Pete,
Selector switches may be hard because you may be trying to transfer the entire switch as one piece into the program. That can be a difficult thing to visualize.
On the other hand, I usually just think of the indivudual functions that the switch performs, and ask, "what contacts are actually needed"? For example for a HOA, I know from experience that I only need two contact blocks, one closed for Hand, one closed for Auto. If both are open, well that must be Off! You probaly keep seeing the whole switch, but in a PLC program, you really only need to worry about ONE function of the switch for EACH rung or rung BRANCH. Forget the others, and do one at a time. For example, on the A-B-C selector, start with "C", the so-called "fill bypass" (where the conveyor runs continuously, but no boxes are filled), add a parallel branch with the "C" position contact input, and then ask, "is any other output needed besides the conveyor motor, and is anything else needed to make the conveyor run as described? The answer is NO (for the "C" position)!
Then once "C" position contact of the selector is working, go up to "A", the "continuous fill", where the boxes fill automatically without restarting the conveyor each time. Think of all the input conditions for "A" position to work.
Tip: Remove every input contact from your Run Light output rung (except the Start, Stop, and seal-in). Think in terms of "functions" or uses, rather than in terms of physical equipment. What output functions are needed? Well, we need one to indicate if the system is active (regardless of conveyor motor being on/off). That can be O:2/2, and you only need the Start, Stop, and seal-in to control that. Then we need one to turn on the conveyor as needed. That rung is where all the limiting contacts need to be placed. Remember what I said when you were first starting work on the Silo Simulator? It was that, knowing what was coming up with the selector switch, it would be easier, at the beginning, to set up a "System Run" output O:2/2, and use that as a Start/Stop command for the conveyor output. With a O:2/2 contact as an input on your Conveyor Motor Output O:2/0, things will get a whole lot simpler to visualize. One advantage is that the Stop, Start, and motor seal-in contacts all go away to another rung, and you will have only one "system run" (run light) contact in their place.
A System Run output is sensible in this case. When the selector is set to A Continuous Fill, the graphic screen indicator light O:2/2 should be on to show that the system is running unattended in automatic, even if the conveyor is stopped at the moment. This is a safety feature not mentinoned in the LogixPro exercise, but I think this was the intention. What other reason is there to have two outputs, one for Conveyor Motor, and one for "RUN"? If you use the Conveyor output O:2/0 to turn on the light, it is only ON when the motor is running. This could be misleading in a real-world plant, where an indicator should show if the system is safe (completely off), or not.
