TTL timing fundamentals: For standard 74xx TTL logic, what is a typical order-of-magnitude propagation delay from input change to valid output change under nominal loading?

Introduction / Context:
Propagation delay is a key timing parameter that dictates maximum operating frequency and setup/hold margins in synchronous systems. For legacy standard TTL (not the faster Schottky variants), the typical delay is on the order of tens of nanoseconds.

Given Data / Assumptions:

• Standard 74xx TTL family (not 74S/74AS/74F).
• Nominal supply 5 V, moderate load (e.g., 15 pF, 400 µA source, etc.).
• We seek an approximate, representative value.

Concept / Approach:
Datasheets for classic 74xx TTL commonly show propagation delays around 10 ns per gate under typical conditions. Newer “Fast TTL” and Schottky TTL families reduce this substantially, but standard TTL remains near this number.

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