Industrial networks can be complex. As early as the 1980s, it was clear that digital control of a system required communication that was as error-free as possible between the system participants. To gradually automate a system, as many devices as possible should therefore be connected to the system control. However, as many devices as possible usually meant a very high amount of cabling work, as the wiring was always carried out in parallel and all participants were individually wired to the control.
The desire for serial cabling continued growing. The Fieldbus technology introduced at the time therefore communicated via just one cable and transferred information – in the form of bits – one after the other, rather than in parallel.
A Fieldbus is a bus system that connects sensors and actuators to exchange information with a control computer (PLC). This means that a data connection from a PLC to different device participants is created using bus cables.
Most Fieldbuses are based on the master-slave process. The master is responsible for process control, while the slaves perform the communication tasks assigned to them.
In Part 1, the international standard IEC 61158 (Industrial communication networks – Fieldbus specifications) defines the concept behind Fieldbuses and, in its other parts, standardises the various systems that can be set up with Fieldbuses.
A convenient solution: the benefits of Fieldbus technology
Fieldbuses only communicate via ONE cable. This offers the following advantages:
| Advantage | Explanation |
|---|---|
| Simplicity | Communication is carried out via ONE cable, which makes the planning project for the installation of a Fieldbus system quick and easy. A single cable means less installation work and simplified control cabinets. |
| Reliability | The short signal paths within the Fieldbus system increase the reliability of communication. The master-to-slave communication path almost automatically creates collision protection. |
| Fault protection | A Fieldbus system offers increased protection against interference, especially with analogue values. In addition, due to the simplicity of the system, troubleshooting can be easily implemented in the event of faults. |
| Standardisation | The protocols for the Fieldbus systems are standardised and can work with different devices from different manufacturers. As a customer, you do not need to commit to a single manufacturer. |
| Flexibility | The system can be easily expanded and sensors and actuators can be added flexibly. |
| Retrofitting | Fieldbus technology can often be well integrated into Ethernet systems, making it possible to change technology easily. |
The right choice: properties of Fieldbuses
A Fieldbus system is rarely selected based on technical criteria, but rather on the type of programmable logic controller (PLC) used. Every PLC manufacturer therefore favours and optimises Fieldbus technology for their applications so that sensors and actuators can exchange information without any interference or delay.
All Fieldbuses have the same basic function of cyclic transmission of input and output data. The technical differences between the various Fieldbus systems are:
- Maximum achievable cable length
- Maximum number of data bytes per data package
- Scope of functions
- Topology shape (e.g. star, ring, tree)
- Transmission medium (copper, optical fibres, wireless)
Package-oriented communication via Fieldbus protocol standards
In order for field devices to be able to talk to their controllers and vice versa, a common language is required. The framework conditions for fault-free communication are defined and standardised in what are known as (network) protocols.
Depending on the technical requirements that a network must meet in your application environment, different bus systems can be used.
The following sections consider simple S/A wiring (which connects sensors and actuators with one another), as well as more complex Fieldbuses (which establish a network between decentralised peripheral devices and controller devices).
Find out more about the key protocol standards for Fieldbus technology below:
Simple sensor/actuator wiring
If you need to connect conventional sensors and actuators to superordinate network levels, copper S/A cables as well as a range of different bus components are generally available. In the case of simple sensor/actuator wiring, only simple voltage levels or currents are transmitted. There is no package-oriented communication via protocols.
More complex network wiring
As a smart extension: IO-Link
IO-Link is the first globally standardised IO technology and is used for communication between sensors and actuators. This technology is internationally standardised and defined in the IEC 61131-9 standard.
The technology is based on point-to-point communication using a 3-conductor sensor-actuator connection. The IO-Link is not a Fieldbus, but rather a connection technology. This is Fieldbus-independent and can be integrated into all Fieldbuses. An IO-Link system consists of a master and one or more devices (sensors and actuators). The master acts as an interface to the central PLC and controls the connected devices. Due to bidirectional communication, IO-Link enables advanced diagnostics of sensors and actuators. This makes it possible, for example, to remotely maintain devices.
IO-Link is characterised by its high transmission speed of 4.8 kBaud, 38.4 kBaud and 230.4 kBaud. The data is quickly sent one after the other using the IO-Link protocol. The connection technology requires little space and thus enables the miniaturisation of smart communication between the sensor and actuator.