Fieldbus Basics


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Fieldbus Basics

The Fieldbus systems created in the 1980s are nowadays indispensable within industry. As a fixed component of complex machinery and installations, they are primarily used in manufacturing automation. However, the fieldbus is also used in process and building automation, as well as in automotive engineering.

Sensors and actuators (so-called “field devices”) as well as motors, switches, drives, or lamps are connected with programmable logic controllers (SPS) / master and process controllers with the help of wire-bound and serial fieldbuses. As such, the fieldbus supports the rapid exchange of data between individual system components even over great distances. Even strong external loads cannot influence the robust digital signal transmission system. As the fieldbus communicates only via a cable, it has been possible to decrease the wiring considerably in comparison to parallel wiring.

A fieldbus functions in the so-called master-slave operation. While the master is in charge of control of the processes, the slave stations work the individual partial tasks.

Fieldbuses differ according to their topology (star, line, tree or ring), their transmission medium, and -depending on the type - different transmission protocols (message-oriented procedure or summation frame procedure). The individual fieldbuses also differ in regard to the reachable cable length, the max. number of data bytes per telegram and the function scope. As such, additional functions such as the alarm handling, diagnosis, and lateral traffic between individual bus participants are not possible for each fieldbus.

Examples of Fieldbuses

Interbus:

The interbus with transmission rates of up to 2 Mbps is characterised by especially high transmission security and a short, constant cycle time. It is divided into subsystems and consists of the remote bus, the installation remote bus and the local bus arranged in a ring topology. As the names already suggest, the remote bus serves to connect up to 254 subscribers which are located at large distances from each other.On the other hand, the local bus connects subscribers that are located close to each other to the system.

Profibus:

The PROFIBUS is used in manufacturing engineering and automation. It has an unlimited number of subscribers and data transmission rates between 9.6 kbps and 500 kbps. It has a hierarchical structure with the sensors/actuators levels, field levels and the process level. In master-slave operation, the token passing access procedure is used. Here, slaves may only access the profibus upon the master’s request.

In addition to the two examples cited here, there are other fieldbus variants as well as fieldbuses on an Ethernet basis such as EtherNet/IP or EtherCAT

Fieldbus: A Good Example of Communication in Action

In the case of fieldbus, the communication functions in terms of technology as well as in terms of cooperation with the involved partners. However, this was completely different when the fieldbus was first developed. Now that all parties (manufacturers, organizations, consumers) have exchanged ideas with regard to the different requirements and expectations, today there are even national and international standardizations and normalizations, as well as periodic examinations in test laboratories.

Advantages and Disadvantages of Fieldbuses compared to Parallel Wiring

Fieldbus Advantages

  • Speed : Because of the reduced wiring expenditure, fieldbus systems can be planned and installed more quickly. Fieldbuses communicate via just one cable.
  • Reliability : Short signal paths increase the availability as well as the reliability of the systems.
  • Interference reliability : Fieldbuses offer increased protection against interference, especially in the case of analog values.
  • Uniformity : Because of standardized bus protocols and unified connection technology, equipment from different manufacturers can be used and exchanged. As such, it not necessary that all individual components are from the same manufacturer.
  • Flexibility : Even expansions and changes can be executed easily and quickly with fieldbuses. This way, the systems can be adapted variably to new requirements and they can also be used in the future.

Fieldbus Disadvantages:

  • Complexity : As a fieldbus represents a complete system, qualified personnel are required for its use.
  • Costs : The individual fieldbus components are considerably more expensive
  • Hazards in case of bus faults : The guide system can be cut off from the sensors and actuators. To prevent this, redundant bus systems should be used if necessary

From the Fieldbus to the Ethernet-based Communication Network

The original aim of the introduction of the industrial Ethernet was unification of the communication infrastructure from the guide level to the sensor/actuator bus. Unfortunately, exactly the opposite occurred: At present, the number of Ethernet solutions developed until now is even higher than the number of fieldbuses. This makes a comparison of the different systems nearly impossible for the user.

When decision is made for an Ethernet-based solution, the existing fieldbus solutions can be integrated step-by-step into them. This is something which is frequently done by proxies or gateways. However, this is not easy. The actuators must be controlled with a predictable delay time. For this, the fieldbuses use deterministic access procedures such as time division multiple access (TDMA). However, the Ethernet uses a stochastic access procedure and as such, it is not deterministic.

source - kunbus