i posted a thread earlier about changing STL to LAD, i want to create a OB just for my use, not to be downloaded in the PLC. even if it was, it won't be called, so it wouldn't affect anything, i think. my question is, i know siemens uses certain OB's, FB's, FC's and so on for certain functions, am i free to create any OB i want? my program is using OB 1, 80, 82, 85, 86, 87, 121, 122. i want to use OB4, is this gonna cause any problems? thanks!