Hi,
I'm currently working on a model, where I am using an input via a CombiTimeTable via an external file. These are volume flow values for certain times. When the values change, the model linearly interpolates between them. This is working fine so far. The problem is that I also need the derivative of these values. The system should be differentiable 1 time.
Just setting der(input_variable) doesnt work, so I tried to do it with annotation(derivative = function_name). Without the CombiTimeTable it worked well but when using it I allways get an error while compiling:

Warning: Failed to differentiate equation pumpExternal1.inp = combiTimeTable1.offset[1] + Modelica.Blocks.Sources.CombiTimeTable.tableTimeIpo(combiTimeTable1.tableID,combiTimeTable1.columns[1],time)
Warning: Failed to differentiate equation when initial() then
combiTimeTable1.tableID = Modelica.Blocks.Sources.CombiTimeTable.tableTimeInit(if  NOT combiTimeTable1.tableOnFile then "NoName" else combiTimeTable1.tableName,if  NOT combiTimeTable1.tableOnFile then "NoName" else combiTimeTable1.fileName,{{}},combiTimeTable1.startTime,combiTimeTable1.smoothness,combiTimeTable1.extrapolation)
end when
Error: Failed to reduce the DAE index.

error: No executable generated.C:/temp/sme_1404806824_13977.exe
error: No settings file generated.C:/temp/sme_1404806824_13977_init.sim


Note that I'm using Wolfram System Modeler. I'm not sure whether this behaviour is modelica oder programm specific.