Networks · Automation

Fieldbuses: common standards and their features

Fieldbuses connect sensors, actuators and controllers. The choice of a standard depends on the topology, the transmission medium, the application area and the regional distribution.

Fieldbuses are the backbone of communication in automation technology and vehicles. They connect sensors, actuators and control units, and ensure reliable transmission between participants. Their purpose: to simplify communication in complex systems and reduce wiring. They can be distinguished according to several criteria — topology, transmission medium, application areas and distribution.

The right topology for the environment

The topology describes the structure of the network in which the devices are connected, and it varies from one bus to another. The bus topology, where all devices are connected to a common line, is widespread: Modbus, CANopen and DeviceNet use it, the latter two being based on CAN. The star topology, where each device is directly connected to a central node, is offered as an option by PROFIBUS and EtherCAT. The tree topology, a combination of the two previous ones, allows a flexible structure and is also supported by PROFIBUS and EtherCAT. Some variants of EtherCAT also use a ring topology, where the devices form a closed loop.

The transmission medium

The medium used is another distinguishing feature. Copper cable is widespread and cost-effective: PROFIBUS and CAN use it. Optical fiber offers high transmission speeds and good immunity to interference, but remains less frequent — optionally in PROFIBUS, for example. Wireless media offer flexibility and easy installation, at the cost of susceptibility to interference; they are found in some variants of PROFIBUS. Repeaters and bridges also make it possible to transmit various fieldbuses optically or wirelessly over certain sections, in order to bridge difficult stretches.

Ethernet, known for its speed and wide distribution, is used by Modbus (over Ethernet), EtherCAT, POWERLINK and many other “Industrial Ethernet” networks. Note, however, that conventional switches and topology components cannot be used in these systems, as they often have high requirements for deterministic communication with low latency.

Application areas and distribution

• PROFIBUS is particularly widespread in manufacturing and process automation. Its flexibility in topology and its reliability make it very popular in Europe.
• Modbus is used in industrial automation and building automation. Its simple implementation and global distribution, especially in North America, make it a popular standard.
• CANopen, based on the CAN protocol, is mainly used in mechanical engineering and vehicle technology. Its robustness and real-time capability are particular strengths; the limited network size is a weakness.
• DeviceNet, also based on CAN, is mainly used in manufacturing automation and in North America.
• EtherCAT offers a flexible topology on an Ethernet medium. This flexibility and its speed make it popular worldwide, especially in Europe and Asia.

The decisive criteria for choosing the right fieldbus are the application area and the required data rate, real-time capability and robustness, as well as the costs of components and implementation.

In summary