ECSAM is a method for modeling, analysis, and derivation of requirements of embedded computer systems and their software. Previous ECSAM papers described its application in the conceptual modeling of system internals, providing white box models. The current paper extends the use of ECSAM to the systematic derivation of the system's external specifications taking into consideration the behavior of the environmental systems. The application of the method described in the paper results in a black box external model, which can serve as a basis for the development of operational specifications of the system in terms of system modes and its behavioral processes. The resulting external and internal system models are inherently consistent. The external specifications are translatable into use cases and operational scenarios, which can successively be transformed into operational and test requirements. The approach is demonstrated on a simplified Home Alarm System (HAS). The approach can be automated using CAS2E tools, to systematically generate operational and test requirements from a system's black box external model.