Hi Jez,

You might be interested to know that the tilde as a logical negation predates any of the other notations you are likely to encounter (or use) today. In undergraduate school I took a course called Introduction to Logic, with a textbook of the same name. That textbook, by Irving M. Copi, was originally published in 1953 and since word processors and personal computers were still a few decades away he used what was available on his keyboard.

The course (by the way) was a philosophy course, not an electronics course, though some universities count the same course as a mathematics course. And yes, it included a complete treatment of DeMorgan's Theorem, plus all of those other things like Modus Ponens, Modus Tollens, Tautology, and both the Hypothetical and Disjunctive Syllogisms.

Anyway, since that was my first exposure to logical notation, and since Copi used the tilde as a logical negation, to this day it is my preferred notation. Whether describing an active low pin, or a NGT edge triggered pin, I denote them with a tilde. I even usually use a tilde when I'm writing longhand in my notebook.

Joe