Interoperability in HVDC systems could be supported with open upper-level control and protection (C&P) software, while hardware-near C&P functions stay black-boxed and proprietary. Methodologies like model-based systems engineering and graph theory can assist in defining the boundary between open and closed software. Most likely, partially open C&P software in HVDC is not hindered by legislation, but has to be addressed in contractual agreements. Also, a new responsibility matrix for testing is proposed.