Hello, I have came accross to a very strange problem when monitoring functions in STEP7. Let's say there is function FC10 in the PLC program. This function is called a couple of times in the proram. It has input and output data. Inputs to this functions are input variables (binary contacts) I0.0 and I0.1 (example). When I open one of the function calles for monitoring/Debugging I see that these input variables (inside the function) are pulsing (going ON and OFF). Of course in the really I0.0 and I0.1 are not pulsating. I thought that at first that I need to use call up path to see what is going on. However, I wan't able to monitor this function using call uppath to see real states of the functions. What I did was the following: 1. Debug ->Operation and then set Test operation 2. Debug ->Call enviroment and then check Call up path 3. In the list I see FC10 and OB1, I check OB1 as well (FC10 is already checked). I save, go to OB1 in LAD editor right click and choose monitor. Again monitored state showing inputs to first call of FC10. I was unable to monitor inputs and outputs of FC10 in any other call. Is it possible to use call up path and monitor functions? If so, can you please exaplain me what I'm doing wrong? I have used call up path successfully with FBs, but not sure about FCs. Now back to the original problem. I have created VAT table and put all input variables to all FC calls (there are three FC10 calls). VAT shows that no input variables are pulsating. Also when monitoring in OB1 (where FC10s are called), there are no changes in input variables. But when I open FC10 for monitoring I see pulsating input variables. That is very strange and cannot explain this. Also, on PLC on input modues I see no changes on LEDs) Did anyone had similar experience? If I resolve the problem with monitoring function call that I want I'll be able to figure out what is happening inside the function. I must admit, that these pulsating variables lead me to wrong tranck when I tried to follow PLC program to determine what is happening in the process. Thank you for your help.