I read about a quarter of the thread, so if i am repeating anything, I apologize. plc supervisory programs have some
valuable time and money saving attributes! As far as trouble shooting, you have trending where you can see how any temps, psig's and levels ect.. were at when the system was running fine and look at what had changed since. You can actually view the ladder logic as the program is running and see what should be energized according to the program! As far as money saving, you have internal(software based) timers and counters. And writing the program is based on ladder diagram logic. I took a 60 hour plc course at the community collage and it helped me a lot!