Noise margin measures the voltage allowance between guaranteed driver levels and receiver thresholds. High noise margin is VOH(min) − VIH(min); low noise margin is VIL(max) − VOL(max). Fan-in counts a gate's inputs. Fan-out concerns how many loads an output can drive within electrical and timing limits.
Worked example
With VOH(min) = 4.4 V and VIH(min) = 3.5 V, the high margin is 0.9 V. With VIL(max) = 1.5 V and VOL(max) = 0.4 V, the low margin is 1.1 V. These are illustrative specifications, not limits for a particular device.
What to watch for
A static current-based fan-out estimate may miss capacitive loading and transition-time problems. Large loads can make an otherwise compatible signal too slow.
Try it yourself
What high margin follows from VOH(min) = 2.9 V and VIH(min) = 2.0 V?
Answer: 0.9 V. A positive margin provides some tolerance, but the complete connection still needs other electrical checks.
How to study this topic
Use the numerical values here as teaching examples. For a physical design, consult the actual component data sheet at the intended supply, load and temperature. Logical equivalence alone does not establish electrical compatibility.
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Digital Electronics