We will present a study regarding the security of interconnection of office IT and industrial IT: New needs; new risks due to the evolution of DCS technology; the architecture selected and tested on different sites (installation of a firewall for interconnecting the networks, how to use existing infrastructure for remote users); and the administrative problems. The particular case of PI: needs for writing back results to the DCS and the problems of doing it with a standard PI interface, and the solution we designed in order to accept it without compromising the security.