74LS139 as demultiplexer — The dual 2-to-4 decoder (74139) functions as a demultiplexer when the data is applied to which pin function?

Introduction / Context:
A demultiplexer routes a single data input to one of several outputs based on select lines. Many TTL decoder chips can be repurposed as demultiplexers by feeding data into their enable pins while using the select pins to choose the output line. The 74LS139 contains two independent 2-to-4 decoders with active-LOW outputs and an active-LOW enable per section.

Given Data / Assumptions:

• Each 74139 section has inputs: Enable (active-LOW), Select A, Select B.
• Outputs are active-LOW (Y0–Y3).
• Goal: steer one data signal to a chosen output line.

Concept / Approach:
To use the 74139 as a demultiplexer, apply the data to the enable input. When the enable is asserted (LOW), exactly one output goes LOW based on the select lines; when the enable is deasserted (HIGH), all outputs remain HIGH. Thus the LOW-active output mirrors the data level (active when data is LOW) on the selected output line.

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