The promise
Profinet is set to become a big player in process automation

Profinet is an Ethernet-based communication standard for automation technology that has been developed by Profibus International. With several advantages over fieldbus systems, and easy integration into established IT infrastructures, it will win a considerable share of the automation market. However, Profinet lacks feature such as bus powering and intrinsic safety, so it will supplement rather than replace fieldbuses. As a result, the integration of Profibus PA and other fieldbus systems into Profinet is a priority.

The key to wide adoption of Profinet is the integration of fieldbus systems already used for process automation into Profinet. This article examines the current state of digital communications in process automation, outlines the benefits of Profinet, and explains how Profinet can be integrated into existing process automation systems. Conventional wiring carrying 4–20 mA signals, and often Hart digital communications, still dominates process automation.

Even though fieldbus technologies have grown fast in recent years, in this sector fieldbuses are still not used anything like as widely as in factory automation. According to Profibus International, out of around 15.4 million Profibus nodes installed by the end of 2005, just 2.8 million are used in process automation, and only around 530,000 of these are associated with the PA profile [1].
Fieldbuses have been slow to catch on in the process industries for two reasons. First, many process applications have very high reliability requirements. Second, the lifetime of a process plant is usually at least 10 years—several times that of a factory automation system. As a result, investment decisions in process automation are relatively conservative, and any discussion of the future of Profinet must take account of this.
Introducing Profinet
Profinet [2] is an umbrella term for two technologies: Profinet CBA and Profinet IO. Profinet CBA (Component Based Automation) divides an automation system into individual components which communicate via Ethernet. Profinet IO is used for input/output data. Devices are classified into three groups: an IO Controller, an IO Supervisor, and IO Devices. Communication setups are based on layer 2 (MAC addresses) and UDP/IP [3]. Because it is based on industrial Ethernet, Profinet can be integrated into existing Ethernet infrastructures. This makes it easier to link the systems used for production, manufacturing execution (MES) and management, and simplifies the introduction of technologies such as e-mail and web services into future automation devices. The functional scope of Profinet is significantly greater than that offered by current fieldbus protocols. In addition to the component model it allows time-equidistant transmission of IO data (Isochronous Real Time (IRT) communications) for demanding tasks such as drive control. Other advantages include special functions for redundancy and safety-related issues, and the ability to adjust the frequency of cyclic data transmissions. Thanks to these benefits, the coming years are expected to see a great increase of the use of Ethernet-based communication systems, and particularly Profinet [4].
Special needs of process automation
The fieldbus systems established in process automation have, however, features that are lacking from their Ethernet-based competitors. At the physical level, these include the ability to power field devices from the bus (the Manchester Bus Powered or MBP system), intrinsic safety and other options for potentially explosive atmospheres, and robust and flexible network topology (linear, star or tree).
In factory automation, the control application is usually able to handle IO data by accessing each controller’s memory directly. Process automation has more complex, system-specific, requirements for both IO data and diagnostics. As a result, control strategies in process automation are engineered using standardized function blocks that are individually configured to suit specific field devices. Profinet does not have the ability to do this alone, so it will have to rely on functionality within fieldbuses such as Profibus PA.
Integrating Profibus PA and Profinet
Though Profinet is not yet capable of replacing established fieldbuses, many signs point to more widespread use of Profinet in process automation. Most process plants, for instance, have areas such as storage, packaging, and transport, that include a high proportion of factory automation components (hybrid applications) and so are ideal for Profinet [5].
The future of Profinet lies in two phases, the first of which is the integration of Profibus PA into Profinet and other fieldbus systems. Integration is essential, because existing fieldbus systems will continue to be used even in new plants. The established fieldbuses have features including bus power, versatile topology and intrinsic safety, as well as a wide range of existing field devices, that Profinet will not be able to match quickly. Integration will protect existing investments and eventually provide a migration path to Profinet.
Figure 1 shows how field devices based on Profibus PA could be linked by Profinet to other devices, terminals and controllers. The technical specifications for such integration are currently being drafted in Profibus User Organisation working groups. Profinet will also be important as a communications backbone in automation systems.
The long-term scenario is to see Profinet-capable devices in process automation. The first of these are likely to be those with general factory as well as process applications, such as flowmeters and analyzers, equipped with the Profibus DP RS-485 interface. The use of Profinet for low-cost devices such as temperature transmitters is much less likely from today’s viewpoint.
Developments such as wireless communications and solutions for data security will aid the spread of Profinet. It will also be necessary to draft specifications for Profinet-capable process devices, in the same way that Profibus application profiles have been developed for PA devices.
In summary, Profinet will be particularly useful for automation systems that include both factory and process components. It has several advantages over fieldbus systems, and above all the ability to be easily linked to management-level IT systems. The fact that Profinet lacks some features of fieldbuses that are required in process automation means that Profinet will not replace fieldbuses, but will work alongside them. For this reason, and to protect existing investments, the current priority is seamless integration of fieldbuses into Profinet. In the long run, Profinet-capable process devices will be available alongside established fieldbus devices.