One of the principal problems of the simulations of programmable controls is the deficiency of consistency and synchrony of the involved inputs when they are not originated in the physical world, but inserted into simulation by manual way by the user. It can occur, thereby, that the user actuates manually, for example, the input which would receive the indication of the high buoy of a tank, forgetting to actuate before one referring to the low buoy, or he/she actuates them in the correct order, but in an interval of only few seconds, that would implicate an unreal inflow filling this tank.
On controls more complex, as, for example, ones of elevators, would be even virtually impossible to supply manually the input signals in correct order and synchrony.
One of the forms of to mitigate this problem is to simulate, beyond of the PLC, likewise the proper plant to be controlled and the interconnection among its simulated sensors and actuators and the virtual PLC, although such simulation is, in general, more difficult to design and implement that of the proper controller equipment.
It's the proposal of the virtual plants, that I initiate to implement in the version 3 of the SimuPLC, with the simulated tanks of liquids .