Profinet comms diagnostics with S7 1200

[pauly]

I am looking to monitor Profinet communications to some drives from a Siemens S7 1200 plc. By monitoring I mean detection of the drives on the bus and communicating. Siemens pointed me in the direction of the GET_DIAG function. I tried this and all I get is an error code of 8082 in the RET_VAL. The explanation of the code does little to suggest the reason for it "Type in the details parameter is not supported with the selected mode"
If any body knows of any other ways of checking Profinet devices are present on the bus I would be grateful to know. Or indeed if any one has used the GET_DIAG function susccessfully I would pleased find out how it was done.
Thanks