Digital logic interpretation: is an invalid (undefined/floating) digital signal treated as logic 0? In practical digital electronics, a signal that is undefined, out of specification, or left floating is sometimes observed on an input. Evaluate the statement: “An invalid digital signal is used as a zero.” Decide whether this statement accurately reflects how digital systems treat undefined levels.

Introduction / Context:
Students often assume that any “not-high” condition in digital electronics is equivalent to logic 0. This question probes your understanding of valid logic levels versus invalid or undefined signals, and how inputs should be handled to avoid metastability, oscillation, or unpredictable behavior.

Given Data / Assumptions:

• The target domain is standard digital logic families (TTL, CMOS) using positive logic.
• “Invalid” means outside guaranteed input thresholds, or floating (no defined drive).
• Logic 0 and logic 1 must satisfy the vendor-specified VIL/VIH limits.

Concept / Approach:
Logic families define voltage windows. For example, a typical TTL input treats 0 to about 0.8 V as LOW and above about 2.0 V as HIGH, with a forbidden region in between. CMOS defines similar windows based on VDD. A signal that lies in the forbidden/undefined region, or a floating input with no bias, is not guaranteed to be read as 0; it can switch unpredictably, pick up noise, or bias to 1 via input structures. Therefore, “invalid equals zero” is an unsafe assumption.

Step-by-Step Solution:

Verification / Alternative check:
Practical verification: drive an input near threshold or leave it floating and observe erratic toggling or increased power consumption, confirming the undefined nature.

Why Other Options Are Wrong:

Common Pitfalls:
Leaving CMOS inputs floating, assuming internal bias will force a stable zero; ignoring the undefined window leads to intermittent faults.

Final Answer:
Incorrect