Inom processindustrin får fältbussar en allt viktigare funktion. Moderna distribuerade styrsystem kopplas ihop med avancerade ändnoder såsom smarta ställverk och frekvensomriktare, som i många fall har egna inbyggda styrsystem. Detta ställer höga krav på säker och tillförlitlig datakommunikation inom en processindustri. Idag används till största del två sorters fältbussteknologier: Seriell kommunikation via RS-485 samt paketbaserad kommunikation baserad på IEEE 802.3, även mer känt som Ethernet. Seriella fältbussar dominerar marknaden än idag trots att det är en föråldrad teknologi jämfört med Ethernet. Anledningarna till detta är flera: Då seriella fältbussar har varit i drift inom processindustrin i över 25 år så är det beprövat och kompetensen att underhålla och felsöka systemet finns ofta ute på industrierna. Driftsäkerhet prioriteras högt och tröskeln att gå över till ett nytt system är hög. En annan anledning är att processerna som styrs generellt sett är långsamma, oftast räcker cykeltider på 1s, samt att datamängden som överförs är relativt liten, så den långsammare transmissionshastigheten i en seriell fältbuss har inte utgjort någon begränsning. Ethernetbaserade fältbussar besitter dock intressanta fördelar över seriella men har även begränsningar som måste beaktas. I början på denna uppsatsundersöks en processindustris generella krav på en fältbuss. Därefter studeras för- och nackdelar med ethernetbaserade fältbussar samt det undersöktes om ett av de protokoll som används för fältbusskommunikation över ethernet, Profinet IO, levt upp till de krav som ställts. En fördel som framkommit med ethernetbaserade fältbussar är att det går att upprätta väldigt robusta nätverk med de protokoll som finns att tillgå. En nackdel är svårbestämd fördröjningstid på grund av best effortprincipen i paketförmedlade nätverk. Profinet IO löser detta genom synkron dataöverföring direkt ut på länklagret.Avslutningsvis så genomfördes ett praktiskt upprättande av en ethernetbaserad fältbuss där ett styrsystem kommunicerar med en frekvensomriktare över ett redundant ringnätverk. / Fieldbuses plays an increasingly more important function in the process industry. Modern distributed control systems coupled with advanced end nodes such as "smart" substations and frequency converters which in many cases have their own internal control systems. This places high demands on secure and reliable data communication within a process industry. Today mainly two types of fieldbus technologies are used: Serial communication via RS-485 and packet based communication based on IEEE 802.3, also known as Ethernet. Serial field buses dominate the market today, even though it is an obsolete technology compared to Ethernet. There are several reasons for this: Serial field buses have been in operation in the process industries for over 25 years, it a proven technology and the skills to maintain and troubleshoot the system are often found out on the floor. Process industries prioritize reliability and the threshold to move to a new system is high. Another reason is that the processes that are controlled are generally slow, usually a cycle time of 1s is sufficient, and that the amount of data transferred is relatively small so the slower transmission rate in a serial field bus has not been a limitation. Ethernet-based field buses, however, possess interesting advantages over serial buses but also has limitations that must be considered. At the beginning of this paper the process industry's general requirements on a fieldbus is examined. Then the pros and cons of Ethernet-basedfield buses are studied. After that it was investigated if one of the protocols used for fieldbus communication over Ethernet, Profinet IO, lived up to the requirements. One advantage that emerged with Ethernet-based field buses is that very robust networks can be established with the protocols that are available. A disadvantage is that it’s difficult to determine the delay due to the best effort principlein packet switched networks. Profinet IO solves this by synchronous data transfer directly out the link layer. Lastly there’s a practical implementation of an Ethernet-based fieldbus, where a control system communicates with a frequency converter over a redundant ring network.

Fieldbuses are defined as a family of communication media specified for industrial applications. They usually interconnect embedded systems. Embedded systems exist everywhere in the modern world, they are included in simple personal technology as well as the most advanced spaceships. They aid in producing a specific task, often with the purpose to generate a greater system functionality. These kinds of implementations put high demands on the communication media. For a medium to be applicable for use in embedded systems, it has to reach certain requirements. Systems in industry practice react on real-time events or depend on consistent timing. All kinds are time sensitive in their way. Failing to complete a task could lead to irritation in slow monitoring tasks, or catastrophic events in failing nuclear reactors. Fieldbuses are optimized for this usage. This thesis aims to research fieldbus theory and connect it to industry practice. Through interviews, requirements put on industry are explored and utilization of specific types of fieldbuses assessed. Based on the interviews, guidelines are put forward into what fieldbus techniques are relevant to study in preparation for future work in the field. A discussion is held, analysing trends in, and synergy between, state of the art and the state of practice. A strong momentum is identified. The traditional communication media Ethernet, not originally intended for time-sensitive industry appliances, are expanding throughout the field, both in research and, maybe most interestingly, in practice. It is mainly motivated through qualities of somewhat lesser technical significance. A plethora of methods have emerged trying to optimize Ethernet for real-time purposes, each one resulting in some drawbacks, which are in turn addressed. In the end of this paper, the large-scale trend of Real-Time Ethernet is questioned and discussed.