K-ST: A Formal Executable Semantics of the Structured Text Language for PLCs

TL;DR

This paper introduces K-ST, a formal executable semantics for the Structured Text language used in PLCs, enabling verification of compiler correctness and uncovering bugs in open-source implementations.

Contribution

The paper presents K-ST, a high-level formal semantics for ST based on IEC 61131-3, validated through extensive testing and comparison with commercial PLC compilers.

Findings

Validated K-ST with 509 programs from Github

Uncovered 5 bugs in OpenPLC compiler

Demonstrated K-ST's effectiveness in verifying compiler correctness

Abstract

Programmable Logic Controllers (PLCs) are responsible for automating process control in many industrial systems (e.g. in manufacturing and public infrastructure), and thus it is critical to ensure that they operate correctly and safely. The majority of PLCs are programmed in languages such as Structured Text (ST). However, a lack of formal semantics makes it difficult to ascertain the correctness of their translators and compilers, which vary from vendor-to-vendor. In this work, we develop K-ST, a formal executable semantics for ST in the K framework. Defined with respect to the IEC 61131-3 standard and PLC vendor manuals, K-ST is a high-level reference semantics that can be used to evaluate the correctness and consistency of different ST implementations. We validate K-ST by executing 509 ST programs extracted from Github and comparing the results against existing commercial compilers (i.e., CODESYS, CX-Programmer, and GX Works2). We then apply K-ST to validate the implementation of the open source OpenPLC platform, comparing the executions of several test programs to uncover five bugs and nine functional defects in the compiler.