Interbus is a Master/Slave bus system. Who is Master and who is Slave?
KUKA and Siemens also mean to swap low and high byte (on kuka side) - otherwise you will get wrong values.
What input and output modules are you using (normally only slave card and master card is needed)?
As SkyeFire mentioned:
make sure Interbus is working
proper outputs are sent and read as proper input
And after that start dealing with your EXT configuration