Interpreting logic symbols — determining active level from bubbles When reading a decoder's logic symbol, how can you determine whether an output is active-HIGH or active-LOW from the drawing alone?

Introduction / Context:
Schematic symbols use small visual cues to convey polarity (active level). Decoders, encoders, and flip-flops often show bubbles on pins to indicate inversion. Correctly interpreting these “bubbles” is essential to avoid wiring a system with the wrong polarity and getting inverted behavior.

Given Data / Assumptions:

• Standard ANSI/IEC logic symbolism applies.
• A small bubble (circle) denotes inversion at that pin.
• Active-HIGH means the function asserts at logic 1; active-LOW means it asserts at logic 0.

Concept / Approach:
A bubble indicates an inverting relationship. On an output, the presence of a bubble means the output is asserted when the internal “true” signal is LOW at the pin—i.e., the output is active-LOW. Similarly, a bubble on an input means that signal is interpreted in its complemented sense (active-LOW input). No bubble implies active-HIGH behavior.

Step-by-Step Solution:

Verification / Alternative check:
Compare with truth tables of standard decoder ICs (e.g., 74xx138 with active-LOW outputs) whose symbols show bubbles on outputs.

Why Other Options Are Wrong:

• Bubbles never denote active-HIGH; that would contradict the inversion meaning.
• “Square indicates …” is not a standard polarity marker.

Common Pitfalls:
Forgetting that a bubble flips the sense of the signal and that multiple bubbles in series can cancel the inversion.

Final Answer:
A bubble indicates active-LOW.