Safety Related Real Time Programming

• Published in: IFAC Proceedings Volumes Vol. 33; no. 7; pp. 63 - 68
• Main Authors: Halang, W.A., Tsai, G.
• Format: Journal Article
• Language: English
• Published: 01.05.2000
• Subjects: Safe controllers; safe languages; safety integrity levels; software verification; safety integrity levels; software verification; Safe controllers; safe languages
• ISSN: 1474-6670
• DOI: 10.1016/S1474-6670(17)39934-2

Programs employed for purposes of safety critical control must be verified rigorously, i.e., subjected to formal safety licensing, which constitutes a very difficult and hitherto not satisfactorily solved problem. The essential issues and fundamental principles of safety related programs and computer applications are elaborated, and the importance of the human element in their development process is pointed out. At any time, utmost simplicity should be strived for, and self-discipline should be exercised. To each of the four safety integrity levels as defined by 1EC 61508-1 is assigned, respectively, a set of static and inherently safe language constructs, as well as a typical programming language or method, whose syntax enforces observation of the prevailing restrictions and rules. This is done in accordance with simplicity and comprehensibility of the verification methods available for the selected programming paradigms to meet the requirements of the individual safety integrity levels and, thus, the trustworthiness of the corresponding results. The programming methods cause/effect tables and function block diagrams on the basis of verified libraries assigned to the two upper safety integrity levels SIL 4 and SIL 3 are the only ones so far allowing, at the present state of the art, to verify automation software, which has to meet high safety requirements, in easy and economic ways. For the lower safety integrity levels, textual languages are introduced, viz., for SIL 2 a partial language enabling formal program verification, and for SIL 1 a static language with safe constructs for asynchronous multitasking. To formulate sequential function charts, an inherently safe language is defined.