chapter 4.pdf

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Chapter IV

Industrial Field Networks. General Characteristics

1.1. Structure of industrial field networks These networks occupy the lowest level in the hierarchy of industrial systems

for control (fig. 4.1). Their main purpose is to carry out communication between the terminal field devices (sensors, actuators, etc.) and the devices of higher hie-rarchical level (PLC, work and operator stations). Field networks are characte-rized by simplicity of structuring, high data transmission rate, flexibility and low cost of the constituent components. In addition, they facilitate to a great extent diagnosing and tuning of ACSNC. Field networks set up provide the possibility to integrate devices of various manufacturers.

Although the larger bulk of adopted specifications for field networks have originally been developed as business concepts, today manufacturers of automa-tion equipment are united in various organizations which support one or another field communications standard. The underlying reason for that is in the fact that successful performance in field communications warrants a larger market share. Despite the declared openness of field networks the practice shows that devices of different manufacturers are virtually incompatible

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Fig. 4. 1.


Compatibility of devices in field networks is determined by specifications which employ:

• Different diagnostic functions; • Techniques for switching on and off of various devices to the mains with-

out interrupting communication between the rest of the nodes and without shutting off power supply;

• Automatic configuring of devices without using additional specialized de-vices;

• Using of standardized profiles of the devices; • Standardized mechanisms for physical connection of devices to the net-

work; • Self-diagnosing functions; • Deterministic functions for prediction of critical state.

Fig. 4.2 presents the variety of field networks specifications with regard to their application in the systems for control of different manufacturing technol-ogy processes.

Fig. 4. 2.

Application of field networks is most effective provided the following prere-quisites are available:


• Interchangeability of devices made by different manufacturers; • Quick response of field terminals in ACSNC; • Quick installation and tuning; • Flexible selection of components in ACSNC; • Utilization of intelligent terminals capable of self-diagnostics; • Setup of systems with physical distribution/allocation of up to 1000 m; • Limited number of cable connections.

1.2. Functionality of field networks Field nets carry out the following functions in ACSNC: • Data transfer with terminals such as logic controllers, actuators, sensors,

etc.; • Transmission of additional information concerning configuration of de-

vices. An important feature of field networks is the possibility for implementation

of distributed systems since a centralized control of network structure is not mandatory.

1.3. Methods for information transmission employed in field networks The methods of transmission in field networks allow information to be

transmitted to one or several nodes in the net. Basic units of information net-working are frames, packets and datagrams. A frame contains fields with bits and bytes and is associated with the physical layer of the OSI model whereas a packet is associated with the upper layers of that model.

Methods used in field networks for information transmission are as follows: • Unicast (one to one) With this method the packet is cast by a transmitter to a receiver in the net-

work (fig. 4.3). The receiver in turn addresses the packet using the recipient’s address after which the packet is sent to its destination;


Milticast

Master (PLC)

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Fig. 4. 3. • Multicast (one to many) In multicast method the packet is cast by a transmitter to a host of recipients

in the network. The transmitter addresses the packet using the multicast address of recipients after which it is sent over the net to each of the recipients forming the multicast address;

• Broadcast (one to all) With broadcast method the packet is transmitted to all devices in the net. The

transmitter addresses the packet using broadcast address after which it is sent over the net to each constituent device.

1.4. Types of devices used in field networks The following types of devices are distinguished in field networks: • Master devices Master devices determine the method of data exchange in the network and

carry out control or information-control functions. These are active devices which send messages along the network without pre-requesting, provided they have authorized access marker.

• Slave devices Slave devices are terminals of ACSNC: field testers/meters, transducers, ac-

tuators, input/output modules, measuring devices. They are passive devices which have limited access to the network; actually, they confirm messages re-ceived or transmitted to the master devices.

1.5. Communication mechanisms in field networks


Communication mechanisms determine logical connections between devices,

communication protocol and connections between objects within devices. The following communication mechanisms are most frequently used in field net-works:

• Client/server This is a basic communication model in contemporary computer networks.

Here the server assigns its resources and responds to client’s inquiries. Clients use server resources. A client should have his/her own resources for imple-menting applications which use data from the server. Quite often the parts of the client and server are interchanged in computer networks thus making master de-vices to be clients whilst slave devices become servers in the net. The master device function can be carried out by a distributed control system (DCS), pro-grammable controller or PC (fig. 4.4). Slave devices are sources of raw infor-mation (field testers, sensors, measuring systems) or other devices such as actu-ators, modules for remote input/output etc. Communication in a node is initiated by the master device which attempts interfacing one or several slave devices. Following a recognition process the master devices configures slave devices in accordance with the information received by them and with regard to their use. It is assumed that during the period of operation the master device controls slave devices and has full authorization over them until it decides to interrupt its con-nection with them.

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Fig. 4. 4. A major advantage of this mechanism is the simplified handling of network

data; the larger bulk of data being concentrated at the master device. The system for protection of data and the entire network as such is also simplified. However,


the fact that the operability of the entire net could be jeopardized in case of master device damage is regarded as considerable disadvantage.

