This book describes the design of a low complexity, fault-detecting computer architecture for utilisation in programmable logic controllers (PLCs) for process control purposes. The cyclic operating mode of PLCs and a specification level graphical programming paradigm based on interconnecting application-oriented standard software function modules are architecturally supported. Thus, by design, there is no semantic gap between the specification, programming and machine execution levels enabling the safety licensing of application software by diverse back translation, an extremely simple but rigorous method.