Interfacing logic families — can 74HC CMOS directly drive 74ALS TTL? Consider a 74HC-series CMOS gate driving the input of a 74ALS TTL gate at 5 V. Is a direct logic-level interface acceptable without special translation?

Introduction / Context:
Mixing logic families is common in maintenance or incremental redesigns. The key question is whether voltage levels and currents are compatible in both directions. 74HC (CMOS) and 74ALS (Advanced Low-Power Schottky TTL) are often used together on 5 V rails.

Given Data / Assumptions:

• Supply VCC = 5 V for both families.
• Typical 74HC output: VOL <~ 0.1–0.33 V at modest sink current; VOH >~ 4.4 V at light load.
• Typical 74ALS input thresholds: VIL(max) ≈ 0.8 V, VIH(min) ≈ 2.0 V; very low input currents.

Concept / Approach:
For a valid interface, the driver's VOH must exceed the receiver's VIH(min), and the driver's VOL must be below the receiver's VIL(max), with current margins respected. A 74HC output HIGH of about 4.4–5.0 V easily exceeds the 2.0 V VIH(min) of 74ALS. A 74HC output LOW is well below 0.8 V. Input currents of 74ALS are small, so fan-out is not a limiting factor in most cases.

Step-by-Step Solution:

Verification / Alternative check:
Contrast the reverse direction: 74ALS driving 74HC inputs can be marginal unless using HCT (TTL-threshold CMOS) because TTL VOH(min) ≈ 2.4 V is close to 74HC VIH(min) ≈ 3.5 V. Hence the common guidance: use HCT for TTL-to-CMOS, but HC can drive TTL fine.

Why Other Options Are Wrong:

• “No” ignores level margins that are met.
• “Pull-ups” or “series resistors” are not required for DC logic-level compatibility.
• “Schmitt-trigger inputs” are unnecessary for steady logic-level interfacing.

Common Pitfalls:

• Confusing HC↔TTL in both directions; direction matters.

Final Answer:
Yes