TTL input thresholds and undefined region In standard TTL (transistor–transistor logic) families, which input-voltage range is considered invalid or undefined for reliable logic interpretation?

Introduction / Context:
Digital inputs in TTL (transistor–transistor logic) operate correctly only when the applied voltage clearly corresponds to a logic LOW or a logic HIGH. This question tests your recall of the formal LOW, HIGH, and undefined (invalid) ranges for classic TTL input thresholds.

Given Data / Assumptions:

• Family considered: standard TTL logic conventions.
• Typical guaranteed thresholds: VIL(max) = 0.8 V (upper bound for LOW) and VIH(min) = 2.0 V (lower bound for HIGH).
• Supply assumed near 5 V, which is typical for TTL.

Concept / Approach:
A digital input is recognized as LOW if the voltage is at or below VIL(max). It is recognized as HIGH if the voltage is at or above VIH(min). Any input that lies between these two limits is not guaranteed to be interpreted consistently; it falls into the invalid or undefined region. Designers avoid operating inputs in this middle band to ensure noise margin and reliable switching.

Step-by-Step Solution:

Verification / Alternative check:
TTL datasheets list guaranteed input logic-level thresholds with noise margins. The gap between 0.8 V and 2.0 V provides tolerance to noise and device variation. Inputs in this band can cause oscillation or indeterminate logic state.

Why Other Options Are Wrong:

• 0 to 0.8 V: This is the valid LOW region, not invalid.
• 1.2 to 1.6 V: This interval lies inside the undefined range but does not name the whole invalid range.
• 2.0 to 5.0 V: This is the valid HIGH region for TTL inputs.
• Not applicable: Threshold specifications always apply; the undefined band is real.

Common Pitfalls:
Confusing CMOS input thresholds (which depend on VCC fraction) with fixed TTL thresholds; assuming any mid-level value will be “interpreted somehow,” which risks metastability or chatter.

Final Answer:
0.8 to 2.0 V