Welcome Lindsey,
Congratulations on being the first ONC poster, we have had plenty of HNC posters but you are the first ONC one.
You will normally get told to do the research yourself, then, if there is something you don't understand, post what you do know and others will help you out.
Have a look at this site.
Click Here , look at the EdExcel prog. logic controller section, that could possibly answer your question, this is the HNC syllabus, not ONC, but it might help anyway.
You can then post any specific questions, but be warned, lazy students are quickly shot down in flames.
A good book to get hold of, from your college library maybe, is Programmable logic controllers by W Bolton. That book will provide you with all the answers necesary.
Paul
P.S To my American friends, the ONC is the certificate taken before the infamous HNC.