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IRC5 as profibus slave of Siemens S7 plc

Does anyone has experience in using an IRC5 as profibusslave of an S7 controller?
I am able let the communication work, but the strange thing is that I cannot use the
system function sfc14- sfc15 : read an write consistently from the slave.
It works if I use the perhiral bytes directly,  but not with sfc14-15, do I need to do something special in the gsd?