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Important Digital Logic and Design Short Questions

Chapter: 16
Question 1:
What is the significance of the active-low outputs in the 74LS139 MSI Decoder?
Answer: In the 74LS139 MSI Decoder, the active-low outputs mean that the selected output is driven to logic 0, while all other outputs remain at logic 1. This is important for ensuring that only the desired output is activated based on the input conditions.
Chapter: 16
Question 2:
What is the main advantage of using a Look-Ahead Carry Generator over traditional ripple-carry ALUs?
Answer: The main advantage of using a Look-Ahead Carry Generator is the significant reduction in propagation delay, as it allows for the pre-computation of Carry bits, ensuring faster overall processing compared to traditional ripple-carry ALUs where the Carry must ripple through each bit sequentially.
Chapter: 16
Question 3:
Describe how the 74HC85 4-bit Comparator can be cascaded to form a 12-bit Comparator.
Answer: The 74HC85 4-bit Comparator can be cascaded by connecting the cascading outputs of one comparator to the cascading inputs of the next. This allows multiple 74HC85s to work together, enabling the comparison of larger binary numbers, such as 12-bit quantities, by comparing bits in segments.
Chapter: 16
Question 4:
How does an Iterative Circuit-based Comparator implement the A=B function?
Answer: In an Iterative Circuit-based Comparator, the A=B function is implemented by passing the comparison results of individual bits through cascading inputs and outputs. If all corresponding bits of A and B are equal, the final output will be 1, indicating A=B; otherwise, the output will be 0.
Chapter: 16
Question 5:
What is the role of the MSI 74LS139 Decoder's enable input (G)?
Answer: The enable input (G) in the MSI 74LS139 Decoder is used to activate or deactivate the decoder. When G is set to 0, the decoder is enabled and can activate one of its outputs based on the inputs. When G is set to 1, all outputs are disabled and remain at logic 1.
Chapter: 16
Question 6:
Explain the process of cascading two 2-bit Comparators to form a 4-bit Comparator.
Answer: To cascade two 2-bit Comparators to form a 4-bit Comparator, the outputs of the most significant Comparator are used to determine if further comparison is necessary. If A>B or A<B in the most significant bits, the result is finalized. If A=B, the least significant bits are compared to determine the final output.
Chapter: 16
Question 7:
What is the purpose of the NOT gates at the inputs A and B in the 74LS139 2-to-4 Decoder circuit?
Answer: The NOT gates at the inputs A and B in the 74LS139 2-to-4 Decoder circuit are used to reduce the unit load on each input. This configuration ensures that each input presents a single unit load, optimizing the circuit's performance.
Chapter: 17
Question 8:
What is the purpose of the 74XX138 3-to-8 Decoder in logical circuits?
Answer: The 74XX138 3-to-8 Decoder is used to decode 3-bit binary inputs into one of eight outputs, with active-low outputs and three enable inputs.
Chapter: 17
Question 9:
How many enable inputs does the 74XX138 Decoder have, and what are their activation states?
Answer: The 74XX138 Decoder has three enable inputs: one is active-high, and the other two are active-low. All must be activated for the decoder to function.
Chapter: 17
Question 10:
How does the 74XX138 Decoder reduce the load at the inputs?
Answer: The 74XX138 Decoder uses three extra NOT gates connected at the inputs to reduce the four unit load to a single unit load.