InfoPath Dev

Hi I have a 2 text boxes, txtA and txtB. Also, I have secondary data source where list contains with code and description. txtA is open for user to type codes and corresponding description from list should gets display to txtB. Does anyone knows how to achieve this by using simple infopath rules? Cheers  

Need help on infopath Here my situation- I have repeating section; inside which i have created repeating table; inside which i have given user option insert new row. Each row has 5-10 fields among them I have drop down, text box and check box. Each field has several action & formatting rules on them. Every time, user add new row time, ...