Which fieldbus do you go for?

With all the fieldbus options available, choosing the right bus – or buses – for your application is no trivial task. Andy Pye provides this guide

There have emerged literally hundreds of fieldbuses developed by different companies and organisations all over the world. The term fieldbus covers many different industrial network protocols. Most fieldbus protocols have been developed and supported by specific PLC manufacturers.

At the lowest level are the sensor networks, which were originally designed primarily for digital (on/off) interface. These are fast and effective, but with only limited applications beyond relatively simple machine-control. AS-I (actuator/sensor interface) is popular in Europe, while Seriplex is a US development.

AS-I is perhaps the simplest and least expensive fieldbus. AS-I was developed by a consortium of European automation companies, which saw a need for networking the simplest devices at the lowest level. AS-I is easy to configure and low in cost. It is most often used for proximity sensors, photoeyes, limit switches, valves and indicators in applications like packaging machines and materials handling systems. AS-I is designed for small systems employing discrete I/O. It allows for up to 31 slaves, which can provide for up to four inputs and four outputs each for a total of 248 I/O.

Filbus was developed by GESPAC as a comprehensive remote I/O system, based on distributed intelligence and peer-to-peer communication. Firmware functions are built into each Filbus I/O module and allow basic capabilities such as pulse count, delay before action and sending/receiving messages to/from other modules on the network.

Bitbus was originally introduced by Intel as a way to add remote I/O capability to Multibus sytems. This origjnal fieldbus is one of the most mature and most broadly used networks today. Bitbus allows programs to be downloaded and executed in a remote node for truly distributed system configurations.

Deterministic control WorldFIP provides a deterministic and reliable scheme for communicating process variables (generated by sensors and executed by actuators) and messages (events and configuration commands) at up to 1Mbit per second on inexpensive twisted pairs cables. FIP uses an original mechanism where the bus arbitrator broadcasts a variable identifier to all nodes on the network, triggering the mode producing that variable to place its value on the network. Once on the network, all modules which need that information 'consume' it simultaneously. This concept results in a decentralised database of variables in the nodes and remarkable real-time characteristics. This feature eliminates the notion of node address and makes it possible to design truly distributed process control systems.

At the next level are the device buses, which provide analogue and digital support for more complex instruments and products. DeviceNet interfaces very well with programmable controllers. However, it is still only a local, device-level network, operating typically with up to 64 points within about 300m. Profibus DP and Interbus are somewhat comparable to DeviceNet and are excellent general-purpose device-level networks.

Profibus is a fieldbus network designed for deterministic communication between computers and PLCs. It is the most widely accepted international networking standard. Nearly universal in Europe and also popular in North America, South America and parts of Africa and Asia, Profibus can handle large amounts of data at high speed and serve the needs of large installations. Based on a real-time capable asynchronous token bus principle, Profibus defines multi-master and master-slave communication relations, with cyclic or acyclic access, allowing transfer rates of up to 500kbit/s. The physical layer (two-wire RS485), the data link layer 2 and the application layer are all standardised. Profibus distinguishes between confirmed and unconfirmed services, allowing process communication, broadcast and multi-tasking.

Profibus DP is a master/slave polling network with the ability to upload/download configuration data and precisely synchronise multiple devices on the network. Multiple masters are possible in Profibus, but the outputs of any device can only be assigned to one master. There is no power on the bus.

Interbus was one of the very first Fieldbuses to achieve widespread popularity. It continues to be popular because of its versatility, speed, diagnostic and auto-addressing capabilities. Physically, it has the appearance of being a typical line-and-drop based network, but in reality it is a serial ring shift register. Each slave node has two connectors, one which receives data and one which passes data onto the next slave.

Controller Area Network (CAN) is a fast serial bus that is designed to provide an efficient, reliable and very economical link between sensors and actuators. CAN uses a twisted pair cable to communicate at speeds up to 1Mbit/s with up to 40 devices. Originally developed to simplify the wiring in automobiles, its use has spread in machine and factory automation products.

CAN provides standardised communication objects for process data, service data, network management, synchronisation, time-stamping and emergency messages. It is the basis of several sensor buses, such as DeviceNet (Allen-Bradley), Honeywell's SDS or Can Application Layer (CAL) from CAN in Automation - a group of international users and manufacturers which is over 300 companies strong. CANopen is a family of profiles based on CAN which was developed within the CAN in Automation group.

Error detection The extensive error detection and correction features of CAN can easily withstand the harsh physical and electrical environment presented by a car. SDS was developed by Bosch for networking most of the distributed electrical devices throughout an automobile, initially for eliminating the large and expensive wiring harnesses in Mercedes. DeviceNet is a manifestation of CAN adapted for critical factory networking purposes.

