It is quite simple, just use a photocell on each section but it will depend on if the conveyors can handle more than one pallet or box or whatever, for example a simple pallet conveyor where the largest pallet or box fits onto one of the conveyors, just use the PE on the end of the following conveyor to stop the preceeding conveyor when that conveyor detects a pallet, it gets a little more complicated if the conveyors can hold more than one pallet/box. you may need to do some extra logic for example one conveyor section can hold say 3 pallets then you need to think about can pallets sit across two of the conveyor sections.
You need to be a little bit more specific, perhaps a simple drawing showing the conveyor sections, PE positions etc.