The invention relates to a system for writing a simulation program for simulating an automation installation. To facilitate the writing of a simulation program for a complex automation installation, the system comprises - means for defining a respective resource object (1, 3) for at least two components of the automation installation, wherein each of these resource objects (1, 3) has an associated program fragment (2, 4) with computer program code for simulating the functionality of the associated component and wherein each of these resource objects (1, 3) has at least one port (30, 40) whose value can be engaged and/or read in by the program fragment (2, 4) of the associated resource object (1, 2), - means for defining port data (31, 41, 61, 71) for each port (30, 40, 60, 70), which define characteristics of the values which can be interchanged on the associated port (30, 40, 60, 70), - means for identifying a first port (30) of a first resource object (1), which is provided for interchanging values with a second port (60) of a second resource object (3), on the basis of the port data (31, 61) associated with the first and second ports (30, 60), - means for defining precisely one variable which can be engaged with the values to be interchanged between the first and second ports (30, 60), and - means for integrating the program fragments (2, 4) associated with the first and second resource objects (1, 3) into the simulation program (5) and for engaging the variables with the values to be interchanged between the first and second ports (30, 60).