Fieldbus & Industrial Networking


Emerging communications standard for rugged domains

Jan 2000 Fieldbus & Industrial Networking

The Lontalk protocol from Echelon Corporation in the USA has been approved by ANSI and the IEEE as an industrial standard for control networks. This protocol supports all 7 layers of the ISO Open Systems Interfacestandard,and is hence very suited to multiple media communicators. Lontalk is hence a natural candidate for extending field networks into rugged domains. Apart from this, the Lontalk protocol and the associated Lonworks technology offers many benifits to end users, system integrators and product developers, sice it provides a real platform for open interoperable systems.

Success in the marketplace for new technologies is usually achieved by riding the wave of industry-wide trends. Some of the current worldwide trends in the industrial information sector include:

* The increasing importance of information technology to provide the tools for effective management.

* An increasing demand to replace isolated information systems for different functions with integrated systems for the entire enterprise.

* The replacement of all manually driven and paper-based information systems with automated electronic systems.

* A movement away from closed, proprietary technologies towards interoperable technologies based on open standards.

Rather than having a technology focus, the focus should be on satisfying the underlying needs of the market. Generic needs for the industrial control markets that are in line with the above trends include:

* Safeguarding investment in capital outlay.

* Reducing the risk of relying on a single supplier.

* Ease of installation, maintenance and expansion.

* Support for the integration of multifunction systems and integration with information systems.

Rugged domains are the one area where communication standard satisfying the above criteria has been missing from the options available to end-users. The ideal scenario would be not to use a separate standard for rugged domains, but to use one communication standard for all field applications, including rugged domains. The achievement of this objective may however still be a long way off.

Open standards - a framework for plug-and-play

The computer industry has demonstrated that open, standards-based technologies are critical to meet the requirements of diverse end-users, to allow multi-vendor solutions and to ensure expandability. Given the number of potential stakeholders in an industrial plant, it is clear that only an open standards-based solution for distributed plant information networks will meet these requirements. The network architecture should hence adopt existing standards where applicable.

A further typical requirement is for the system to be scalable, and to allow additions and modifications at low cost, eg the monitoring of additional machines should not require extensive rewiring or modification of software for the existing installation. It should even be possible to move machines (and change the configuration of the data capturing network) on a regular basis without requiring the assistance of the system supplier.

An important requirement is that several different end-users (typically managers with different functional responsibilities) must have access to the same information. In the case of customers with more than one plant, as well as customers outsourcing some of these functions, the benefits of an integrated information system can only be fully realised by linking the system to a wide-area network (WAN). The general case for which the system architecture should hence be designed is that of a distributed plant information system (DPIS).

The Internet Protocol (IP) has proved itself as a clear and undisputed standard for digital communications, both in the LAN and WAN environments. It offers specific features that are crucial to the information network, including scalability, security, platform independence and ease of integration with other applications. On the level of local data capturing, Lonworks has emerged as a technology that can seamlessly extend the information network to machine and sensor level. It is an advanced control networking technology that has been designed for the purpose of taking open networking beyond the computer to operational levels. Currently, it is the only fieldbus contender that fully complies with all seven layers of the ISO Open System Interface standard. Cost-effective interfaces exist from leading suppliers like Toshiba to link Lonworks to IP networks. The use of Lonworks provides the system operator with remote access not only to the captured data, but also to the low-level operational status of the control network itself, as well as of the devices being monitored.

The ideal scenario for system integrators and end-users would be the support of open standards by all equipment manufacturers. In the case of Lonworks the active support of leading component suppliers facilitate the implementation of open standards. Interoperability between the products of different vendors is guaranteed through the use of identical low-level building blocks, eg the Neuron chip in which the Lontalk protocol has been embedded. In addition, an event-driven, C-based embedded programming language called Neuron C, which supports the development of control network applications, supports the technology. More powerful control or data processing applications can be implemented by interfacing the neuron to more powerful processors through a so-called microprocessor interface program (MIP).

The attribute for which Lonworks has perhaps become the best known is the support of multiple physical media to implement a single network across all domains. Routing and addressing schemes are available to support the logical breakdown of the hierarchy in larger systems. Third party vendors have developed a multiplicity of software support tools for installation, debugging, maintenance and upgrading of Lonworks networks. The neuron has also been designed with built-in diagnostics to support the debugging and maintenance processes.

Distributed versus centralised control

Fieldbus control networks are not only limited to implementing field communications, but should form the basis for a new control system paradigm. Similar to the migration of computer networks from mainframes to distributed personal computers and work stations, the control industry is moving away from centralised command based systems towards distributed information based systems. Distributed control is based on the sharing of information among a large number of nodes on a network, rather than central generation of commands by a powerful industrial computer or PLC.

Several important advantages can be achieved through the distributed control paradigm:

* Increased flexibility (modifications to functionality only involves isolated nodes and not the software of the central processor).

* Lower cost to expand and modify (nodes can be added incrementally, without some centralised pro-cessor running out of capacity).

* Increased reliability if correctly implemented (single points of failure can be eliminated by using local intelligence).

* The sharing of information between all nodes on the network is simplified.

