Understanding the way a PLC scans will definitely aid you in the proper timing of logic controls for old system as well as new systems.
Learning from any source is great, I would love to spread everything I know but until I finish setting up a online classes that I am developing currently here is a great alternative that I highly recommend to help you learn the fundamentals as cheaply as possible and these are great books for doing so. The way the PLC 500 scans is one of the best ways to understand to root of what everything has grown from, easy lead into RSLogix 5000.
These book are some of the best for your money.
I have these myself as well for teaching in my area.
Here is a few great books to start out with to helping you
All three books are at a discounted price using the links below.
Prices range from $10 to $13 depending on the book, fair price for knowledge.
Use this book for the starting basics:
PLC Programming using RSLogix 500 level 1
Use this book for level 2 of the same topic:
PLC Programming using RSLogix 500 level 2
Use this book for level 3 of the same topic
PLC Programming using RSLogix 500 level 3
I hope this is helpful,