This sounds like a school exercise. First, make a list of the devices that will connected to the PLC. Those are things like the sensors that detect when the elevator is at a floor, when the doors are opened or closed. It also includes the buttons that the users push to call the elevator to a floor, to go up, down, open the doors, close them, etc. Also the actuators that make the elevator go up and down, open and close the door, the lamps that tell the user where the elavator is located, etc. This list of devices is called th I/O list.
Once you have your I/O list compiled, write a description of how you expect the elevator to work. All of the devices in your I/O list should show up somwhere in the description. AS you write the descrition, you may find that you need additional I/O devices.
Once you have a good description, post it here along with your I/O list and we'll offer suggestions.
