Continuous system models are becoming increasingly more important in the modeling and analysis of complex systems. Unfortunately, the runtime simulation costs required to support continuous modeling can be prohibitive to their use. One technique to decrease simulation runtime costs is mixed-domain simulation where the system is modeled by a mixture of discrete and continuous elements. In those regions where highly detailed information is required, continuous models can be used, and discrete models can be used otherwise. In this paper, we present a simulation backplane and its scheduling algorithm that can be used to integrate existing discrete and continuous simulators to form a single unified mixed-domain simulation environment.