Using a 74139 dual 2-to-4 demultiplexer as a decoder: Under what condition does the 74139 behave like a one-of-four decoder that drives exactly one active output line?

Introduction / Context:
The 74139 contains two independent 2-to-4 line decoders/demultiplexers with active-low enable inputs and active-low outputs. As a demultiplexer, it routes a data input to one of four outputs selected by the address lines. As a decoder, it asserts exactly one output (active state) based solely on the address, independent of time-varying data. This question asks what operating condition transforms the demultiplexer behavior into pure decode behavior.

Given Data / Assumptions:

• Each 74139 section has one active-low enable (/G), two address lines (A1, A0), and four active-low outputs (Y0–Y3).
• Active output means logic LOW, consistent with the device’s low-true outputs.
• We desire “decoder mode,” where exactly one output is LOW determined by A1 A0.

Concept / Approach:
In demultiplexer use, the “data” is the enable input level being routed to one of the outputs. To force decoder behavior, fix the data to its active level so that the selected output alone is driven active, while all others remain inactive. Because the 74139’s outputs are active LOW and its enable is active LOW, holding the enable LOW while addressing with A1 A0 yields one LOW output (selected) and three HIGH outputs, i.e., a standard 1-of-4 decoder truth table.

Step-by-Step Solution:

Verification / Alternative check:
Compare to a 74xx139 datasheet decoder truth table: with /G = 0, exactly one output is LOW based on A1 A0. With /G = 1, all outputs idle HIGH, confirming the decoder-like behavior when /G is held LOW.

Why Other Options Are Wrong:

Common Pitfalls:
Forgetting the active-low conventions and assuming HIGH is the active level; mixing demultiplexing (time-varying data) with decoding (static active level).

Final Answer:
Hold the data (enable) input LOW (active) while A1 and A0 select the output