The paper introduces a customized approach to handle failures in IEC 61131-3 programmable controllers.The solution assumes the utilization of a virtual machine as a runtime environment to execute control code in an isolated manner.A formal model of the runtime is presented, employing denotational semantics.Subsequently, the model is expanded by incorporating new procedures that enable the handling of runtime exceptions using ST code constructs.This formal model serves as the foundation for implementing the exception infrastructure in the CPDev development environment.The research presented in the paper, driven by industry demands, aims to facilitate the development of more reliable and resilient control systems, capable of effectively dealing with failures.

Denotational semantic model and its implementation in C/C++ are presented for a virtual machine executing programs written in the CPDev development environment according to IEC 61131 standard.Programs written in IEC ST language are compiled to control-oriented intermediate language designed specifically for the machine.Architecture of the machine and its operation are represented by formal semantic model which assigns abstract algebraic objects to denote machine behaviour.Execution of intermediate language instructions is described in details by denotational semantic equations followed strictly by C/C++ implementations to assure reliability of the machine.