In this paper we present the basis of a broadband fully digital simulator, which would be capable of meeting the challenge obtaining real-time solutions of very large networks using sufficiently small integration time steps such as that power electronic devices( FACTS or HVDC) could be modeled at the device level with sufficient time resolution. One of the challenges in developing a general -Purpose broadband fully digital simulator is the interface of simulation algorithms working with very different time steps and time frequency domains. For examples, Transient stability programs, which can represent very large systems, use relatively large time steps and balanced representation of the network, whereas electromagnetic transients simulators model relatively small systems and small time steps. Interfacing the two simulators will result in a broadband simulator capable of modeling large systems, but including detailed representation where required.The simulator would provide for studies ranging from fast transient to transient , dynamic, and voltage stability, with applications including closed-loop protection and control studies, as well as on line dynamic security assessment.