To a certain exent, you can monitor whether a custom function has been executed by checking on the color of the {CusF} symbol and the last executed function number shown on the online monitoring screen. The most sure footed check of whether a custom function has been executed is to put a "PAUSE" statement where you want the program to halt so that you can examine the values of the internal variable at that instant. The online monitoring continue to work even though the PLC program is halted. You can resume execution by releasing it from the Pause state.
