A structured text to MISRA-C translator and issues with IEC 61131-3 standard

Programmable Controllers (PLC) are being increasingly used in control systems for Nuclear Power Plants. A PLC, qualified for safety applications, is attractive because it saves a lot of time in verification and validation effort as compared to development of embedded control systems. We developed a tool, ST to MISRA-C translator, required for developing application program using Structured Text (ST) in a qualified PLC. The syntax and semantics of ST programming language is defined in the IEC 61131-3 standard. During the development of the translator, we found some issues with ST language syntax and semantics not pointed out by other researchers. These issues are discussed in this paper along with proposed changes in the ST language grammar for its unambiguous parsing.