The process plant illustrated in the figure comprises two tanks, namely a storage tank and a feeder tank, interconnected in series through a pipeline. The primary objective of the process is to maintain the level in both tanks. To achieve this, the tanks are equipped with low-level and high-level sensors that monitor the levels. An outlet valve, operated based on the process requirement, controls the outlet flow. Thus, if any of the low-level sensors are triggered, the inlet valve opens, while any high-level sensor activation results in the closure of the inlet valve.

In the ladder diagram, the first rung is used to initiate the Master Start and Stop function. The second rung is responsible for controlling the Inlet Valve. This valve is energized when LLST1 or LLST2 goes high, which is determined by the level sensor of the storage tank or the level switch of the feeder tank, respectively. Similarly, the valve is closed when both the high level of storage (HLST1) and the feeder tank (HLST2) are detected, as shown in the last rung.

The inputs for the ladder diagram are Start, Stop, LLST1, LLST2, HLST1 and HLST2. The output for the diagram is the Inlet Valve. As shown in the figure, all these input and output variables are defined by default. The run time cases for this experiment are given in the following Table. The first runtime test shows that the inlet valve will be opened if any of the low-level sensors in both tanks are detected. In the next runtime test case, the inlet valve will be closed if both the high-level sensors are True. The outlet valve will be kept open constantly in all the test cases.