10 Instrumentation/computer networks

 10.7 Digital fieldbuses

‘Fieldbus’ is a generic word that describes a range of high-speed, bus-based, network protocols that support two-way communication in digital format between a number of intelligent devices in a local area network. All forms of transmission are supported including twisted pair, coaxial cable, fibre optic and radio links. Compared with analogue networks that use 4–20 mA current loop data transmission, fieldbus-based systems have many advantages including faster system design, faster commissioning, reduced cabling costs, easier maintenance, facilities for automatic fault diagnosis (which also improves safety), the flexibility to interchange components derived from different suppliers, and estimated reductions of 40% in installation and maintenance costs. However, it should be noted that cost savings alone would not be sufficient justification for replacing an analogue system with a fieldbus-based one if the analogue system was operating satisfactorily.

Intelligent devices in an automated system comprise of a range of control elements, actuators, information processing devices, storage systems and operator displays as well as measurement devices. Hence, any fieldbus protocol must include provision for the needs of all system elements, and the communication requirements of field measurement devices cannot be viewed in isolation from these other elements. The design of a network protocol also has to cater for implementation in both large and small plants. A large plant may contain a number of processors in a distributed control system and have a large number of sensors and actuators. On the other hand, a small plant may be controlled by a single personal computer that provides an operator display on its monitor as well as communicating with plant sensors and actuators.

Many different digital fieldbus protocols now exist, and names of some of the more prominent ones include Profibus (Germany), WorldFIP (France), P-net (Denmark), Lonworks (USA), Devicenet (USA), IEEE 1118 (USA), Milbus (UK), Canbus, Interbus-S and SDS. These differ in many major respects such as message format, access protocols and rules for performance prediction. In recognition of the difficulties inherent in attempting to connect devices from different manufacturers that use a variety of incompatible interface standards and network protocols, the International Electrotechnical Commission (IEC) set up a working part in 1985 that was charged with defining a standard interface protocol, which was to be called the IEC Fieldbus. However, at the time of the IEC initiative, a number of companies were already developing their own fieldbus standards, and commercial interests have continually blocked agreement on a common, internationally recognized standard. In the meantime, some countries have adopted their own national fieldbus standard. Also, the European Union established a European standard (EN50170) in 1996 as an interim measure until the appearance of the promised IEC standard. EN50170 has adopted Profibus, WorldFIP and P-net as the three standards authorized for use, and the intention is to allow their use until six years after the IEC standard has been published. The inclusion of three protocols rather than one in EN50170 is unfortunate, but three protocols are much better than the 50 protocols that were in use prior to the adoption of EN50170.

In the meantime, efforts to achieve a single fieldbus standard have continued, with development now being carried out by a body called the Fieldbus Foundation, which is a consortium of instrument system manufacturers and users supported by the IEC. The present approach is to define a protocol whose basic architecture is in two levels, known as upper and lower. The lower level provides for communication between field devices and field input–output devices whilst the upper level enables field input–output devices to communicate with controllers. These two levels have quite different characteristics. The lower level generally requires few connections, only needs a slow data-transfer rate and must support intrinsically safe working. On the other hand, the upper level requires numerous connections and fast data transfer, but does not have to satisfy intrinsic safety requirements. In the fieldbus standard proposed, the lower level will conform with the specifications for the Application Layer in ISO-7Ł and the upper layer will satisfy the specifications for the Physical and Data Link Layers in ISO-7. Three standard bus speeds are currently specified for the Foundation Fieldbus lower level of 31.25 kbit/s, 1 Mbit/s and 2.5 Mbit/s. Maximum cable lengths allowed are 1900 m at 31.25 kbit/s, 750 m at 1 Mbit/s and 500 m at 2.5 Mbit/s. For the upper Foundation Fieldbus layer, a high-speed ethernet is currently being developed that will provide a data transfer rate up to 100 Mbit/s

At the time of writing, it appears that we may now be very close to achieving an international fieldbus standard. The Fieldbus Foundation published a draft standard in 1998 known as IEC61158. This was agreed by a majority vote, but with dissent from a number of major instrument manufacturers. In the confirmed standard, which is expected in 2000, there are likely to be supplements that will allow devices operating under other protocols such as Profibus to be interfaced with the IEC61158 system.

