Hello Greger,
Proview as of today has support for MODBUS communication on a serial line with the RTU format. It is not really implemeneted as a I/O-system like for example Profibus. You have to define the messages yourself using RemTrans-communication (general concept for all type of communication for a lot of protocols, UDP for example). The documentation for this is a little poor and also old.
You should have a look at swedish documentation for Proview v4.4 and a document called \"Kommunikation\". You could also have a look at the Object handbook and all the objects for the Remote-classes (especially the RemnodeMODBUS, RemTrans, RemoteConfig, RemTransSend and RemTransRcv). An updated documentation for this is under creation.
In the near future Proview will also have support for MODUS/TCP implemented as a real I/O-system.
/Robert