Hi David,
It appears that you are trying to create a FM for generic datasource.
Could you let us know if you used the sample FM 'RSAX_BIW_GET_DATA_SIMPLE' provided by SAP for this?
Basically E_T_DATA must be declared in TABLES tab and it must be of same structure as you provide in the extraction structure field in RSO2 when creating generic datasource.
For example: If I am creating a datasource ZDS based on FM and my extraction structure name is ZEXTRACT and extraction function module name is ZEXTFM.
Then in my ZEXTFM under tables tab E_T_DATA must of type ZEXTRACT.
Can you share code of your MYZFM then we can be more specific about the error?
There are lot of documents present in forum where you can read about the generic datasources based on FM.
Please let me know if you have any questions.
Cheers...Amit