 

10.8 Communication protocols for very large systems

Once a system gets too large to be covered by a local area network, it is generally necessary to use telephone lines. These provide communication over large distances within a protocol that is often called a wide area network. Public telephone lines are readily available, but there is a fundamental problem about their use for a wide area network. Whilst instrumentation networks need high bandwidths, public networks operate at the low bandwidth required to satisfy speech-based telephony. High bandwidths can be obtained by leasing private telephone lines but this solution is expensive and often uneconomic.

The solution that is emerging is to extend LAN technology into public telephone networks. A LAN extended in this way is renamed a metropolitan area network (MAN). An IEEE standard for MAN (IEEE 802.6) was first published in 1990. Messages between nodes are organized in packets. MANs cover areas that are typically up to 50 km in diameter, but in some cases links can be several hundred kilometres long. Use of the public switched telephone network for transmission is most common although private lines are sometimes used.

Both ring and bus networks lose efficiency as the number of nodes increases and are unsuitable for adoption by MAN. Instead, a protocol known as distributed queue dual bus (DQDB) is used. DQDB is a hybrid bus that carries isochronous data for the public switched telephone network as well as providing the data bus for a MAN. For handling data on a MAN, DQDB has a pair of buses on which data, preceded by the target address, circulates in fixed size packets in opposite directions, i.e. there is a clockwise bus and an anticlockwise bus. All stations have access to both buses and the protocol establishes a distributed queue. This ensures that all stations have access to the bus on a fair basis. Thus, the stations have their access demands satisfied in the order in which they arise (i.e. a first-come, first-served basis) but commensurate with ensuring that the bus is used efficiently. Fibre-optic cables are commonly used for the buses, allowing data transmission at speeds up to 140 Mbit/s.

 

10.8.1 Protocol standardization

Many years ago, the International Standards Organization recognized the enormous problems that would ensue, as the size of networks increased, if a diversity of communication protocols developed. In response, it published the Open Systems Interconnection seven-layer model (ISO-7) in 1978. This provides standard protocols for all aspects of computer communications required in a large-scale system, that is, management and stock control information etc. as well as instrumentation/process control networks.

The ISO seven-layer model defines a set of standard message formats, and rules for their interchange. The model can be applied both within local area networks and in much larger global networks that involve data transmission via telephone lines. Whilst the standards and protocols involved in ISO-7 are highly complex, the network builder does not need to have a detailed understanding of them as long as all devices used in the network are certified by their manufacturer as conforming to the standard. The main functions of each of the seven layers are summarized below:

Layer 1: Physical protocol: Defines how data are physically transported between two devices, including specification of cabling, connectors, I/O ports, modems, voltage levels, signal format and transfer speed.

Layer 2: Data protocol: Establishes paths to ensure data can be exchanged between two devices, and provides error detection and correction (by retransmitting corrupted data).

Layer 3: Network protocol: Controls the flow of data in packets between all devices in a network.

Layer 4: Transport protocol: Allows a user-task on one computer to communicate with another user-task on a different computer transparently of network characteristics, thus ensuring high reliability in data exchange.

Layer 5: Session protocol: Synchronizes communication activities during a session, and maintains a communication path between active user-tasks (a session is defined as the period of time during which two user-tasks remain connected).

Layer 6: Presentation protocol: Provides for code conversion as necessary, so that user-tasks using different data formats can communicate with each other.

Layer 6: Presentation protocol: Provides for code conversion as necessary, so that user-tasks using different data formats can communicate with each other.

It should be noted that only layers 1, 2 and 7 are usually relevant to instrumentation and plant control systems.

The Manufacturing Automation Protocol (MAP) was conceived by General Motors in 1980 in order to support computer integrated manufacturing. It conforms with ISO7, and is a similar attempt at providing a standard computer communications protocol for large systems.

 

10.9 Future development of networks

Network design and protocol are changing at a similar rapid rate to that of computer systems as a whole. Hence, it would be impossible in a text of this nature to cover all current developments, and, in any case, any such coverage would rapidly become out of date. The past few pages have covered some aspects of the general concepts and design of networks, and this will prove useful in helping the reader to understand the mode of operation of existing networks. However, network specialists should always be consulted to obtain up-to-date information about the current situation whenever a new network is being planned.