What is the Boolean expression for a four-input OR gate?
Answer: A
Correct answer: NAND
By De Morgan's law \( \overline{A}+\overline{B} = \overline{A\cdot B} \), which is the NAND function. It is the expression for a NAND written with the inversion pushed inwards onto the variables.
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In Boolean algebra, \( A \cdot (B + C) = A\cdot B + A\cdot C \).
Answer: A
Correct answer: True
The statement is true. This is the distributive law, and it is the same identity as in ordinary algebra, where multiplication distributes over addition. What has no ordinary counterpart is its dual, which also holds in Boolean algebra: \[ A + B\cdot C = (A + B)(A + C). \] Addition distributes over multiplication, which is false for numbers and true here - a good illustration that Boolean algebra is a different system despite the familiar notation. Every Boolean identity has such a dual, obtained by exchanging AND with OR and 0 with 1. The pairing runs through the whole set: \( A + 0 = A \) with \( A \cdot 1 = A \), \( A + 1 = 1 \) with \( A \cdot 0 = 0 \), and De Morgan’s two theorems with each other. The distributive law is what allows an expression to be converted between sum-of-products and product-of-sums form, which is the first step in most simplification.
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A latch is level-triggered while a flip-flop is edge-triggered.
Answer: A
Correct answer: True
The statement is true, and it is the standard distinction between the two. A latch is transparent while its enable input is at the active level: as long as the enable is high, changes at the data input pass straight through to the output. A flip-flop responds only at the instant the clock changes - on the rising or falling edge - and ignores its input at every other moment. That difference matters greatly in synchronous design. Because a flip-flop samples at a single instant, its output is stable for a whole clock period, which makes timing analysis tractable: one need only check that data arrives before the setup time and holds past the hold time. A latch’s transparency window makes the same analysis far harder and can allow a signal to race through two stages in one cycle. Latches are still used where their transparency is wanted, and internally a flip-flop is often built as two latches in a master-slave arrangement.
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Which of the following program is used to copy files to or from another using time sharing system over a single link ?
Answer: C
Correct answer: UUCP
UUCP - Unix to Unix Copy - copies files between machines over a single link, and was the basis of early mail and news distribution. TFTP is a trivial transfer protocol and UART is a hardware device.
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How many crosspoints are needed in a single-stage switch with 40 inputs and 50 outputs ?
Answer: D
Correct answer: 2000
A single-stage crossbar needs one crosspoint for every input-output pair:
\[ 40\times 50 = 2000 \]
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Which of the following digits are known as the terminal number of the network user address
Answer: C
Correct answer: 8-12
Digits 8 to 12 identify the terminal, following the four-digit DNIC and the three-digit area code.
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What is the maximum burst length on an 155.52 Mbps ATM ABR connection whose PCR value 200,000 and whose L value is 25 msec ?
Answer: B
Correct answer: 12 cells
The burst is limited by the interval L and the peak cell rate, and the excess over what the line itself would deliver in that time works out at 12 cells for the figures given.
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The secondary device in a multipoint configuration sends data in response to which of the following event ?
Answer: C
Correct answer: Poll
A secondary never transmits unbidden. It sends data only in response to a poll - the primary's invitation to do so. Select is the opposite direction, the primary announcing that it has data to send.
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Which among the following transmission systems provide the highest data rate to in individual device?
Answer: A
Correct answer: computer bus
A computer's internal bus runs at rates no external line approaches: it is short, parallel and driven at the processor's own clock. Telephone and leased lines are limited by the distance they must cover and by the medium available.
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Which Boolean identity is known as the idempotent law?
Answer: C
Correct answer: \( A + A = A \)
The idempotent law states that combining a variable with itself changes nothing: \[ A + A = A, \qquad A \cdot A = A. \] This is one of the clearest departures from ordinary arithmetic, where \( A + A = 2A \) and \( A \cdot A = A^{2} \). In a two-valued algebra there is no 2 and no square, so the operation simply returns the value unchanged. The other options are the remaining basic identities, each with its own name. \( A + \bar{A} = 1 \) is the complement law, with dual \( A \cdot \bar{A} = 0 \). \( A + 0 = A \) is the identity law, with dual \( A \cdot 1 = A \). And \( A + 1 = 1 \) is the null or annulment law, with dual \( A \cdot 0 = 0 \). Each pair is a dual of the other, obtained by exchanging AND with OR and 0 with 1 - a symmetry that runs through the whole of Boolean algebra and halves the amount to be memorised.
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