Logic input thresholds (TTL example): For standard 5 V TTL-compatible inputs, what is the guaranteed range for a valid logic LOW level (VIL)?

Introduction / Context:
Digital logic families define input threshold bands for recognizing logic LOW and logic HIGH. For TTL-compatible devices operating at 5 V, the guaranteed LOW input range (VIL) is standardized to ensure robust interfacing and noise margins across process, temperature, and supply variations.

Given Data / Assumptions:

• 5 V TTL or TTL-compatible CMOS inputs.
• Manufacturer thresholds reflect worst-case guarantees.
• We seek the guaranteed valid LOW input range (VIL).

Concept / Approach:
Typical TTL specifications define VIL(max) = 0.8 V and VIH(min) = 2.0 V. Any input voltage ≤ 0.8 V is recognized as a valid LOW; any ≥ 2.0 V is a valid HIGH. Values between are undefined. These limits exist to allow noise margin on both LOW and HIGH states and to accommodate device variations.

Step-by-Step Solution:
Identify family: TTL-compatible at 5 V.Recall thresholds: VIL ≤ 0.8 V, VIH ≥ 2.0 V.Therefore, the valid LOW input range is 0.0 V to 0.8 V.

Verification / Alternative check:
Consult any 74LS/74HCT datasheet: the DC characteristics table lists VIL(max) = 0.8 V at nominal supply and rated temperature range.

Why Other Options Are Wrong:
0.0–0.4 V is too conservative and unnecessarily reduces margin.

0.4–0.8 V omits legitimate LOWs below 0.4 V.

2.0–5.0 V is the HIGH band, not LOW.

0.4–2.4 V spans the undefined region and portions of both bands.

Common Pitfalls:
Confusing input thresholds with output VOL/VOH specs; outputs guarantee VOL ≤ 0.4 V (at rated IOL) which is different from the input VIL threshold.

Final Answer:
0.0 V to 0.8 V