I found on Search engine of AltaVista discussion concerning PPI protocol description with contact to control.com site. I have the same problem:
Former using communication to OMRON CPM2 and changing PLC to Siemens S7/200 i don't know, how to commumicate with it. Some dll's are accessible to solve this problem, but I need only know ASCII sequence to send to COMx, witch tells PLC to send some bytes back to PC - READ partial content of V-memory and ASCII sequence to WRITE some data to PLC's V-memory (with appropriate control Char's). Thank You !
JTom.

 

the PPI protocol is functionally similar to Omron HostLink, but unlike HostLink, is unnecessarily complicated in its messaging sequence and is not free: if you look at previous discussion on this same site, you will know that Siemens asks some 3000USD just to let you have the hardcopy of the specifications, and with a lot of nondisclosure agreements to sign. Being an user of both S7-200 and Omron PLCs, i try whenever i can to use the latter, just because the protocol for communication is freely avaliable and described in detail.

If you do not want to spend your money on a ActiveX control, the only way to proceed may be to use a port sniffer while connecting the S7-200 with MicroWIN, and note the sequence of command and response. A good and free port sniffer is available at http://www.sysinternals.com, or you may connect at http://www.runmode.com, the guy here is very knowledgeable about Siemens HW and he posted an introductory document on his experience with the protocol.