PID Equation Derivation Help

Dear Experts,
1. Can u plz Explain the steps to derive the PID equation and
2. How to use PID block in AB RSlogix 500 or 5000.....
3. Plz suggess any tutorial for PID Concept, PID derivation and using PID in Programs in AB 500 & 5000

best advise is to do a search on this site for posts done by Ron Beaufort he has posted several excellent articles on PID also hes done thosde articles for the AB hardware/software