Fieldbus
A fieldbus is a member of a family of industrial digital communication networks used for real-time distributed control. Fieldbus profiles are standardized by the
International Electrotechnical Commission as IEC 61784/61158.
A complex automated industrial system is typically structured in hierarchical levels as a distributed control system. In this hierarchy the upper levels for production managements are linked to the direct control level of programmable logic controllers via a non-time-critical communications system. The fieldbus links the PLCs of the direct control level to the components in the plant at the field level, such as sensors, actuators, electric motors, console lights, switches, valves and contactors. It also replaces the direct connections via current loops or digital I/O signals. The requirements for a fieldbus are therefore time-critical and cost-sensitive. Since the new millennium, a number of fieldbuses based on Real-time Ethernet have been established. These have the potential to replace traditional fieldbuses in the long term.
Description
A fieldbus is an industrial network system for real-time distributed control. It is a way to connect instruments in a manufacturing plant. A fieldbus works on a network structure which typically allows daisy-chain, star, ring, branch, and tree network topologies. Previously, computers were connected using RS-232 by which only two devices could communicate. This would be the equivalent of the currently used 4–20 mA communication scheme which requires that each device have its own communication point at the controller level, while the fieldbus is the equivalent of the current LAN-type connections, which require only one communication point at the controller level and allow multiple of analog and digital points to be connected at the same time. This reduces both the length of and total number of cables required. Furthermore, since devices that communicate through a fieldbus require a microprocessor, multiple points are typically provided by the same device. Some fieldbus devices now support control schemes such as PID control on the device-side instead of forcing the controller to do the processing.History
The most important motivation to use a fieldbus in a distributed control system is to reduce the cost for installation and maintenance of the installation without losing the high availability and reliability of the automation system. The goal is to use a two wire cable and simple configuration for field devices from different manufacturers. Depending on the application, the number of sensors and actuators vary from hundreds in one machine up to several thousands distributed over a large plant. The history of the fieldbus demonstrates how these goals have been approached over time.Precursors of fieldbuses
General Purpose Interface Bus (GPIB)
Arguably the precursor field bus technology is HP-IB as described in IEEE 488 in 1975. "It became known as the General Purpose Interface Bus, and became a de facto standard for automated and industrial instrument control".The GPIB has its main application in automated measurements with instruments from different manufacturers. It is a parallel bus with a cable and connector with 24 wires, limited to a maximal cable length of 20 metres.
Bitbus
The oldest commonly used field bus technology is Bitbus. Bitbus was created by Intel Corporation to enhance use of Multibus systems in industrial systems by separating slow i/o functions from faster memory access. In 1983, Intel created the 8044 Bitbus microcontroller by adding field bus firmware to its existing 8051 microcontroller. Bitbus uses EIA-485 at the physical layer, with two twisted pairs - one for data and the other for clocking and signals. Use of SDLC at the data link layer permits 250 nodes on one segment with a total distance of 13.2 km. Bitbus has one master node and multiple slaves, with slaves only responding to requests from the master. Bitbus does not define routing at the network layer. The 8044 permits only a relatively small data packet, but embeds an efficient set of RAC tasks and the ability to develop custom RAC tasks. In 1990, the IEEE adopted Bitbus as the Microcontroller System Serial Control Bus.Today BITBUS is maintained by the BEUG - BITBUS European Users Group.
Computer networks for automation
Office networks are not really suited for automation applications, as they lack the upper-bounded transmission delay. ARCNET, which was conceived as early as 1975 for office connectivity uses a token mechanism and therefore found later uses in industry.Manufacturing Automation Protocol (MAP)
The Manufacturing Automation Protocol was an implementation of OSI-compliant protocols in automation technology initiated by General Motors in 1984. MAP became a LAN standardization proposal supported by many manufacturers and was mainly used in factory automation. MAP used the 10 Mbit/s IEEE 802.4 token bus as a transmission medium.Due to its scope and complexity, MAP failed to make a big breakthrough. To reduce the complexity and reach faster processing with reduced resources the Enhanced Performance Architecture MAP was developed in 1988. This MiniMap contains only levels 1, 2, and 7 of the Open Systems Interconnection basic reference model. This shortcut was taken over by later fieldbus definitions.
The most important achievement of MAP is Manufacturing Message Specification, the application layer of MAP.
Manufacturing Message Specification (MMS)
The Manufacturing Message Specification is an international standard ISO 9506 dealing with an application protocol and services for transferring real time process data and supervisory control information between networked devices or computer applications published as a first version in 1986.It has been a model for many further developments in other industrial communication standardizations such as FMS for Profibus or SDO for CANopen. It is still in use as a possible application layer e.g. for power utility automation in the IEC 61850 standards.
Fieldbuses for manufacturing automation
In the field of manufacturing automation the requirements for a fieldbus are to support short reaction times with only a few bits or bytes to be transmitted over not more than some hundreds of meters.MODBUS
In 1979 Modicon defined a serial bus to connect their programmable logic controllers called Modbus. In its first version Modbus used a two wire cable with EIA 485 UART signals. The protocol itself is very simple with a master/slave protocol and the number of data types are limited to those understood by PLCs at the time. Nevertheless, Modbus is still one of the most used industrial networks, mainly in the building automation field.PROFIBUS
A research project with the financial support of the German government defined in 1987 the fieldbus PROFIBUS based on the Fieldbus Message Specification. In practical applications, it proved too complicated to handle in the field. In 1994 Siemens proposed a modified application layer with the name Decentralized Periphery which reached a good acceptance in the manufacturing industry. As of 2016, the Profibus is one of the most installed fieldbuses in the world and reached 60 million installed nodes in 2018.INTERBUS
In 1987 Phoenix Contact developed a serial bus to connect spatially distributed inputs and outputs to a centralized controller. The controller sends one frame over a physical ring, which contains all input and output data. The cable has 5 wires: besides the ground signal, two wires for the outgoing frame and two wires for the returning frame. With this cable is it possible to have the whole installation in a tree topology.The INTERBUS was very successful in the manufacturing industry with more than 22,9 million devices installed in the field. The Interbus joined the Profinet technology for Ethernet-based fieldbus Profinet and the INTERBUS is now maintained by the Profibus Nutzerorganisation e.V.
CAN
During the 1980s, to solve communication problems between different control systems in cars, the German company Robert Bosch GmbH first developed the Controller Area Network. The concept of CAN was that every device can be connected by a single set of wires, and every device that is connected can freely exchange data with any other device. CAN soon migrated into the factory automation marketplace.DeviceNet was developed by the American company Allen-Bradley and the ODVA as an open fieldbus standard based on the CAN protocol. DeviceNet is standardised in the European standard EN 50325-2. Specification and maintenance of the DeviceNet standard is the responsibility of ODVA. Like ControlNet and EtherNet/IP, DeviceNet belongs to the family of CIP-based networks. CIP forms the common application layer of these three industrial networks. DeviceNet, ControlNet and Ethernet/IP are therefore well coordinated and provide the user with a graded communication system for the management level, cell level and field level. DeviceNet is an object-oriented bus system and operates according to the producer/consumer method. DeviceNet devices can be client or server or both. Clients and servers can be Producer, Consumer or both.
CANopen was developed by the CiA, the user and manufacturer association for CANopen, and has been standardized as European standard EN 50325-4 since the end of 2002. CANopen uses layers 1 and 2 of the CAN standard and extensions with regard to pin assignment, transmission rates and the application layer.