TTL noise immunity What is the standard noise margin per level for classic TTL logic (based on the usual VIH/VIL and VOH/VOL specifications)?

Introduction / Context:
Noise margin quantifies how much unwanted voltage variation a logic level can tolerate without causing an error. TTL families publish typical thresholds and guaranteed output levels from which noise margins are derived.

Given Data / Assumptions:

• VOH(min) ≈ 2.4 V, VOL(max) ≈ 0.4 V.
• VIH(min) ≈ 2.0 V, VIL(max) ≈ 0.8 V.
• Noise margin (high) = VOH(min) − VIH(min); noise margin (low) = VIL(max) − VOL(max).

Concept / Approach:
Compute each margin using given specs. High-level margin: 2.4 − 2.0 = 0.4 V. Low-level margin: 0.8 − 0.4 = 0.4 V. Therefore, the standard TTL noise margin per level is approximately 0.4 V.

Step-by-Step Solution:

Verification / Alternative check:
Family comparison charts consistently list TTL noise margins near 0.4 V, while CMOS at 5 V often has larger margins because VIH and VIL are fractions of VCC.

Why Other Options Are Wrong:

• 5.0 V: That is the supply voltage, not noise margin.
• 0.0 V: Would imply no noise tolerance.
• 0.8 V: Does not match calculations from standard specs.

Common Pitfalls:
Mixing input and output thresholds or using typical instead of guaranteed values. Always base margins on guaranteed min/max, not typicals.

Final Answer:
0.4 V