HMI (WinCC) - MPI - PLC (SIMATIC S7-300) How to communicate?

Dear All,

I have a problem in communication between WinCC and PLC.
My case is shown below:

I have written a program in STEP 7, made a HMI by WinCC 6.0and use the S7-PLCSIM to test both of these programs: the result is ok.

I have down loaded program in STEP7 (Simatic manager\...\ blocks\down load) into CPU 315-2DP (SIMATIC S7-300).The communication type is MPI.

Iuse someexternal switches to test the down loading,the lamps of CPU is on and off according to my switching, it means thatthe program is well down loaded.

My problem is thatthe HMI can not communicate with the PLC, theHMI in the screen is fail of control when I switch on and off the external switches.

I'm just a beginner in PLC programming, so would you be so kind to show me the way to solve my problem as well as how to connect HMI (WinCC) with PLC for control and accessing data.

Thank you very much!
My email: bangkokhanoi2007@yahoo.com
Your sincerely
T2007