Driving capability (fan-out) Approximately how many 74LS-TTL logic gate inputs can be reliably driven from the output of a single standard 74TTL gate?

Introduction / Context:
Fan-out is the number of standard inputs a gate output can drive while meeting guaranteed logic levels. Because different TTL subfamilies have different input currents, the fan-out depends on both the driver family and the load family.

Given Data / Assumptions:

• Driver: standard 74TTL output.
• Loads: 74LS-TTL inputs, which draw less current than standard TTL inputs.
• Use guaranteed VOL/VOH and IOL/IOH vs IIL/IIH numbers.

Concept / Approach:
Fan-out (LOW) ≈ IOL(max of driver) / |IIL(max per load)|; Fan-out (HIGH) ≈ IOH(max of driver) / IIH(max per load). 74LS inputs typically require much less current than standard TTL, so a standard TTL output can drive more LS inputs than standard TTL inputs. Typical textbook/handbook values round to about 20 LS loads per standard TTL output.

Step-by-Step Solution:

Verification / Alternative check:
Reference logic charts in textbooks: standard TTL → fan-out 10 for standard TTL loads, and about 20 when driving LS inputs due to reduced current demand.

Why Other Options Are Wrong:

• 10: Applies to standard-to-standard TTL loading, not LS loads.
• 200 / 400: Far exceed current limits; would violate VOL/VOH specs.

Common Pitfalls:
Ignoring both HIGH-state and LOW-state limits; true fan-out is the minimum of the two calculations. Also, temperature and supply variations can reduce margins.

Final Answer:
20