Which one of the following logic expression is incorrect?
Answer: B
Correct answer: 1 ⊕ 1 ⊕ 0 = 1
XOR gives 1 when an odd number of its arguments is 1. Here two of the three are 1, an even number, so the result is 0, not 1. Checking the others: \(1\oplus 0 = 1\) (one 1), \(1\oplus 1\oplus 1 = 1\) (three 1s), and \(1\oplus 1 = 0\) (two 1s).
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How many truth table entries are necessary for a four-input circuit?
Answer: D
Correct answer: 16
A truth table needs one row per input combination, and four inputs give \( 2^4 = 16 \).
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Power is connected to pins 7 and 14 of a 7408 quad two-input AND gate IC to allow voltage for all four AND gates on the IC.
Answer: B
Correct answer: True
The 7408, like the rest of the 14-pin family, takes VCC on pin 14 and ground on pin 7, and that single supply serves all four gates in the package.
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When does the output of a NOR gate = 0?
Answer: C
Correct answer: Whenever a 1 is present at an input
NOR is the complement of OR. OR is 1 as soon as any input is 1, so NOR is 0 in that case, and it is 1 only when every input is 0.
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Which of the following expressions is not equivalent to X ' ?
Answer: D
Correct answer: X NOR 1
Work each one out. \( X\,\text{NAND}\,X=(X\cdot X)'=X' \); \( X\,\text{NOR}\,X=(X+X)'=X' \); \( X\,\text{NAND}\,1=(X\cdot 1)'=X' \). But \( X\,\text{NOR}\,1=(X+1)'=1'=0 \), which is a constant, not \(X'\).
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A major advantage of ECL logic over TTL and CMOS is ________.
Answer: B
Correct answer: high speed
ECL keeps its transistors out of saturation, so they switch very fast; the price is a large standing current and therefore high power dissipation. Speed is what it is chosen for, and power is what it costs.
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The logic gate that will have HIGH or "1" at its output when any one (or more) of its inputs is HIGH is a(n):
Answer: A
Correct answer: OR gate
OR gives 1 when at least one input is 1, which is the condition described. A NOR gate would give 0 in that case, and an AND gate needs every input HIGH.
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Which of the following equations would accurately describe a four-input OR gate when A = 1, B = 1, C = 0, and D = 0?
Answer: B
Correct answer: 1 + 1 + 0 + 0 = 1
In Boolean algebra the plus sign is OR, not arithmetic addition, and OR gives 1 if any operand is 1. So the sum of several 1s is still 1, and the result is a single bit rather than 01 or 00.
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An example of a universal building block is:
Answer: D
Correct answer: NOR gate
NOR and NAND are the two universal gates: either alone can build NOT, AND and OR, and therefore any Boolean function. AND, OR and XOR cannot produce a complement by themselves.
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Which one of the following set of gates are best suited for 'parity' checking and 'parity' generation.
Answer: B
Correct answer: EX-NOR or EX-OR gates
XOR gives 1 when an odd number of inputs is 1, so a chain of XOR gates across a word computes its parity directly. That single property serves both purposes: generating the parity bit to be sent, and checking it on arrival. XNOR does the same with the opposite sense.
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