Logic-family compatibility: The ________ series of CMOS ICs are pin, function, and TTL-level compatible with the 74-series logic devices.

Introduction / Context:
When mixing logic families, input threshold compatibility and pin/function equivalence are critical. Designers often select parts from families that are explicitly TTL-compatible to drop into 74-series designs without level translators.

Given Data / Assumptions:

• 74HCTxx parts are CMOS internally but accept TTL-level inputs.
• Pinouts and functions mirror many 74xx devices.
• ALS is a TTL subfamily; 2N is for discrete transistors; ECL is a different logic technology altogether.

Concept / Approach:
The “HCT” designation stands for High-speed CMOS, TTL-compatible. These devices are designed to be drop-in compatible (functionally and often pin-wise) with standard 74-series TTL logic while offering CMOS power advantages.

Step-by-Step Solution:

Verification / Alternative check:
Compare 74HC vs 74HCT input threshold specs; only HCT specifies TTL-compatible VIH/VIL, confirming compatibility with legacy TTL outputs.

Why Other Options Are Wrong:

• ALS: A low-power Schottky TTL subfamily, not CMOS with TTL thresholds.
• CMOS (generic): 74HC is CMOS but not necessarily TTL-level compatible.
• 2N: Transistor prefix; not a logic family.
• ECL: Different logic technology with distinct levels and pinouts.

Common Pitfalls:
Assuming all CMOS (HC) inputs accept TTL levels interchangeably; ignoring VIH/VIL specs; overlooking pin compatibility across specific part numbers.

Final Answer:
HCT