very good link for digital logics
"An Introduction to Digital Logic - Signals and Gates"
http://www.facstaff.bucknell.edu/mastascu/elessonshtml/logic/logic1.html#NAND
External Links:
1. http://www.qcsltd.com/papers/pubs.htm
2. http://www.findwhitepapers.com/software-development/software-testing/