Fieldbus
A fieldbus is a member of a family of industrial digital communication networks[1] used for real-time distributed control.
The fieldbus[2] links the PLCs of the direct control level to the components in the plant of the field level such as sensors, actuators, electric motors, console lights, switches, valves and contactors and replaces the direct connections via current loops or digital I/O signals.
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.
"It became known as the General Purpose Interface Bus (GPIB), and became a de facto standard for automated and industrial instrument control".
Use of SDLC at the data link layer permits 250 nodes on one segment with a total distance of 13.2 km.
[6] Office networks are not really suited for automation applications, as they lack the upper bounded transmission delay.
The Manufacturing Message Specification (MMS) is an international standard ISO 9506[8] 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.
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.
In 1979 Modicon (now Schneider Electric) defined a serial bus to connect their programmable logic controllers (PLCs) called Modbus.
In 1994 Siemens proposed a modified application layer with the name Decentralized Periphery (DP) which reached a good acceptance in the manufacturing industry.
[11] In 1987 Phoenix Contact developed a serial bus to connect spacially distributed inputs and outputs to a centralized controller.
In process automation traditionally most of the field transmitters are connected over a current loop with 4-20 mA to the controlling device.
This allows not only to transmit the measured value with the level of the current, but also provide the required electrical power to the field device with just one two-wire cable of a length of more than a thousand meters.
[15] A special standard for instrumentation IEC/EN 60079-27 is describing requirements for the Fieldbus Intrinsically Safe Concept (FISCO) for installations in zone 0, 1 or 2.
The study led to the European Eureka initiative for a field bus standard in June 1986 that included 13 partners.
FIP has lost ground to Profibus which came to prevail the market in Europe in the following decade - the WorldFIP homepage has seen no press release since 2002.
The two-wire bus cable of these segments takes over not only the communication, but also the power supply of the participants (MBP transmission technology).
The market of building automation has also different requirements for the application of a fieldbus: The BatiBUS defined in 1989 and used mainly in France, the Instabus extended to the European Installation Bus (EIB) and the European Home Systems Protocol (EHS) merged in 1999 to the Konnex) (KNX) standard EN 50090, (ISO/IEC 14543-3).
[19] Going back to the 1980s, unlike other networks, LonWorks is the result of the work of computer scientists from Echelon Corporation.
The BACnet standard was initially developed and is now maintained by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) starting in 1987.
The result of this meeting was the initial form of the IEC 61158 standard with eight different protocol sets called "Types".
[22] There were many competing technologies for fieldbuses and the original hope for one single unified communications mechanism has not been realized.
[23] The experts have decided that the structure of IEC 61158 is maintained according to different layers, divided into services and protocols.
In order to find the corresponding subpart of the individual parts of the IEC 61158 standard, one must know the corresponding protocol type for a specific family.
Depending on the actual profile the safety protocol does provide measures like counters, CRCs, echo, timeout, unique sender and receiver IDs or cross check.
In the case of our example for the PROFIBUS-DP, the specification of the relevant standards would therefore have to be Compliance to IEC 61784-1 Ed.3:2019 CPF 3/1 Requirements of fieldbus networks for process automation applications (flowmeters, pressure transmitters, and other measurement devices and control valves in industries such as hydrocarbon processing and power generation) are different from the requirements of fieldbus networks found in discrete manufacturing applications such as automotive manufacturing, where large numbers of discrete sensors are used including motion sensors, position sensors, and so on.
Already in the year 2000 the International Electrotechnical Commission (IEC) decided that a set of controller-device interfaces (CDIs) will be specified by the Technical Committee TC 121 Low-voltage switchgear and controlgear to cover the device networks.
It is difficult to make a general comparison of fieldbus performance because of fundamental differences in data transfer methodology.
In the comparison table below it is simply noted if the fieldbus in question typically supports data update cycles of 1 millisecond or faster.
As of 2008[update], in process control systems, the market is dominated by Foundation Fieldbus and Profibus PA.[41] Both technologies use the same physical layer (2-wire Manchester-encoded current modulation at 31.25 kHz) but are not interchangeable.
