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E-Zine April 2006PROFIBUS originated from the early work of the IEC fieldbus standards committee as a proposal for fieldbus protocol in 1989. PROFIBUS-PA adopted the well-defined Physical Layer of IEC 61158 part 2 that is identical to that of FOUNDATIONT Fieldbus H1. The upper layer (H2) for PROFIBUS-PA is PROFIBUS-DP. The details of node characteristics for PROFIBUS-PA are defined in a data record called a GSD (Gerätestammdaten: equipment master data). PROFIBUS-PA nodes performs local signal conditioning or the processing of local measurement data using attributes specified in a local database called an Electronic Device Description (EDD) provided for each device by its supplier. The EDD is written to a specification created by the PNO. Both GSDs and EDDs are available for download from the PROFIBUS web site (PROFIBUS.com). GSDs are structured text files describing the instrument itself, such as its range, data structure, options supported, supplier, and model number. Typically, the GSD is used by display (HMI) software to provide device information. Typically, no configuration information is contained in the GSD. EDDs are structured text files that contain configuration information written in Electronic Device Description Language (EDDL). This language was changed in 2004 to conform to IEC 61804-2 that is shared by both HART and FOUNDATIONT Fieldbus. Most of EDD has been taken from the experience of FOUNDATIONT Fieldbus with its Device Descriptions, which were based on similar constructs from HART. Previous attempts by the PNO to expand GSDs into Device Descriptions have been discontinued. Suppliers are developing the new EDDs and placing them on the PROFIBUS web site. PROFIBUS can utilize an instrumentation specification method called Field Device Tools (FDT) that is loosely related to the EDD. While EDD defines the exact syntax for a database structure for the field instrument, FDT calls for another data structure called a Device Type Manager (DTM) that defines the field device attributes to the host system. The FDT/DTM data structure is built to allow host systems to more easily define the database necessary for intelligent field devices. The user uses the device configuration tools that may internally use EDDL or FDT/DTM methods. The most important element is that the configuration software and the intelligent field device must use the same root EDD specification. PROFIBUS-DP is the upper layer for PROFIBUS-PA networks. DP/PA Linking Devices are available to allow a single PROFIBUS-DP network to aggregate the data collected by many PROFIBUS-PA network segments, and to distribute the control signals from the unit controllers to the control valves on the PROFIBUS-PA segments. Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control
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