Thanks a lot , now I understand , it seems the communication topic between plc's and/or ET's is a vast topic and it needs a lot of search and reading Siemens documents , for further understanding. for example when or not to use the DP_Send/DP_Recv or load/transfer , DPWRT/DPRT , i_GET/I_PUT ........ and this is depending on what type of plc's and ET's and operating modes and via cps or not .....
I hope one day you make tutorials on this topic and highlighting the use of this commands, in projects.
And finally i want to say : Realpars is the Perfect place to learn PLC .
Thanks again Realpars .