Which of the following gates has the exact inverse output of the OR gate for all possible input combinations?
Answer: A
Correct answer: NOR
NOR is the complement of OR by definition, so its output is the exact inverse of the OR output for every input combination. NAND inverts AND, not OR, and a NOT gate takes only one input.
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A NOR gate output is LOW if any of its inputs is LOW.
Answer: B
Correct answer: False
NOR gives 1 when all inputs are LOW, so a LOW input on its own does not force the output LOW - a NOR output is LOW when any input is HIGH.
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Which of the following gates are added to the inputs of the OR gate to convert it to the NAND gate ?
Answer: A
Correct answer: NOT
By De Morgan's law \( (AB)' = A' + B' \), so a NAND is an OR gate whose inputs have been inverted. Putting a NOT gate on each input of an OR therefore gives NAND.
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A one-to-four line demultiplexer is to be implemented using a memory. How many bits must each word have ?
Answer: A
Correct answer: 1 bit
A one-to-four demultiplexer sends a single data bit to one of four outputs, so the memory needs four locations - addressed by the two select lines - each holding just the one data bit.
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A truth table illustrates how the input level of a gate responds to all the possible output level combinations.
Answer: B
Correct answer: False
The statement has the two the wrong way round. A truth table shows how the output level responds to all possible input combinations, not the reverse.
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The term "hex inverter" refers to:
Answer: B
Correct answer: six inverters in a single package
The prefix counts the circuits in the package: hex is six. The 7404 hex inverter holds six independent one-input inverters, not one inverter with six inputs.
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If a 3-input AND gate has eight input possibilities, how many of those possibilities will result in a HIGH output?
Answer: A
Correct answer: 1
An AND output is HIGH only when every input is HIGH, and of the eight combinations of three inputs exactly one has all three at 1.
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The time required for a pulse to decrease from 90 to 10 per cent of its maximum value is called
Answer: B
Correct answer: Decay time
Decay time, or fall time, is measured between the 90 per cent and 10 per cent points on the falling edge, mirroring rise time on the leading edge. The 10 and 90 per cent marks are used because the extremes of the transition are hard to locate.
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A small dot or circle printed on top of an IC indicates
Answer: D
Correct answer: Pin 1
The dot, and the notch beside it, mark pin 1; the pins are then numbered anticlockwise from it when the chip is viewed from above. Power and ground pins differ from one device to another, so no fixed pin can be assumed for them.
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What is the largest number of data inputs which a data selector with two control inputs can have ?
Answer: B
Correct answer: 4
The control inputs select which data input is passed through, so k control lines address \( 2^k \) data inputs. With two control inputs that is \( 2^2 = 4 \).
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