Digital logic decoders — what do the address and enable inputs actually do? In combinational logic, a decoder activates exactly one output line based on its binary address inputs, often gated by enable pins. What is the primary purpose of a decoder's inputs?

Introduction / Context:
Decoders are fundamental combinational logic blocks that convert an n-bit binary code at their inputs into one-of-2^n active outputs. They are used for address decoding in memory systems, chip selects, multiplexed display driving, and many other digital applications. Understanding the role of the address inputs versus any enable pins is essential for correct design and troubleshooting.

Given Data / Assumptions:

• A standard n-to-2^n decoder has n address inputs (often labeled A, B, C, …).
• Many families include one or more enable inputs (active-HIGH or active-LOW) that globally gate the outputs.
• Only one output should be active at a time when the device is enabled.

Concept / Approach:
The decoder's address inputs select which single output line becomes active. If the device has enable inputs, they merely permit or inhibit that normal decoding action. When disabled, all outputs take the inactive state regardless of the address inputs. Thus, the primary purpose of the decoder's inputs (the address lines) is to cause the correct output gate to assert according to the input code.

Step-by-Step Solution:
Identify the role of address inputs: select exactly one output line.Identify the role of enable pins: gate the entire device on or off.Match the options: the statement that focuses on selecting the correct output gate via inputs is most accurate.Therefore choose option (a).

Verification / Alternative check:
Look at common parts such as 74HC138 (3-to-8) or 74LS154 (4-to-16). Their data sheets show truth tables where the outputs follow the address inputs only when enable conditions are satisfied; otherwise, all outputs are inactive. This confirms the interpretation above.

Why Other Options Are Wrong:

• (b) Enable pins can disable outputs, but that is not the primary purpose of the address inputs.
• (c) No standard decoder activates all outputs simultaneously; that would defeat decoding.
• (d) Mixes two different roles and is misleading about “activate all outputs.”

Common Pitfalls:

• Confusing address inputs with enable inputs; they serve different functions.
• Forgetting the active-LOW polarity of some enables, leading to miswiring.

Final Answer:
To allow the decoder to respond to the inputs to activate the correct output gate.