Communication between S7-400 and S7-300 via PROFIBUS DP

Hello tan;
If one of the CPUs can be set as a Profibus-DP slave, then it is very easy, follow the following example from Siemens Support:
http://support.automation.siemens.com/WW/view/en/6518822

If both must be masters (they already have DP slaves configured in your project, for example) then consider using a DP/DP coupler.This special slave has 2 seperate slave interfaces, one for eah master's network, and can exchange data between the 2 masters (up to 244 bytes in, 244 bytes out).