Siemens PLC

Hello every body !!!!
I'm a new member. I have a problem when using Visual Basic to comunicate PLC with PC.I don't know the way to open the serial port, so PLC can't read data from PC and PC can't read data from PLC.
If yuo know and care that problem, please show me !!!
Thank you for your help !!!

Are you talking about S7-200 or S7-300/400 ?

I dont know so much about S7-200.

For S7-300/400 you need some hardware that connects your PC with the MPI port of the PLC.
And you need some kind of library to make the connection from your program to the PLC.
Siemens has Prodave which is a DLL library.
There are also various OPC servers.