Herllo Saeed;

Look in the forum, there are many threads dealing with this subject. Use the search box at the right of this page to find them.

Ths is one of the discussions you would have found:

https://support.industry.siemens.com/tf/ww/en/posts/communication-of-s7-1200-with-s7-300-via-cpu-343-1-lean/136420?page=0&pageSize=10

Hope this helps,

Daniel Chartier