Networking the field level with Fieldbuses

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.

Find out more about sensors, actuators and the field level

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:

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 protectionA 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.
StandardisationThe 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.
FlexibilityThe system can be easily expanded and sensors and actuators can be added flexibly.
RetrofittingFieldbus technology can often be well integrated into Ethernet systems, making it possible to change technology easily.

However, the advantages of the Fieldbus technology are contrasted with the disadvantages. These include longer response times and higher costs, as the individual Fieldbus components are considerably more expensive than alternative components for communication systems.

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.

Basic information on protocol standards

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.

AS-i (Actuator Sensor Interface) is the world’s only standardised Fieldbus system for the lower process level. Using just one port and only one cable with two buffered fibres, this simple connection system can network up to 31 sensors and actuators/AS-i modules with the controller level. In this context, the AS-i cable is responsible not only for exchanging data, but also for supplying power to the slaves. A cyclic transfer rate of ≤ 5 ms can be achieved here with a cable length of 100 m. As a result, this open industry standard (which is not tied to a specific manufacturer) enables fast, efficient cabling in industrial automation, which ultimately reduces costs.

The AS International Association e.V. promotes the expansion and standardisation of the AS-i protocol standard. LAPP is a member of this association and plays an active role in further developing the standard.

More complex network wiring

PROFIBUS (Process Field Bus) is a standard for Fieldbus communication, and one of the world’s current leading Fieldbuses. It enables data to be exchanged cyclically between devices on the lower field level (decentralised peripheral devices, slaves) and higher-level controller units (central masters). It has now become a key communication protocol used in manufacturing technology, as well as process technology and engineering. The master-slave system can use both twisted pair cables and fibre optic cables.

If higher speeds are required, the successor PROFINET  with its Ethernet capability can be used.

LAPP is a member of the user organisation PROFIBUS Nutzerorganisation e.V. (PNO) and plays an active role in further developing PROFIBUS.
The following two variants of PROFIBUS are primarily used.

Fieldbus communication in manufacturing technology can be handled using either a mono-master set-up or a multi-master system (where several masters are connected to a bus). The masters query input information from the slaves at cyclic intervals and return output information. Different versions of the PROFIBUS DP system are available:

  • DP-V0 for cyclic communication,
  • DP-V1 additionally for acyclic communication,
  • DP-V2 additionally for isochronous real-time functionality, use of time stamps, and slave-to-slave communication.

PROFIBUS DP offers a transfer rate of 12 Mbit/s and can combine up to 126 devices (masters and slaves) in a network.

PROFIBUS PA is used for communication in process automation applications. As the power is limited, PROFIBUS PA is ideally suited for use in potentially explosive atmospheres. The transfer rate of 31.25 kbit/s allows for long cable paths and a high degree of resistance to electromagnetic interference.

The names of the PROFIBUS-compliant products from LAPP’s UNITRONIC® range (for cables) and EPIC Data range (for PLUG connectors) include the designation “BUS DP” for PROFIBUS DP and “PA” for PROFIBUS PA.

The CANopen standard has taken the CAN (Controller Area Network) Fieldbus system that was originally developed for the automotive industry and has expanded it to include a communication profile. This makes CANopen an open protocol standard for use not only in automation technology, but also many other industries, such as medicine, railways, etc., predominantly in Europe.

In the serial CAN system, the data is exchanged in the form of telegrams on both a cyclic and event-driven basis, and the exchange is managed via the first two layers of the OSI model. The network participants with equal status independently transfer their messages to the bus at a secure data transmission rate of up to 1 Mbit/s, over a cable length of 40 m. In the process, data is exchanged in real time.

In the higher-level CANopen system, an application layer is integrated to provide a communication profile. By means of standardised interfaces, this profile is able to individually address the device and application profiles of the device classes used in the network. As a result, the communication profile controls which telegrams are used to exchange which device data.
CANopen is maintained by CiA (CAN in Automation).

In Asia, CC-Link is the leading open industry standard with pronounced deterministic characteristics, used for exchanging data between the controller and manufacturing level. It is available in different variants. In the standard variant, CC-Link is a Fieldbus system enabling a data transmission rate of up to 10 Mbit/s over a total cable length of 100 m, as well as real-time communication with up to 64 devices in the network. If the network speed is lower, the transmission distance can be extended to up to 1,200 m. This means CC-Link is also suitable for large-scale applications. The simple structure of this standard suffices for a wide range of automation technology tasks. This network technology is also increasingly being used in Europe, as CC-Link-compatible products from different manufacturers can be integrated very easily.

You can find more information about the Ethernet-based variant of CC-Link under CC-Link IE .

CC-Link is standardised by the CLPA (CC-Link Partner Association) user organisation.

LAPP is a member of the CLPA and plays an active role in further developing CC-Link.


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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.