trends in lans for plant automation

8/7/2019 TRENDS IN LANs FOR PLANT AUTOMATION

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TRENDS IN LANs FOR PLANTAUTOMATIONDick SchmidtNational Sales ManagerMining& Metals IndustriesAllen-Bradley Company (Milwaukee, Wisconsin)

Industry today faces unparalleled challenges in a marketplace that has suddenly becomeworld-wide. With this change has come world-class pressures in productivity, productquality, and time to market. Old methods of doing business do not apply to this newenvironment, characterized as it is by huge cost differentials in labor, materials anddistribution. Industry must respond to these challengesby not only working harder, butby working smarter. Plant of the future concepts like computer aided manufacturing(CAM), computer aided design (CAD), and computer integrated manufacturing(CIM)are adirect result of this "smarter" orientation.These leading edge concepts depend heavily upon integration of separate automatedprocesses into a cohesive whole. By contrast, early automation programs made their startwith manufacturing units, conveyors, and subsystems. Over time, the automationboundaries expanded to include complete batching, blending and material handling systems,where a series of manufacturing units functioned as a synchronizedwhole. Any number ofintelligent devices can now reside within those expanded boundaries: programmablecontrollers, computers, robot controls, etc. And for true integrated automation, all ofthese devices require interconnection and intercommunication.COMMUNICATION NETWORKS AS INTEGRATIONTOOLSData communication networks within manufacturing provide the tools for achievinginterconnection and intercommunication. They are not the end goal of an operation'sintegration efforts, but rather the essential method by which the goalcanbe achieved.The success of a plant-wide information management system often depends on itscommunicationsnetworks. As an operation's "nervous system", communication networks havethe potential to integrate-and optimize-the functionality of each individualcomputer-based application,aswell as the system as a whole.Today, the installation of plant-wide LANs is increasing in a variety of diverseindustries. While many LANs are in standard industries suchas automotive, many othersare found in applications such asmining & metals, petrochemical or food processing. Thedecision to go with a plant-wideLAN is stimulated by many potential benefits. Most ofthose benefits, economic or otherwise, are based upon flexible communications, and thetransparent migration of computer-based applications between mainframe host computers,workstations, and cell control architecturesThe decision of any management team to install aLAN must focus on the technical andeconomic alternatives, beginning with an in-depth look at LAN services, media, andnetwork types.

At the low end, local area networks provide fundamental data transport services toinformation processing equipment within a specific area, such as an office,factory,complex of buildings, or a campus. From there, LAN functions can be expanded to provide

CH2667-4/89/0000-0251$1.00 01989 IEEE 25 1


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additionalservices such as programmable device messaging, electronic mail, filetransfer, resource sharing, virtual terminal, and distributed data base access.

A LAN cable system provides the physical service of a signal transmission. Signaltransmission media include many different types,such as broadband coaxial, basebandcoaxial, and fiber optic cable. A recent study suggests that by 1989baseband willaccount for 39%of the market for industrialLANs,while broadband will account for morethan a third.- Broadband refers to a wide bandwidth (up to 450 MHz ) communication network. BroadbandLANsuse frequency division multiplexing to divide a single physical channel into anumber of smaller independent frequency channels. Each of these smaller channels can beallocated different bandwidths, and thus canbe used simultaneouslyfor many differentservices, i.e., data, voice, video, telemetry (real-time control data), and imaging(facsimile).- Baseband systems, on the other hand, are single channel systems; that is, they resideat the base of the channel bandwidth and transmit over a single physical transmissionmedium. Baseband systems are designed mainly for manufacturing and single-purposeapplications. The most common baseband L A N s are data highways for manufacturing controloperations, and Ethernet LANs for office information processing environments. Carrierband is a specific type of baseband implementation in which data transmission ismodulated on a frequency to improve signal-to-noise characteristics.

W O R K TYPESThe classificationof networks presents somewhat of a hierarchy - the more complex auser-need becomes, the more involved the type of network.- Dedicated links. Dedicated links are fixed-link systems that can communicate across abuilding or across the country. One device - always the same device - can exchange datawith another device connected to the same link.- Terminal ServerLocal Area Networks. Here, any point in the network can contact anyother point in the network, much like a basic telephone system. Unlike a dedicated,point-to-point communication link, the network poses no restrictions on who initiates thecall, where that call is made from, or where the call is placed.- Personal Computer Networks. These add more functionality to Terminal Server Networks.Using the telephone analogy, again,the user who can send and receive from anywhere andto any placed now has access to other serviceson the line: weather and time reports, forexample, or to translation servicesfor an international call On a personal computernetwork, there are many servicesspecifically tied to personal computers and theirability to transfer large amounts of information. Among others, these services mightincludea file service (data stored elsewhere but immediately accessible to the computeruser), and print service (immediate interface with print capabilities for all availablefile material).- Automation Control Networks. These are essentially data highways, or processcontroller networks designed to transfer information from one control-type device toanother. Another common connection would be plant floor controllers interfaced to hostor supervisory computers Primarily,automation control networks allow machines tocommunicate, and at the same time allow people to interrogate, modify, and change theoperations of those machines.

