Whenever a computer simulation of a model of anyphysical process or a system is done in parallel to its physical counterpartcan be referred to as the real time simulation. Thevirtual representation of physical system runs simultaneously and for the sameduration as the physical system.

They may share common input variables and comeout with comparable output. One good example for a RTS can be operationof the Fuel Injection System of a modern day computer controlled car engine,the onboard computer (Engine Control Unit) calculates the duration of operationand the interval between each operation based upon the throttle input, camshaftposition, inputs from Oxygen Sensors, inputs from NOx sensors etc. all of whichare measured in real time. Another suitable example are the computer games whereoutcomes are generated based on user inputs in real time. A computer does all the computations using anoperating system which eventually does all of its calculations in the form of0’s and 1’s. All the differential equations, state equations or any mathematicalfunctions representing a physical system will converted to a discrete system of0’s and 1’s, these will be solved by the computer simulation software usingtheir own solvers. The solvers use different numerical methods to do thecomputations and each may take different amount of time and produce resultswith different accuracy.

To work with RTS the simulation associated wouldbe for discrete time with a constant step size. Variable time-steps simulationis not suitable for RTS and hence the time is incremented in equal step sizescalled Simulation Time Steps and the simulation itself is often called FixedTime Simulation.As mentioned earlier the differential equationsand the mathematical functions representing the model are solved to perform thei/o operations and to obtain the output of the model. However during a’discrete non real time simulation’ the actual time required to solve theaforementioned equations and functions may be more or less than the simulationtime step. But in case of real time simulation it is necessary that (apart fromthe precise modeling of the physical system) all the computations are donewithin the simulation time step so that the model under test can accuratelyrepresent the functioning and perform all the I/O operations of its equivalentreal or physical system. If the computations are not complete within thesimulation time step the real time simulation results are not accurate which isalso referred to as ‘overrun’. Moreover if the computations are done before thesimulation time step is complete then the remaining time, called the’idle-time’ is simply lost, which is in contrast to the acceleratedsimulations where the remaining time would be used to perform the computationsof the subsequent time step.