input bits.

A full adder logic circuit will have __________ a) Two inputs and one output b) Three inputs and three outputs c) Two inputs and two outputs d) Three inputs and two outputs

Answer: d Explanation: A full adder circuit will add two bits and it will also accounts the carry input generated in the previous stage. Thus three inputs and two outputs (Sum and Carry) are there. In case of half adder circuit, there are only two inputs bits and two outputs (SUM and CARRY).

A universal logic gate is one which can be used to generate any logic function. Which of the following is a universal logic gate? a) OR b) AND c) XOR d) NAND

Answer: d Explanation: An Universal Logic Gate is one which can generate any logic function and also the three basic gates: AND, OR and NOT. Thus, NOR and NAND can generate any logic function and are thus Universal Logic Gates.

How many truth table entries are necessary for a four-input circuit? a) 4 b) 8 c) 12 d) 16

Answer: d Explanation: For 4 inputs: 24 = 16 truth table entries are necessary.

The code where all successive numbers differ from their preceding number by single bit is __________ a) Alphanumeric Code b) BCD c) Excess 3 d) Gray

Answer: d Explanation: The code where all successive numbers differ from their preceding number by single bit is gray code. It is an unweighted code. The most important characteristic of this code is that only a single bit change occurs when going from one code number to next. BCD Code is one in which decimal digits are represented by a group of 4-bits each, whereas, in Excess-3 Code, the decimal numbers are incremented by 3 and then written in their BCD format.

The output of a logic gate is 1 when all the input are at logic 0 as shown below: INPUT OUTPUT A B C 0 0 1 0 1 0 1 0 0 1 1 0 INPUT OUTPUT A B C 0 0 1 0 1 0 1 0 0 1 1 1 The gate is either _________ a) A NAND or an EX-OR b) An OR or an EX-NOR c) An AND or an EX-OR d) A NOR or an EX-NOR

Answer: d Explanation: The output of a logic gate is 1 when all inputs are at logic 0. The gate is NOR. The output of a logic gate is 1 when all inputs are at logic 0 or all inputs are at logic 1, then it is EX-NOR. (The truth tables for NOR and EX-NOR Gates are shown in the above table).

How many AND gates are required to realize Y = CD + EF + G? a) 4 b) 5 c) 3 d) 2

Answer: d Explanation: To realize Y = CD + EF + G, two AND gates are required and two OR gates are required.