Kieran9,
Referring to your #3 question; What is distributed control? If you are referring to adding more I/O drops through a distributed network off of one PLC's CPU, for control then basically you have two or more racks. In the first rack will be your Power Supply, CPU, various input & output cards, or analog cards,etc, and a Distributed I/O Drop card that will link Rack 1 and its CPU to another rack that has a Power Supply, a Distributed I/O Drop card (to communicate with first rack), and other various input/output cards..etc for contolling more input/output points then you have available because of slot limitation on first rack. I hope this is what you may be asking about and helps you get an idea. Check with Modicon Quantum PLC by Schneider Electric for a reference.