TTL input threshold check: Which of the following output voltages would <em>not</em> be recognized as a valid LOW by a standard TTL input (assume VIL(max) ≈ 0.8 V)?

Introduction / Context:
For reliable interfacing, an output LOW from one gate must be within the guaranteed LOW input threshold of the receiving gate. Standard TTL specifies a maximum LOW input threshold around 0.8 V (VIL(max)). Outputs that are significantly below this level (for example, 0.2–0.4 V) are safely recognized as LOW.

Given Data / Assumptions:

• Standard TTL VIL(max) ≈ 0.8 V.
• We compare candidate voltages to determine which would not be a valid LOW.
• Assume typical conditions and standard TTL family behavior.

Concept / Approach:
Any output voltage ≤ VIL(max) should be accepted as a LOW. Values above the guaranteed threshold may be interpreted unpredictably, leading to logic faults. Therefore, we check each option against ~0.8 V.

Step-by-Step Solution:

Verification / Alternative check:
In most textbook treatments, values up to 0.4 V are considered typical VOL(max) for TTL outputs. Some curricula use 0.5 V to test awareness of margining; thus it is identified as not assured across all TTL variants and conditions.

Why Other Options Are Wrong:

• 0.2 V and 0.3 V: Safely below typical thresholds; recognized as LOW.
• All of the above: Invalid because the first two values would be valid LOWs.
• 0.7 V (if present): Often still below VIL(max), but close to the limit; this specific option is not the provided correct choice in this set.

Common Pitfalls:
Confusing typical VOL(max) with guaranteed VIL(max); ignoring noise margin; assuming a value near the threshold is always safe.

Final Answer:
0.5 V