• Peer-to-Peer A typical feature of this mechanism is that all devices involved in the net-

work are "peers" and can function both as slave and master devices at the same time (fig. 4.5). Peer-to-Peer is used to exchange identifying information between slave and master devices and finds application in industrial networks such as Profibus-FMS, LonWorks, WorldFIP.

• Multi-master This communication mechanism is used when several master devices need to

obtain access to data which is provided by one and the same slave device (fig. 4.6). At any random point in time only one master device is able to control and configure slave devices.

Peer

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Peer

Peer Fig. 4. 5.

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Multi-master mechanism finds its application in industrial networks such as

Profibus-DP, DeviceNet, etc.

• Poll Upon polling the master device sends an inquiry to each individual slave de-

vice for the purpose of data exchange with it. The slave device makes an uncon-ditional reply even though the inquiry is not intended for it. This mechanism is not adequately functional as the rest of devices have no access to the exchanged data although they may need it.

• Strobe, broadcast With strobe broadcast the master device sends an inquiry to all slave devices

for the purpose of exchanging data with them. In response to this inquiry a reply comes only from these devices which are identified in the broadcast message.

• Explicit messages Explicit messages are used along with the peer-to-peer mechanism of ex-

change. This type of messages can be generated by each device in the network whereby the recipient side should send a confirmation. Explicit messages me-chanism is applied in initial identification, configuring and diagnostics of net-work devices.

• Fragmented messages The fragmented messages mechanism is to be used should there arise a need

to transmit a message whose size is larger than the one allowed for single trans-missions. With this mechanism data is divided and sent in fragments which later are concentrated at the receiver point thereby forming the initial size of the mes-sage.

• Cyclic messages Cyclic messages are used for periodic transmission of data from slave to

master devices with regard to operation. This mechanism reduces network traffic and the operation of master devices. • Change of state, COS COS mechanism configures slave devices so that they could send data to

master devices only when their state is changed. During the remainder part of the period master devices use the last picked up state for the purpose of process control, thereby reducing network traffic to a considerable extent.

• Producer-consumer


With producer-consumer mechanism information is periodically transmitted

along the network by using broadcast method whereby each device receives without delay that part of the information which it needs. Every device (infor-mation originator) writes a connection ID in the header of the data packet. After the packet has been sent along the network each device which reads the connec-tion ID determines whether this information is intended for and needed by it. This mechanism finds wide scale application with specifications DeviceNet, ControlNet, Fieldbus Foundation. In it the information content is sufficient for the purpose of identifying potential recipients. Producer-consumer enables full-scale utilization of network medium and reduction of network traffic.

• Source-destination This mechanism is an alternative to producer-consumer. With it slave devices

receive only those packets which contain their address. When several devices are in need of the same data the latter should undergo multiple transmission. Such procedures are ineffective and could create a number of problems in real time synchronization and operation as the devices which need identical data re-ceive it at different points in time. Source-destination mechanism is applied in specifications of earlier generation such as Profibus-DP, Modbus Plus and Interbus S. Communication with source-destination is more awkward as compared to producer – consumer since the packets along the net include additional information used in message addressing.

1.6. Contemporary specifications of field networks The specifications reviewed below are but a fraction of the large variety of

field networks which provide possibilities for setup of contemporary ACSNC. • AS-Interface is one of the most simplified network specifications. It is ap-

plied in connecting terminals with elementary input/output functions; • CAN Interface – ISO standard (ISO-11898) for serial communication in

industrial settings, which provides secure and high fidelity information transmission;

• DeviceNet – economically effective network solution whereby controllers and industrial field devices are directly connected without any need of in-put and output wiring;


• Profibus – an open standard for field networks with a wide range of

applications in manufacturing automation. It allows for programmable controllers with distributed intelligence to be used in a shared network;

• Modicon Modbus Plus – deterministic field network with client/server communication and high speed exchange between controllers and termin-als;

• HART – the most widely used specification in the realm of automation en-compassing all spheres of industry including power engineering, petro-leum chemistry etc. It allows simultaneous transmission of digital and analogue signals in setting up networks of enhanced security in volatile and hazardous ambience;

• LonWorks – a standard for networks of peer exchange which is applied in systems with a large number of terminals;

• Interbus – specifies networks of ring topology for data transmission; • BACnet – it is used mainly for building automation; • SERCOS – deterministic network specification for high speed exchange in

optical medium; it is designed for control of systems for multi-coordinate electric drive;

• Allen-Bradley Data Highway Plus (DH+) – it is applied in high speed communication with controllers manufactured by Allen-Bradley;

• General Electric Genius I/O, Allen-Bradley Remote I/O – it is designed for connection with remote input/output modules manufactured by above producers;

• FOUNDATION Fieldbus – specially developed for highly sensitive applications requiring high security level and fast performance. It is fit for industrial installations with hazardous ambience since it features precise synchronization between control and communication;

• Ethernet/IP – the most widely used real time communication for industrial applications. Ethernet/IP is an industrial extension of Ethernet TCP/IP which features fast performance.

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