The Siemens CP supports socket communications which is one open method I have used in the past with other native socket devices, but the 5/05 only supports AB protocols. Another option (depending on your application) might be to use a common HMI which communicates to both processors and can "bridge" the two. The best way I have found is to put a Woodhead (SST) PROFIBUS module in the 5/05 rack and use PROFIBUS between the two CPUs. The PROFIBUS option costs about as much as a gateway and offers an easy to configure and high speed communications link. I know PROFIBUS is not the Ethernet link you are looking for but it does work good.