UniSim is a graphic tool that can be used to develop and validate portable automation software, which fully complies with the IEC 61131-3 standard. UniSim is based on a graphical editor and on a simulation engine which allows to perform off-line validation of the automation software. Its simulation engine can also be interfaced with off-the-shelf I/O boards, in order to fast prototype automation systems by using a personal computer. Hardware-in-the-loop validation can be performed interfacing the prototype with the real plant or with an emulator. Furthermore, UniSim adopts the new XML Formats for IEC 61131-3, which provides an open interface for data exchange among different systems. Thanks to this choice the code developed with UniSim can be easily ported among different commercial platforms, increasing software reusability. Although the current release of UniSim is a work in progress version, it is already available and it can be effectively used to develop small and medium automation size projects.