Increasingly field buses are used in a variety of applications as the end users make large investments on fieldbus installations. Therefore the availability of these fieldbuses is of paramount importance in smooth plant operation. Although the fieldbus standards specify the type and format of the diagnostics data, the extent, location and sequence of diagnostics data within a controller is entirely vendor dependent. As the number of differing controller makes increase in an installation, so does the problem of accessing fieldbus diagnostics data. This research work defines a unified framework for representing the fieldbus diagnostics data in the form of a data block, and develops diagnostics Function Blocks to provide the required diagnostics data at the controller and higher levels.