Using the FM POSTING INTERFACE DOCUMENT

Hello

Im very upset  the fact that i dont want to develop a BDC  and use instead the function module

   POSTING_INTERFACE_DOCUMENT

It is working perfect  as long as you only get  crédits and débits    40  and 50 posting key

but as soon as  special account   lets say posting key 29  and  the field UMSKZ  having a value    for special accounts

then everything  is not working.

I wrote a BDC and it is working fine because i can tell what i want

like  if  umskz  = 'U'

           call screen 303   with date  zfbdt

       else

           call screen 300

I do not know if there is a way to control this

Like a said i run thousands of trans with normal accounts   and the FM is perfect and fast

But im resigned to use the BDC

So im thinking that im missing some knowledge of the FM Capabilities

and i need your valuable help

Thanks

Tahar