At the next level are the 'control' networks, which include ControlNet, developed by Allen-Bradley and utilised by Honeywell, overlapping with some of the functionality intended to be provided by Profibus-FMS and Fieldbus SP-50; Profibus FMS uses the same physical layer as Profibus DP but allows multi-master, asynchronous, peer-to-peer communication. FMS and DP can operate simultaneously on the same network.

ControlNet was conceived as the ultimate high-level fieldbus network and was designed to meet several

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high performance automation and process control criteria. Of primary importance is the ability to communicate with each other with 100% determinism, while achieving faster response than traditional master/slave poll/strobe networks.

LONWorks operates over greater distances and is a practical peer-to-peer network, extensible to many thousands of points, though it can be comparatively slow and more complex.

Profibus PA is a specification for using Profibus in intrinsically safe applications. It provides improved performance at the fieldbus level, for instruments and controls, replacing the features and functions which are provided by HART, originally developed for transmitter calibration and diagnostics. The primary differences are in speed, complexity and distance. LONWorks also extends into this realm.

Foundation Fieldbus is a sophisticated, object-oriented protocol that uses multiple messaging formats and allows a controller to recognise a rich set of configuration and parameter information ('device description') from devices that have been plugged into the bus. Foundation Fieldbus even allows a device to transmit parameters relating to the estimated reliability of a particular piece of data. Foundation Fieldbus uses a scheduler to guarantee the delivery of messages, so issues of determinism and repeatability are solidly addressed (determinism means knowing absolute worst-case response times with 100% certainty). Each segment of the network contains one scheduler.

Dominant technology In the United States, Device-Net, Profibus DP, and Foundation Fieldbus H1 have taken their places as the dominant open systems for connecting industrial devices. DeviceNet is the leader in automotive, material handling, and semiconductor applications, while Foundation Fieldbus is taking the lead in process control. Profibus is strong in both realms, as well as being the dominant technology in Europe.

Since its introduction, DeviceNet has grown rapidly in the UK and, according to DeviceNet UK Chairman Richard McLaughlin, now boasts more than 320 companies worldwide actively developing compatible products within the Open DeviceNet Vendor Association (ODVA). These include sensors, actuators and smart devices.

Against this backdrop, frustrating for many, came the re-emergence of the Ethernet. Ten years ago, no serious design engineer would have suggested using Ethernet for networking factory floor devices. Ethernet, the technology for office automation, was developed more than 20 years ago as a high-speed serial data-transfer network. It has become a worldwide standard, with more than 85% of all installed network connections being Ethernet. But it was deliberately ignored for industrial applications, and for good reasons; its lack of determinism and robustness made it a feeble, unpredictable companion for the shop floor.

Yet today, the scene has almost reversed. Why such a radical change? Over the past few years there have been many enhancements to the Ethernet standard, especially in areas of determinism, speed, and message prioritisation. So there is no longer any reason why Ethernet cannot be used to build deterministic fieldbus networks that are cost-effective and open. And since Ethernet is already the network choice for business computing, its presence at the control level will make sensor-to-boardroom integration a reality.

Another good reason manufacturers are looking at Ethernet is the coming explosion of factory floor data traffic. As smart sensors and various devices on the plant floor eat up the available bandwidth over the next four years, manufacturing plant information generated by PLCs and control systems is expected to increase from 10 to 30 times the current level.

Ethernet, with its internet-friendly TCP/IP protocol, is ideally positioned. It is popular, plummeting in price and being propelled by sheer market demand. So Ethernet is poised to penetrate deep into the factory network hierarchy, down to the I/O level. That makes some PLC manufacturers uncomfortable. Even the recently arrived fieldbus systems are beginning to feel the impact (some say threat) of Ethernet.

Furthermore, the DeviceNet, Profibus and Foundation Fieldbus protocols are all available or in development as application layers for Ethernet. And most PLCs now offer Ethernet as a standard networking option in addition to their fieldbus of choice.

High Speed Ethernet (HSE) is a 100 Mbit Ethernet standard that uses the same protocol and objects as Foundation Fieldbus H1, on TCP/IP. The next generation of Ethernet is called Gigabit Ethernet, which is capable of 1 Gbits/sec. This will bridge the gap between the necessity of industrially hardened wiring capability and the growing need for process data via business LANs and the Internet.

Most firms cannot afford to have a DeviceNet or Profibus specialist on staff who thoroughly understands the network protocol. Even if a company could afford such a person, it is unlikely fieldbus would be their specialty. However, almost every company has a network administrator who is well versed and specialises in Ethernet protocol, making Ethernet all the more attractive for industrial control.

Q: “Industrialists have the expectation in mind that they can use Ethernet, that they are going to use it, and application barriers that present themselves can be worked through. Manufacturers are pressing for an Ethernet Plan. Industrial Ethernet will happen, and it will only be a spirited marketplace standardisation shakeout, combined with badly needed guidance from the Industrial Ethernet Association (IEA) that remain to complete its destiny.” Lance Gordon, Senior Analyst, Frost & Sullivan