Function block diagrams (FBD) are widely used for implementation of programmable logic control (PLC) in safety critical domains and in the conventional factory automation. With the growing software intensity of such systems, the size and complexity of the PLC FBD applications is growing. The implicit execution order of PLC FBD can be ambiguous for developers, causing misinterpretation of the control programs behaviour. This work aims at reducing this ambiguity, investigating reimplementation of FBDs in a new programming language of IEC 61499, which has explicit mechanism for defining the execution order. A method is proposed for generation of IEC 61499 FBDs from the PLC FBDs. We also present a tool that implements our approach and which is complemented with an automated tester to prove the equivalence in the behaviour of the source and generated systems.