On change of a DropDownList assign a value to a textbox

Hello, I have a DropDownList feeded by a sqldatasource and I'd like when the user clicks on it and select an option from there should assign some value to a textbox, but it's not doing it.

This is my DropDownList info:

<asp:DropDownList ID="cmbPadre" runat="server" DataSourceID="SqlDataSourcePadre"

DataTextField="Modulo" Go to the complete details ...