"Dr. DMX" himself offers this DMX 512 Primer. It's not the simplest thing to understand but its well written and you can figure it out
He also has this nice little chart page that shows how systems should work.
If you've got $30, some time, and want to REALLY understand DMX get ahold of the book"Practical DMX" by Nick Mobsby It's a great easy to read and understand book that fully explains DMX.