Which Boolean expression correctly represents the Difference (D) output of a Full Subtractor?

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RRB JE ECE 22 Apr 2025 Shift 1 CBT 2 Official Paper
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  1. D = A ⊕ B ⊕ Cout
  2. D = A ⊕ B ⊕ Bin
  3. D = A ⊕ B
  4. D = A AND B

Answer (Detailed Solution Below)

Option 2 : D = A ⊕ B ⊕ Bin
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Detailed Solution

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Explanation:

The correct option for the Boolean expression that correctly represents the Difference (D) output of a Full Subtractor is Option 2: D = A ⊕ B ⊕ Bin.

A Full Subtractor is a combinational logic circuit used to perform the subtraction of three bits: the minuend (A), subtrahend (B), and the borrow-in (Bin). The Full Subtractor has two outputs: the Difference (D) and the Borrow-out (Bout).

Full Subtractor Truth Table:

A B Bin D (Difference) Bout (Borrow-out)
0 0 0 0 0
0 0 1 1 1
0 1 0 1 1
0 1 1 0 1
1 0 0 1 0
1 0 1 0 0
1 1 0 0 0
1 1 1 1 1

The Difference (D) output can be derived from the truth table. The Boolean expression for the Difference (D) in a Full Subtractor is given by:

D = A ⊕ B ⊕ Bin

The XOR (⊕) operation is used because it outputs true (1) when an odd number of inputs are true (1). This property makes it ideal for calculating the difference in a subtraction operation.

Analysis of Other Options:

  • Option 1: D = A ⊕ B ⊕ Cout - This option is incorrect because Cout (Carry-out) is not part of the inputs for calculating the Difference (D) in a Full Subtractor. The correct inputs are A, B, and Bin (Borrow-in).
  • Option 3: D = A ⊕ B - This option is incorrect because it does not account for the Borrow-in (Bin) input, which is essential in the subtraction process in a Full Subtractor.
  • Option 4: D = A AND B - This option is incorrect because the AND operation does not correctly represent the difference calculation. The correct operation involves the XOR function to handle the subtraction logic.
  • Option 5: (Blank Option) - This option is not applicable as it does not provide any expression.

In summary, the correct Boolean expression for the Difference (D) output of a Full Subtractor is D = A ⊕ B ⊕ Bin, as it correctly considers all necessary inputs and their respective operations to perform the subtraction.

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