Among RTL, DTL, TTL, and HTL logic families, which family offers the highest noise immunity under standard definitions?

Introduction / Context:
Noise immunity indicates a logic family's tolerance to unwanted voltage fluctuations without misinterpreting logic levels. Different families have different input thresholds and margins. High Threshold Logic (HTL) is designed with large switching thresholds to reject significant noise, especially in industrial environments. The question evaluates recognition of this property across classic families.

Given Data / Assumptions:

• Families considered: RTL (resistor–transistor), DTL (diode–transistor), TTL (transistor–transistor), HTL (high threshold logic).
• Assume typical implementation and supply ranges.
• Noise immunity refers to DC input noise margins derived from VIH/VIL and VOH/VOL.

Concept / Approach:

HTL uses elevated threshold devices and often different supply arrangements to achieve large separation between logic low and logic high input levels. This directly increases allowable superimposed noise without causing logic errors, which is advantageous in electrically noisy industrial settings with long cables and transients.

Step-by-Step Solution:

Verification / Alternative check:

Industrial control literature cites HTL for robust noise rejection, while TTL is general purpose and RTL/DTL are older with smaller margins.

Why Other Options Are Wrong:

RTL and DTL have modest noise immunity and are rarely chosen for harsh noise environments today. TTL has better margins than RTL/DTL but does not match HTL's high thresholds.

Common Pitfalls:

Confusing speed or fan-out with noise immunity, or assuming the most common family (TTL) must also have the best margins.

Final Answer:

HTL