Industrial Ethernet networks will increasingly be used in automation and control systems. In order to integrate existing solutions based on PLCs and fieldbus connected devices, appropriate migration strategies are required. The paper describes a proxy solution for integrating existing PLC-based solutions into PROFINET IO. The PLCs may be connected to the proxy using any communication protocol, including fieldbus and Ethernet. Besides runtime and engineering aspects of the proxy, security considerations are included to support and to enhance the security cell-concept.