Introduction / Context:
In designing decoders, encoders, and code converters, you must harmonize signal polarities across the system. Choosing the wrong active level can invert logic, break enables, or force unnecessary inversion stages late in the design.
Given Data / Assumptions:
- Many standard logic parts offer active-LOW outputs (common in decoder ICs).
- Some subsystems expect active-HIGH enables; others expect active-LOW chip selects.
- Consistency reduces extra hardware (inverters) and confusion.
Concept / Approach:
Decide early if your design will use active-HIGH or active-LOW outputs. This governs part selection (e.g., 74154 active-LOW outputs) and how downstream loads interpret signals. Aligning polarities from the outset simplifies logic equations and minimizes glue logic.
Step-by-Step Solution:
Inventory subsystem expectations: which lines should assert HIGH vs. LOW.Select devices whose output polarity matches those expectations.Update truth tables and timing with the chosen polarity.Avoid late-stage fixes by planning polarity at the schematic stage.
Verification / Alternative check:
Cross-check with datasheets: many decoders and muxes explicitly state active-LOW outputs or enables.
Why Other Options Are Wrong:
Schmitt-trigger inputs: Useful for noisy signals but not the first system-wide decision.TTL vs. CMOS levels: Important, but polarity mismatches cause more pervasive logic errors.PCB color: Irrelevant to signal behavior.
Common Pitfalls:
Mixing active-HIGH and active-LOW conventions without clear documentation.Forgetting that some outputs default to active-LOW with inversion bubbles in schematics.
Final Answer:
Whether outputs are active-HIGH or active-LOW
Discussion & Comments