For certain class of problems notably in cyber-physical systems it is necessary for simulations to be indistinguishable from computational-physical systems with which they interact. Hard real-time simulation offers controlled timing which lends it to be composed with systems operating in physical-time. Accurate real-time simulation equipped with distinct input/output modularity for simulation and software systems is proposed. To demonstrate this approach, a new model for composing ALRT-DEVS (a hard real-time simulation platform) with computational-physical systems is proposed. It provides an abstract communication model for hard real-time simulation and software systems to interact. Experiments are developed where a simulated control switch model (with single and multiple inputs and outputs) operates independent mechanical relays. In another experiment, the control switch commands are communicated with software capable of operating simulated and mechanical relays in real-time. In this setting, simulated software, physical, or computational-physical systems may be interchanged with their actual counterparts.