Anton,
The RS-232 ports are not isolated.
The RS-485 port (per page 20 of the cFP-20xx Quick Start Guide http://www.ni.com/pdf/manuals/323334b.pdf ), has a maximum voltage rating of 100 Vrms and a transient overvoltage rating of 740 Vrms.
Regards,
Aaron
LabVIEW Champion, CLA, CPI