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SOLVING THE INTEGRATION PROBLEMA major obstacle to interconnection of programmable (intelligent) devices is theproliferation of different manufacturers' devices within a facility. Each manufacturer'sproduct has its own protocoleffectivebut incompatible: DEC uses DECNET,IBM usesSNA,Gould Modicon uses Modbus, Allen-Bradleyuses Data Highway, etc. Integrationof thisdiverse population of devices and protocols is a massive task.Integration of dissimilar devices from different manufacturers has typically requiredgreat expense in the form of custom software, custom hardware, and redundant cabling. Infact, a study made in the early 1980's by General Motors concluded that 50% of automationproject costs were communications related. Furthermore, the resultant communicationsystems were often inflexible to change or expansion.Thisgave rise to the development of a Manufacturing Automation Protocol (MAP ) initiatedand driven by General Motors as a way to achieve standardized, large-scale multivendorcommunications. The realization of a MAP standardwill enhance industrial progressbecause off-the-shelf products from different vendors willbe able to interconnect andintercommunicate.Numerous existing communication standards and standardization activities are in forcetoday, and form the basis of the MAP effort.At the international level, the International Standards Organization(ISO) functionsas aworld standards control-point and clearing house. Standards aimed at internationalsupport are processed through IS0 for final balloting and approval. European groups suchasECMA and CCllTwork closely withISO.At the national level, primary focus has been on ANSI, IEEE, and NBS (National Bureau ofStandards). Of particular note is the IEEE work done in Project 802 and the detailingwork NBShas provided for emergingIS0 standard protocols.THE OS1MODELThe Open System Interconnect (OSI) model was developed by IS 0 in order to break thecomplex networking task into pieces or modules. The model defines an architecture orblueprint- for exchanging information among systems through standard protocols. Themodel provides a toll for the description, design and implementation of standardizedcommunication protocols. Within the model, network communication is divided into seven"layers". Each layer is largely independent of the communicationon the networkBeginning with the lowest layer, they are:- Physical. Defines the transmission medium and the electrical and mechanical means forthe uscr to interface with the medium.- Data Link. Establishes, maintains, and releases access to the physical medium;packages messages for transmission, and checks the integrity of received messages.- Network. Routes messages; manages intra- and inter-net message flow.- Transport. Makes and maintains the connectionbetween stations. It is the lowestlayer where station-to-station conversation exists.- Session. Establishes and controls the communication "session" between two stations onthe network.- Presentation. Provides a common representation and format of the information exchangedbetween the two stations on the network

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TheOS1 model does not define new methods of system data exchange, but rather provides afunctional blueprint to follow in implementing communication products. Its primarybenefit to the MAP effort is: by identifying the seMces that each layer must provide,it encourages standardization and allows it to proceed quickly; it also assures theflexiiility needed to incorporate changing technology.As standards are agreed upon for a specific layer, vendors can incorporate thosestandards in their products. They do not have to wait for development of standards forthe entire model. This feature also means that overall network communication can beupgraded as new and superior technologies are developed for a particular layer. Upgradesat one layer will not affect the other layers, but will upgrade communicationsas awhole.IEEE 802 PROJECTThe IEEE 802 project was originated roughly sixyears ago in response to pressures by thevendor community for establishing LAN standards at the bottom two OS1 layers (Physicaland Data Link). Out of project 802 has come a family of physical layer and media accesstechnique (MAC) standardswith a common data link control sublayer. Different physicaland MAC specificationsare provided to address different LAN requirements.The different physical/MAC combinations are: 802.3 (CSWCD),802.4 (token bus), and8025 (token ring). The 802.3 technology is considered to be well suited for the officeenvironment using baseband cabling. The 802.4, on the other hand, is considered to bethe best choice for manufacturing due to its deterministic, robust characteristics. The802.4 specifications define both broadband and carrier band media The 8025specificationswere primarily driven by IBM and seem suited to the office applications.An important note to make regarding this family of standards is the common data linksublayer. The concept underlying the family approach is that common layers (LLC-7) canbe used with physical and MAC technology chosen to best meet LAN applicationrequirements.UPPER LAYER STANDARDSAt the upper layers of the OSI, numerous standards are currentlyin place:- At the network layer, the I S0 Internet Specificationdefines a protocol for networklayer 3 of ISO/OSI model. The specificationis designed to provide a facility totraverse multiple networks, i.e., network level addressing, and packet routing andcontrol.- At the transport level, the IS0 Transport Specificationprovides for reliableend-toend message exchange. Its concern is connections, i.e., packet sequencing,assembly/disassembly and flow control.- At the session layer, the IS0 Session Specificationprovides control and management ofend-to-end message exchange. Specific concerns include data synchronization, messagesize control and connection management.- The IS 0 application layer defines the protocol for interfacing to user applicationprograms:-- CASWASCE (Common Application Service Elements) provides clean presentation layer

