Which table shows the logical state of a digital circuit output for every possible combination of logical states in the inputs ?
Answer: B
Correct answer: Truth table
A truth table lists the logical output for every combination of logical inputs. A function table does the same job in terms of electrical levels - HIGH and LOW - which is the distinction the next question turns on.
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Which combination of gates does not allow the implementation of an arbitrary boolean function?
Answer: A
Correct answer: OR gates and AND gates only
A set of gates is functionally complete only if it can produce complementation. AND and OR alone can never invert a signal, so functions involving a complement cannot be built. Each of the other three sets can invert: NOT is given outright, XOR inverts when one input is held at 1, and NAND is universal by itself.
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If a 3-input OR gate has eight input possibilities, how many of those possibilities will result in a HIGH output?
Answer: C
Correct answer: 7
An OR output is LOW only when every input is LOW, and of the eight combinations of three inputs just one is all-zero. The remaining seven give a HIGH output.
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The terms "low speed" and "high speed," applied to logic circuits, refer to the ________.
Answer: C
Correct answer: propagation delay time
The speed of a logic family is measured by how long a change takes to travel from input to output, which is the propagation delay. Rise and fall times describe the edges of the signal, and clock speed is a property of the system rather than of the gate.
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The output of an AND gate with three inputs, A, B, and C, is HIGH when ________.
Answer: C
Correct answer: A = 1, B = 1, C = 1
An AND gate is HIGH only when every input is HIGH; a single 0 forces the output to 0.
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Which of the following gates is known as coincidence detector ?
Answer: A
Correct answer: AND gate
An AND gate outputs 1 only when every input is 1 at the same moment, so it signals the coincidence of all its inputs. That is why it is used as a coincidence detector in counting and timing circuits.
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A logic probe is placed on the output of a gate and the display indicator is dim. A pulser is used on each of the input terminals, but the output indication does not change. What is wrong?
Answer: B
Correct answer: The output of the gate appears to be open.
A logic probe shows a dim indication when the node is neither HIGH nor LOW but floating, and a floating node does not respond to pulses on the inputs because it is not connected to the gate's output at all. A bad ground would affect the probe's reading everywhere, not just here, and a tristate output would be driven when enabled.
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Which of the following gates would output 1 when one input is 1 and other input is 0 ?
Answer: D
Correct answer: both (a) and (c)
With inputs 1 and 0, OR gives \(1+0=1\) and NAND gives \((1\cdot 0)'=0'=1\). AND gives \(1\cdot 0=0\), so it is excluded.
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In a Boolean equation the use of the + symbol represents the OR function.
Answer: A
Correct answer: True
In Boolean algebra the plus sign denotes OR. It is not arithmetic addition, which is why \( 1+1=1 \) rather than 2.
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A CMOS IC operating from a 3-volt supply will consume ________.
Answer: A
Correct answer: less power than a TTL IC
CMOS draws significant current only while switching, so its static dissipation is minute compared with TTL, which draws current continuously through its output stage. Lowering the supply to 3 V reduces it further still, since dynamic power varies with the square of the supply voltage.
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