In addition to Raf's comment, if you are creating a delta datasource, then below will not work too as update mode your datasource supporting is 'F' = full load.
CASE i_updmode.
WHEN 'F'.
Also you should copy the RSAX_BIW_GET_DATA_SIMPLE function as the one you used is for F1 interface which generic datasource do not support.
You will not have i_upmode as input parameter at all, so it is of no use.
Thanks
Amit