How to retrieve data from secondary database

Dear Firebds.

I wish to know, How i access a MS Sql Server data from to SAP, I do understand for the secondary database I am required dbcon table configuration, my problem is this I dont know the dbcon concept and establishing connection string between secondary db and SAP ABAP program through native sql. please help me to understand this problem. Any article, code, suggestion will be the reat help of mine.

thanking you.

Naim