  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.
