Hello Binit;

Many options are available to communicate with these 2 CPUs: Industrial Ethernet, Profinet, Profibus-DP, MPI, Modbus/TCP...

Do you have a preference, or any experience with any of these protocols?

I can suggest an excellent document on CPU-CPU communications with Simatic controllers, to start guiding you:

S7 communication compendium:
http://support.automation.siemens.com/WW/view/en/78028908

The first section presents generalities on the communication blocks between CPUs, and discusses notions like connections, media support for various protocols and the protocols supported natively by Siemens CPUs.

The second section is populated with Selection Aids for various communication scenarios. I have attached an example for Profinet connections between 2 CPUs, which suggests that if you want to use the integrated Profinet ports to echange data, you could choose between "USEND/URCV", "BSEND/BRCV" or "PUT, GET" communication blocks, depending on the amount of data you want to exchange and the type of connection you plan to use. "PUT/GET" blocks are single-sided client-server blocks (no connection are required on each partner) so they are simple to implement; but they are limited to 160 bytes per exchange,  whereas BSend/BRcv blocks can exchange up to 64 kBytes (and are more difficult to program for a first-timer).

Hope this helps,

Daniel Chartier