Jochen Müller and Christian Diedrich outline some of the PROFIBUS PA profile extensions of recent years and focus on how the new Version 3.20 targets simplified handling of fieldbus technology.2
PROFIBUS PA field devices provide many innovative functionalities compared to devices with 4-20 mA technology. In addition to enhanced measuring and actuating properties, the diagnostic capabilities and information regarding device management, in particular, open up the potential for comprehensive plant asset management to a significant extent. PROFIBUS provides powerful communication services for transporting a wide variety of field device information to instrumentation and control systems.
The application profile for PROFIBUS PA devices (PA profile) is the standard for field devices in process automation. It defines vendor-neutral device parameters and functions for a wide range of device types and, thus, forms the foundation for uniform applications, simplified engineering, and increased reliability of production plants based on standardised diagnostic information.
In recent years, PROFIBUS PA has developed solutions in response to the additional fieldbus requirements posed by the market (Fig.1). The requirement to use PA devices in safety-related applications led to the development of PROFIsafe for PA in 2004. Based on initial experience from industrial use, the PROFIsafe technology is proving its acceptance and effectiveness in the field.
The market requirement to implement NAMUR Recommendation 107 on self-monitoring and diagnostics for field devices, in 2005, was also satisfied in PROFIBUS PA through fast, on-time development of a solution. This involves a standardised mapping of diagnostic information in field devices onto descriptive categories applicable across vendors (failure, functional check, out of specification, maintenance required).
The growing importance of plant asset management was one of the triggers for the specification of identification and maintenance functions in 2004 which enable uniform identification of field devices across vendors. Strengths are revealed in the field, for example, when synchronising installed field devices with the plant documentation or warehouse/order data and for cross-vendor access to maintenance-relevant device data and more.
Simplified device integration is based on cross-vendor specifications for compatibility of description files and field devices. The description file and device are compatible if they both have a common set of functionalities. Thus, rules have been defined for further development of device software and its effects on compatibility and designation of the device version and the device description. The validity of these rules is checked as part of the PROFIBUS certification.
The uniform designation is achieved, on the one hand, through standardised parameters stored in the device description and device. This enables integration tools to automatically assign a description file to the device, thereby simplifying engineering procedures (such as during initial installation of a field device). Likewise, as a result of the standardised designation, a rule exists for checking the compatibility of the device and description file, in order, for example, to prevent an incorrect assignment from causing maloperations during a device replacement.
In addition, the designation is made on the device housing in an easy-to-understand manner, so that the device can be explicitly assigned to description files even when it is de-energised. This is relevant, for example, for a device replacement in which a device is taken from the warehouse.
When vendor-specific description files were used, the user was often instructed that an identical field device must be used, thus precluding innovations in device technology development. The new Profile 3.02 now bridges this gap and standardises the ability of devices to take on various roles automatically, ie different device versions.
Simplified device integration is also achieved by the mandatory mapping of diagnostics information of a PA field device onto the cross-vendor categories in NE107. The manufacturer-specific diagnostic information is already mapped in the field device by the manufacturer, so that only the standardised information has to be integrated into the higher-level system, for example overall control system, maintenance system. As a result, changes in the integrated diagnostic information do not result in additional effort in the case of device replacement. A further distinguishing characteristic of PROFIBUS PA compared to other fieldbuses used in process automation is the optimisation of transfer time for upload/download of parameters.
Different quantities of data must be transferred, depending on the phase in the life cycle of a field device. Thus, a portion of the parameters must be adapted during commissioning, a portion during any maintenance activities, and a large portion during device replacement. Depending on the functionality of the devices, it is not unusual for there to be as many as 300 parameters, so that the transfer time of parameters increases in importance.
The new PA Profile 3.02 optimises the transfer by grouping parameters and providing for simplified and efficient access.
Christian Diedrich is leader of the PNO Technical Committee "Communication Profiles" and is based at the Institut für Automation und Kommunikation (IFAK) in Magdeburg, Germany. www.ifak.eu Jochen Müller is Leader of PNO Working Group "Marketing PROFIBUS PA and works with Endress+Hauser Process Solutions, Reinbach, Switzerland. www.process-solutions.endress.com
