1. Initial State
   1. O200
2. Load Carton
   1. O201
   2. Motor
      1. O100
   3. Run Light
      1. O102
3. Fill Carton
   1. O202
   2. Solenoid
      1. O101
   3. Standby Light
      1. O103
4. Eject Carton
   1. O203
   2. Motor
      1. O100
   3. Full Light
      1. O104
   4. Run Light
      1. O102
5. The Silo Machine
6. Stop Button
   1. O204
7. The Silo Machine which is part of the PSIM PLC Simulator and training system is a machine which fills a carton with material contained in the hopper.
8. The machine only has two sensors to detect the position and fill level of the carton and two settings i.e. run the conveyor or open the valve on the silo. The PLC controller has to be programmed to move an empty carton under the silo, fill it and then move it away using LADDER or Relay Logic. A traditional approach to the problem would be to just start coding and get the belt moving, stop the belt at the silo and so on building the program up. This will result in a complex program and a huge trap where the machine will dump the content of the silo with no box underneath it.
9. A more structured approach to the problem is to look at the whole process and break it down into individual tasks or states. What makes the machine move from one state to the next and in each state, what must it do. Most machines and processes can be broken down and analysed in this way and a State Diagram produced. (Watch Linked Video http://www.youtube.com/watch?v=rCa_U6YATTA)
10. A LADDER Logic program can now be created using the state diagram as a guide. In order to do this, you need to create a rung for each state which starts with the previous states flag and the condition to move to the next state. This should be sealled with the current state flag anded with not the next state flag. At the end of the program, use the flags to turn on or off all the physical devices.
11. LADDER Program Sample
12. Bob Manekshaw