Abstract: Control systems for industrial automation feature real-time programs embedded in a variety of machines and processes, showing complexity and safety criticality. Standardization has led to the definition of the IEC 61131 design standard for Programmable Logic Controllers (PLCs). Its formalization for purposes of analysis and design tool support is an active research topic. The synchronous approach to real-time and reactive systems has resulted in formalisms and effective tools for the compilation, analysis and verification of specifications. With the motivation to give access to the latter, this paper presents a synchronous model of the PLC programming languages ST (Structured Text) and FBD (Function Block Diagrams), based on the language SIGNAL.