An Assessment of the IEC 1131-3 Standard on Languages for Programmable Controllers

Programmable Logic Controllers (PLCs) are playing an increasing role in the construction of safety critical systems. The standard IEC 1131-3 deenes a number of interrelated languages for the expression of PLC programs. Here we consider a subset of the IEC 1131-3 languages, that of function blocks, and present an assessment of the standard from the viewpoint of providing a formal semantics to that subset. In doing so, we also provide justiication for our decisions in resolving ambiguities in the standard. Finally, we comment on the overall structure of the standard and, particularly, on how it relates function blocks to the other programming elements.