TTL drive capability: What is the commonly cited fan-out for a typical standard TTL gate under rated conditions?

Introduction / Context:
Fan-out is the number of identical inputs that one logic output can reliably drive while meeting the specified logic levels. In TTL, fan-out is often constrained by the sink current (for LOW) and source current (for HIGH) of the driving gate versus the corresponding input currents of the receiving gates.

Given Data / Assumptions:

• Standard TTL families use specifications like IOL and IIL for LOW-state calculations.
• A widely taught rule of thumb for TTL fan-out is around 10.
• Exact values may vary with subfamily (LS, ALS, etc.), temperature, and supply.

Concept / Approach:
Fan-out (LOW) ≈ IOL(max of driver) / IIL(max per load). With typical numbers (e.g., 16 mA sink vs. 1.6 mA per load), the result is about 10. HIGH-state fan-out may differ, but the canonical figure cited in many textbooks is 10 for standard TTL.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets for common 74xx devices and learning resources confirm the ≈10 fan-out figure as a design heuristic.

Why Other Options Are Wrong:

• 100 or 54: Far above typical TTL assumptions.
• 4 or 1: Too low for standard TTL under normal conditions.

Common Pitfalls:
Assuming DC “infinite” fan-out like CMOS; neglecting frequency-dependent loading and wiring capacitance; forgetting that HIGH-state fan-out and dynamic conditions can reduce the practical limit.

Final Answer:
10