A simple bottle filling process control system is a good example to understand the fundamentals of PLCs. An assembly line consists of a conveyor belt that’s connected to a motor drive, nozzle, and a filling stroke. The conveyer belt moves to one direction with empty bottles on it and the presence of a bottle is captured by a sensor at the point of the nozzle. It stops the conveyor belt, starts the liquid pump to fill the bottle, detects the liquid level of the bottle by another sensor, stops the liquid pump, and starts the conveyor belt again. The process continues again and again until the process control system receives an interrupt.