If the output of a three-input AND gate must be a logic LOW, what must the condition of the inputs be?
Answer: C
Correct answer: At least one input must be LOW.
AND gives 0 whenever any input is 0. That covers seven of the eight input combinations, so the condition is at least one LOW rather than all of them.
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An AND circuit
Answer: B
Correct answer: gives an output when all input signals are present simultaneously
An AND gate outputs 1 only when every input is 1 at the same instant. It holds no state, so it is not a memory circuit, and it is a negative-logic OR rather than a negative OR in the sense of the option offered.
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How many truth tables can be made from one function table ?
Answer: B
Correct answer: Two
A function table records electrical levels only. Assigning HIGH to 1 and LOW to 0 gives the positive-logic truth table; assigning them the other way round gives the negative-logic one. So each function table yields exactly two truth tables.
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The number of two-input multiplexers required to construct a 210 input multiplexer is
Answer: D
Correct answer: 1023
Building a \(2^n\) input multiplexer from two-input multiplexers gives a binary tree: \(2^{n-1}\) at the first level, \(2^{n-2}\) at the next, and so on down to one. The total is
\[ 2^{9}+2^{8}+\cdots+2^{0}=2^{10}-1=1023 \]
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The EXCLUSIVE NOR gate is equivalent to which gate followed by an inverter ?
Answer: D
Correct answer: XOR
XNOR is by definition the complement of XOR, so an XOR followed by an inverter produces it. It is 1 when its inputs agree, which is why it also serves as an equality detector.
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Which logic gate produces a HIGH output only when all of its inputs are HIGH ?
Answer: D
Correct answer: AND
The AND gate outputs 1 only for the single input combination in which every input is 1. NAND is its complement, giving 0 only in that case, while OR needs just one HIGH input.
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TTL operates from a ________.
Answer: D
Correct answer: 5-volt supply
TTL is specified for a nominal 5 V supply, with LOW below about 0.8 V and HIGH above about 2 V.
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The number of full and half-adders required to add 16-bit numbers is
Answer: B
Correct answer: 1 half-adder, 15 full-adders
The least significant bit has no carry coming into it, so a half adder suffices there; each of the remaining fifteen bits must add three quantities - two bits and a carry - which needs a full adder. In general an n-bit ripple adder takes one half adder and \( n-1 \) full adders.
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Fan-out is specified in terms of ________.
Answer: D
Correct answer: unit loads
Fan-out is expressed as the number of standard gate inputs, or unit loads, that one output can drive. It is a current limit underneath, but stating it in unit loads makes it directly usable when connecting gates of the same family.
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What is the difference between a 7400 and a 7411 IC?
Answer: B
Correct answer: 7400 has four two-input NAND gates; 7411 has three three-input AND gates
The 7400 is the quad two-input NAND that gives the whole family its number, and the 7411 is a triple three-input AND. Both fit a 14-pin package: four gates of two inputs need twelve pins, and three gates of three inputs need twelve as well.
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