Jazz,
You can use a Siemens CP5511/CP5512 and Simatic Net PC card (plug and play) to connect via MPI or Profibus at any location on the bus. If the PLC is a soft PLC such as WinLC controller and is installed on a PC670 HMI for example, you can connect via a crossover cable and TCP/IP network connection. Regardless of the protocol you must first set up the interface via the Set PG/PC Interface from the Options tab in Step 7.