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Tribhuvan university CSIT 2065 Syllabus Digital Logic Old Questions 2065

Digital Logic - Old Questions

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Syllabus Notes Old Questions Unit Wise Questions Question Bank

3. Design the full subtractor circuit with using Decoder and explain the working principle.

10 marks
Asked in 2065

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Full subtractor contains 3 inputs and 2 outputs (Difference and Borrow) as shown-


The truth table for full subtractor:


From the above truth table,

For the different functions in the truth table, the minterms can be written as 1,2,4,7, and similarly, for the borrow, the minterms can be written as 1,2,3,7. 

Since there are three inputs and a total of eight minterms. So we need 3-to-8 line decoder. The decoder generates the eight minterms for A, B & Bin.

3-to-8 Decoder

Fig: Full subtractor with 3-to-8 Decoder and NOR gates