Composing dependable real time software of function blocks

Society increasingly depends on programmable electronic systems in life critical automation systems. Programs employed for purposes of safety related control must be safety licensed, which is a very difficult and hitherto not satisfactorily solved problem. To obtain trustworthy software, it is advocated to employ the principle of divide and conquer, and to orient the design of control systems at the comprehension capabilities of humans by using function block diagrams on the basis of verified libraries as the only programming method so far, allowing us to verify automation software easily and economically. Function block libraries specific to application areas can be rigorously verified with formal methods, and single application programs can be safety licensed by diverse back translation, an extremely simple, but rigorous method directly leading back in one step from object code to the graphical specification level. Augmented with timing features, function block diagrams are amenable to a priori schedulability analysis with standard methods.