The characteristic equation of D flip-flop is:
Answer: D
Correct answer: Q= D
A D flip-flop transfers its input to its output at the clock edge, so its characteristic equation is \( Q_{next} = D \). It is the simplest of the flip-flops precisely because the next state does not depend on the present one.
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An exclusive-NOR gate output is HIGH when the inputs are unequal.
Answer: B
Correct answer: False
XNOR is HIGH when its inputs are equal. It is XOR whose output is HIGH when they are unequal.
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What is the circuit number of the IC that contains four two-input AND gates in standard TTL?
Answer: C
Correct answer: 7408
The 7408 is the quad two-input AND. Its neighbours in the family are the 7400 NAND, the 7402 NOR, the 7404 hex inverter and the 7432 OR.
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A clock signal with a period of 1 µs is applied to the input of an enable gate. The output must contain six pulses. How long must the enable pulse be active?
Answer: C
Correct answer: Enable must be active for 6 µs.
Each clock period delivers one pulse, so six pulses take six periods:
\[ 6\times 1\ \mu\text{s} = 6\ \mu\text{s} \]
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Which table shows the electrical state of a digital circuit's output for every possible combination of electrical states in the inputs ?
Answer: A
Correct answer: Function table
A function table records the electrical output - HIGH or LOW - for each combination of electrical inputs, without committing to which level means 1. A truth table is the logical version of the same table, obtained once that assignment is made.
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The inverter, OR gate and AND gate are called decision-making elements because they can recognize some input ______ while disregarding others. A gate recognizes a word when its output is ______.
Answer: A
Correct answer: words, high
The gates are called decision-making elements because they respond to particular input words - whole combinations of input bits - and ignore the rest. The gate signals that it has recognised its word by driving its output high.
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If four 4 input multiplexers drive a 4 input multiplexer, we get a:
Answer: A
Correct answer: 16 input MUX
Each of the four first-level multiplexers selects one of four inputs, giving sixteen inputs in all, and the final multiplexer chooses among their four outputs. Two select lines are needed at each level, four in total, which is right for \( 2^4 = 16 \) inputs.
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A full-adder is a logic circuit which can add two single order bits plus a carry in from a previous adder. Its incomplete truth table is given in the table below. The missing entry in the outputs for SUM and CARRY out are
|
Input |
Outputs |
||||
|
A |
A |
B |
Cin |
Sum |
CarryOUT |
|
0 |
0 |
0 |
0 |
0 |
0 |
|
0 |
0 |
0 |
1 |
1 Answer: D Correct answer: 1 1 For \(A=1, B=1, C_{in}=1\) the three inputs total three. The sum bit is the total modulo 2, which is 1, and the carry out is 1 because the total is 2 or more. In XOR terms, \( 1\oplus 1\oplus 1 = 1 \), and the carry is the majority of the three inputs.
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What does the small bubble on the output of the NAND gate logic symbol mean? Answer: C Correct answer: The output is inverted. The bubble is the standard symbol for inversion wherever it appears, on an output or an input. On the output of an AND symbol it makes NAND; on an OR symbol it makes NOR.
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A logic pulser is used to determine the level of floating in a circuit. Answer: B Correct answer: False A logic pulser injects a pulse into a node to stimulate the circuit. It is the logic probe that reads the level and shows whether a node is HIGH, LOW or floating.
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