`I dont really see PLC caring about whether a contact is physically NO or NC, or if it is pump to full or pump to empty, or spring to position or anything like that. In the description section it is said ``In ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system. If a discrete input is energized, the memory bit is a 1, and a ``normally open`` contact controlled by that bit will pass a logic ``true`` signal on to the next element of the ladder.`` So if the input contacts are physically closed the corresponding input address is high, and have a 1 bit in the internal memory, which happens when the tank is empty as per the description. So that should have been the condition for latching the valve, right?  `