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interfacing.-- FI'AM (File Transfer) provides standard file manipulation services.-- Device messaging provides messaging standards for programmable devices-- Network management provides a network and station management protocol standard.MAP WILL SUPPORT MORE THANONENETWORK TYPEThe concept of carrier band emerged from the MAP effort in 1985 and has become widelyendorsed by users and manufacturers alike. Most users are familiar with baseband, forexample, where the cable supports only one network. Carrier band is a substitute forbaseband signalling where the higher data rates and distances desired require a differenttransmission technique to achieve an acceptable signal-to-noise ratio. Carrier bandprovides this improved noise immunity by modulating the signal onto a carrier upontransmission-hence the name "carrier" band. The cable specified by carrier band iscoaxial, again to improve noise immunity at the higher signalling speeds. MAP carrierband today is specified at 5Mbps.Carrier band provides one network per cable is typically limited to shorter distances(several thousand feet) and includes a fast response time option. As such, thistechnology applies well to cell subnetworks where fast response time and dedicatedcapacity are desired.Broadband, on the other hand, offers many channels over longer distances (several miles)on the same cable. It provides coexistence of different network types suchasvoice,video and data These characteristicsmake broadband ideally suited for use as a"backbone" network for plant area and cell level communications.The MAP philosophy is that most MAP facilitieswill employ broadband backbone andmultiple cell (carrier band) subnetworks. Programmable controllers, numerical controls,and other station level devices would attach to the carrier band subnetworks (Fig. 2).MAP.EPA AND MINI-MAPOver the last two years, MAP has recognized that different network applications requiredifferent functional and performance characteristicsfrom associated network nodes. Forexample, host computer intercommunication may require different network characteristicsthan a process control application. To accomodate these emerging needs,M A P has definedthree network node or protocol configurations- full MAP, EPA MAP, and Mini-MAP.- Full MAP is a configuration in which all sevenMAP layers are supported with noprovision for layer bypass. Thisconfiguration is typical for host computers whichwillsupport communication of large messages (e.g., file transfers) over large networks whichinclude subnetworks (internetworking). In the case of large data transfers andinternetworking,the full seMces of MAP- especially transport and network layerprotocol- are needed.- EPA MAP. Enhanced Performance Architecture, is a configuration in which all sevenlayers are provided plus a "bypass" mechanism from layer7 to layer 2. This bypassfunction essentially places the application layer directly on top of the data link layer,creating a collapsed architecture. The purpose - enhanced performance. Throughbypassing layers3 through6, it is believed that protocol overhead, and hence, time,will be eliminated from message transfers. EPA, therefore, provides optimized

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performance in specific instances such as station-testation transfer within a singlesubnetwork Bypassing intermediate layers introduces these restrictions.However, lack of the network layer protocol requires that all transfers be restricted toa single network or subnetwork No inter-networkingservice is provided. Also, lack ofthe transport layer requires that all messages be restricted in length to not exceed thepacket size offeredby the data link layer. This isdue to the fact that transport'sseMce of packet assembly and sequencing is not present.To summarize, an EPA node offers all seven MAP layers for message transfer throughout thefacility and across multiple networkdsubnetworks EPA also provides a layer bypassmechanism for increased performance transfers on a single networkhubnetwork- In the Mini-MAP configuration, only layers 1,2, and 7 are provided. There is noprovision for layers3 through 6. Mini-MAPis intended to apply to low cost/lowcomplexity and end devices such as terminals, bar code readers, etc. Mini-MAP is similarto EPA except that only the "bypass" mechanism is provided -- only layers 1,2, and 7 areimplemented. I t therefore possessesall the restrictions of EPA in terms of shortmessages and no facility for inter-networking.PROCESS CONTROJ, INFLUENCE 0NMAPThe process control industry has influenced MAP to a great degree. This influence hasfocused on performance and reliability characteristicsof MAP to meet the stringentdemands of process control. Major results of this influence include, in fact, EPA andMini-MAP. The process control industryin particular required the enhanced performancecharacteristics of these configurations.In addition to EPA and Mini-MAP, the data linklayer,IEEE802.2, has been heavily influenced by process control. A partitioned datalink layer option, known as Class3, includes an acknowledgmentfeature for improvedperformance. MAP now referencesClass3 in the EPA and Mini-MAP specifications.STAND ARD AND PROPRIETARYLANsIt would be extremely difficult, if not impossible for MAP (or any standard-basednetwork) to address all networking needs in the foreseeablefuture. For example, therecan be specificfunctionalityin proprietaryLAN - such as redundant cabling - that canbe important to a particularuser application. Until such capabilities are addressed bystandards, the development of proprietary networksis the only course of action one canfollow. In essence, the user must be presented with meaningful choices - he can apply astandard network where it makes most sense for him. And, in cases where it does not makesen= for performance, cost, security or other reasons, he can choose a proprietarynetwork It is likely that a combination of standard networks and proprietary networkswill coexist well into the future because there are user needs for both types of LANs.As has oftcnbeen stated, networks are a tool to achieve a goal-improved automationquality. Naturally, there aredifferenttypes of tools needed for different types ofnetworking functions. Standard networks such as MAP provide many of these tools, andproprietary networks provide other capabilities for specific networking needs.

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trends in lans for plant automation

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