Fieldbus

Key takeaways

Fieldbus belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Fieldbus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Fieldbus from memory before moving on to harder problems.

Reference excerpt

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 (IEC) as IEC 61784/61158. A complex automated industrial system is typically structured in hierarchical levels as a distributed control system (DCS). In this hierarchy the upper levels for production managements are linked to the direct control level of programmable logic controllers (PLC) via a non-time-critical communications system (e.g. Ethernet). 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 (serial connections) 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 (GPIB), 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 (13 bytes), but embeds an efficient set of RAC (remote access and control) tasks and the ability to develop custom RAC tasks. In 1990, the IEEE adopted Bitbus as the Microcontroller System Serial Control Bus (IEEE-1118). 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 (MAP) 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 (EPA) MAP was developed in 1988. This MiniMap contains only levels 1, 2, and 7 of the Open Systems Interconnection (OSI) basic reference model. This shortcut was taken over by later fieldbus definitions. The most important achievement of MAP is Manufacturing Message Specification (MMS), the application layer of MAP.

Manufacturing Message Specification (MMS) The Manufacturing Message Specification (MMS) 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

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Fieldbus

Start with the simplest possible case. Write down what Fieldbus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Fieldbus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Fieldbus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Fieldbus

In research: Fieldbus appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Fieldbus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Fieldbus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer-related introductions in 1988, Industrial computing, Serial buses, so understanding it makes those chapters shorter.
In everyday life: Look for Fieldbus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Fieldbus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Fieldbus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Fieldbus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Fieldbus in simple terms?

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 (IEC) as IEC 61784/61158.

Why does Fieldbus matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Fieldbus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Fieldbus.