The large and complex dependency relationship between software components is at the root of system instability. A seemingly minor fault can propagate along dependency chains and bring down the system. Therefore, the first step in the development of a robust system is the management and control of dependency relations between software components. The two contributions of this paper are: 1) a refinement of the commonly used failure semantics in the context of a real time control system; 2) the development of a Dependency Algebra. Component dependency relationships can be described in terms of a set of refined failure semantics mappings, enabling differentiation and comparison of different relationships and designs. The system-wide evaluation and tracking of these dependency relationships are performed using Dependency Algebra.