Navigating with inthe items of combo box (When Dropdown state)

I am using a combo box on flexgrid ,i wrote some code to navigate to next row same column (flexgrid row=row+1) in combo keydown event,but what i want is to naviagate between the comboitems when combo box is in dropdown state without moving to next row and it should work as default when combo is in closed state

urgentl....