Framework for evaluating fieldbus

alternatives

Based on the above arguments, the following criteria have been defined for evaluating fieldbus alternatives:

1. Can it provide the functional requirements of the end-user market?

2. Does it support interoperability between products from multiple vendors?

3. Can it combine different operating domains into one integrated system?

4. Has it been designed for seamless integration with the information system environment?

5. Is it scalable to any size that may reasonably be required?

6. Are support tools available for the creation of complete system solutions?

7. Does it support the remote management of systems, typically across the Internet?

Based on its strong performance regarding most of these requirements, Lonworks has been accepted by various industries, if not as the de jure standard, then as the de facto standard for new installations. This includes the building automation and transportation industries. The progress of this technology within the industrial market is still hampered by the entrenched position of the large PLC suppliers and of associated communication standards, eg Profibus and Modbus. Some PLC vendors are in the process of actively copying the Lonworks paradigm within their own systems. Although these efforts may provide end-users with some of the advantages of the distributed control paradigm, it still does not overcome the obstacle of lack of interoperability between competing PLC technologies.

The LonTalk protocol

The protocol underlying the Lonworks control networks has been approved as an open industry standard by the American National Standards Institute (ANSI): ANSI/EIA 709.1-A-1999 Control Network Protocol Specification. In addition to the ANSI/EIA 709.1 standard, the networking communication protocol is also approved by such standards bodies as IEEE, ASHRAE, AAR, IFSF, SEMI and CEN. The LonTalk Protocol follows the reference model for open systems interconnection (OSI) developed by the International Standard Organisation (ISO). In the terminology of the ISO the LonTalk Protocol provides services at all seven layers of the OSI reference model as shown below:

Physical channel management (layers 1 and 2).

Naming, addressing, and routing (layers 3 and 6).

Reliable communications and efficient use of channel bandwidth (layers 2 and 4).

Priority (layer 2).

Remote actions (layer 5).

Authentication (layers 4 and 5).

Network management (layer 5).

Network interface (layer 5).

Data interpretation and foreign frame transport (layer 6).

Application Compatibility (layer 7).

The protocol offers four basic types of message services:

* Acknowledged.

* Request/response.

* Unacknowledged repeated.

* Unacknowledged.

The LonTalk Media Access Control (MAC) sub-layer is part of the data link layer of the OSI Reference Model. Many different MAC algorithms exist in networks today. One family of these algorithms is called CSMA (carrier sense multiple access). The MAC algorithm used by the LonTalk protocol belongs to the CSMA family. The CSMA family of media access control algorithms requires a node to establish that the medium is idle before it begins to transmit. However, each algorithm behaves differently once the idle state is detected. Slotted media access is used by p-persistent CSMA, and Echelon's LonTalk CSMA protocol. Echelon's LonTalk protocol uses a new CSMA MAC algorithm called predictive p-persistent CSMA. The LonTalk protocol retains the benefits of CSMA but overcomes its shortcomings for control applications. As in p-persistent CSMA, all Lonworks nodes randomise their access to the medium. This avoids the otherwise inevitable collision that results when two or more nodes are waiting for the network to go idle so that they can send a packet. If they wait for the same duration after back off and before retry, repeated collisions will result. Randomising the access delay reduces collisions. In the LonTalk protocol, nodes randomise over a minimum of 16 different levels of delay called randomising slots. Thus, the average delay in an idle network is eight slot widths. This prevents throughput from degrading past the saturation point.

Application of LonTalk on power lines

Dual carrier frequency communication is implemented via a coupling network to power lines. Some of the important attributes are described below.

Reception

* Able to receive latest and older compatible packets.

* Automatically detects whether receiving a 132 kHz (C-band) packet or a 115 kHz packet.

Transmission

* All transmissions are initiated using 132 kHz packets, which can be received by latest and older transceivers.

* Last two retries of acknowledged service messages are at 115 kHz, eg with three retries set, up to two packets are sent at 132 kHz and up to two packets are sent at 115 kHz.

* Unacknowledged repeat messages alternate between 132 and 115 kHz packets.

* Simultaneously sends a 132 kHz pilot code to keep older nodes synchronised during 115 kHz transmission - a key requirement for CENELEC compliant applications. [EIA-709.1 Control Network Protocol Specification; Draft EIA-709.2-A, Control Network Power Line (PL) Channel Specification.]

Summary and conclusions

It is of crucial importance to take a holistic approach when specifying and designing a communication system for field applications. These systems should not be viewed in isolation, but should be considered as part of the larger integrated information system of an industrial plant. Technology options for field communications and control should support:

* Open interoperable multi-vendor systems.

* Cost-effective access to various operational areas. * Complete support of application development and remote operation.

* Scalability and ease of expansion.

* Complete integration with the information system environment.

The Lonworks technology is one fieldbus contender that complies with all of the above requirements. It is currently the only solution for rugged domain communications with a solid standards-based technological foundation. It can however also play an important role in the industrial plant, both for the implementation of distributed control and as a link between monitoring and control equipment and the IP environment.

Q-KON

(012) 665 0052

dstaal@qkon.com





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