Safety considerations in using IEC 1131-3 programming languages

Five PLC languages are defined in IEC 1131-3, some of which can be used within the same program. In defining a safe subset there are choices to be made as to which basic elements of the languages should form the safe kernel, and which individual languages are most appropriate. There are some safe language issues which the IEC 1131-3 languages have already addressed, for example, the use of pointers and the dynamic allocation of memory. However, to address criteria of definition standardisation, checkability, analysability and being deterministic in the formation of a safe language kernel, the following issues require addressing: the definition of parameter passing mechanisms; the control and use of direct addressing, including mechanisms for limiting the visibility of variables; the typing system to enforce strong typing; and the definition of Boolean literals and the use of overlong identifiers. In conclusion, the languages defined in IEC 1131-3 are small enough to allow definition of a safe subset. There are however some fundamental issues which need to be addressed, such as the parameter passing mechanism and the control and use of direct addressing, in formation of a safe subset