ABSTRACT
For a long period o f time, one o f the major difficulties in the use o f computerbased automation structures was that the safety authorization agencies refused to license such structures as safe enough. For the agencies, only the discrete and semiconductor-based protection circuits have been accepted as safe security provi sions. However, the progress in computer and instrumentation hardware, as wel l as in monitoring and diagnostic software, has helped building computer-based automation structures acceptable even for applications in critical process areas, as far as they guarantee that:
• Failure o f any instrumentation element at the field level, or o f any pro grammable controller or computer at control level, does not cause a haz ardous situation.
