The process plant depicted in the following Figure features two tanks linked in parallel with a streamline. The primary goal of this processing plant is to heat and cool a particular material while maintaining a consistent level in both tanks. The inlet valve flow is always open and remains constant for both tanks. The material is supplied to both tanks through inlet valve 1. If the LLST1 sensor detects a low level in tank 1, the inlet valve 2 will open, whereas outlet valve 1 will close.

On the other hand, if the HLST1 sensor detects a high level in tank 1, the inlet valve 1 will close, and the outlet valve 1 will remain closed. The heater will be turned on to heat the material until it reaches the desired temperature. The temperature will be continuously monitored using a temperature transmitter TT1. Once the desired temperature is achieved, the product will be drained through outlet valve 1. The process in Tank 2 is similar to that in Tank 1, except that the material will be cooled instead of heated. The final cooled product will be drained out.

The inputs for the ladder diagram are Start, Stop, LLST1, LLST2, HLST1 and HLST2. The outputs for the ladder diagram are Inlet Valve 2, Inlet Valve 3, Outlet Valve 1 and Outlet Valve 2. Further, two registers, "Desired Temp T1" and "Desired Temp T2", store the desired heating and cooling temperatures in Tank 1 and 2, respectively.

The first rung in the ladder diagram is for latching with the master start and stop buttons. The second rung is to control Inlet Valve 2. This is operated whenever a low level in Tank1 (LLST1) is detected. The following rung is to control Outlet Valve 1. This is operated whenever the Actual Temperature data of Tank 1 is greater than or equal to the Desired Temperature of Tank 1. Similarly, the third rung is to control Inlet Valve 3. This is operated whenever a low level in Tank2 (LLST2) is detected. The last rung is to control Outlet Valve 2. This is operated whenever the Actual Temperature data of Tank 2 is less than or equal to the Desired Temperature of Tank 2.

The following Table lists the experiment's runtime test cases. As per the test cases, if the LLST1 sensor detects a low level in Tank 1, the Inlet Valve 2 will open, and if the HLST1 sensor detects a high level, the Inlet Valve 1 will close. The heater will be turned on to warm the material until it reaches the desired temperature. Once the Desired Temperature in Tank 1 is achieved, the product will be drained through Outlet Valve 1. The process for Tank 2 is very similar to that of Tank 1, except that the material will be cooled instead of heated.