Abstract— Index Terms— field devices: sensor and actuator of the plan.
CAN: control areanetwork. TTP: time-triggered protocol FTT-CAN: Flexible Time-TriggeredCommunication on CAN, TCN: Train Communication Network. I. introduction of fieldbus system T Fieldbus systems have been a part of automationfor up to twenty years.
In fact, they have made automation what it is today.The term itself was coined in process automation and refer to the processfield, besides, the origin intention was to replace the star like pointconnection between the process control computers and the sensor or actuatorswith a single, serial bus system- hence, the terms “FIELDBUS”. But what is a fieldbus today, and what will itbe in the future? While the reduction of cabling may have been initially ( andin some applications still is) the most important reason for the use offieldbus system go far beyond this. II. definition of the fieldbus The augmented termfieldbus is consisting of two term which are “field” and “Bus”.
The meaning of”field” as defined in industrial world, is a geographical or contextual limitedarea. The industrial state that the field is an abstraction of plant levels. Meanwhile,the context “bus” is a well-known word in computer science as a set of commonline that electrically ( or even optically ) connects various unit (circuit) inorder to transfer the data among the circuit. The origin of the fieldbus was toreplace any point to point between the field and their controller ( like PLC & CNC ) bya digital single link on which all the information is transmitted serially andmultiplexed in time. In the most cases, the fieldbus transfer this informationin small sized packets in serial manner. Choosing the serial transmission hasmany merits in comparison with other kids of transmission such as paralleltransmission.
For instance, the sequential or serial transmission reduce the total required number of the connectingline over greater distance than that of the point to point or paralleltransmission. A set of rules must be defined in order to accomplish datatransfer between the units along the bus. This set of rules is calledCommunication Protocol or just the Protocol.
This is unlike the case of theordinary point-to-point transmission where any two connected entities send andreceive data from each other whenever the data is available. The protocol isresponsible for two important rules on the bus, the mechanism that any unit canacquire or seize the bus (from the network terminology this means the way ofMedium Access), and the synchronization between those multi-units on the bus. The medium access protocol choosing is avirtual steps in designing the DCCS. This is because of the odd nature of thebrusty traffic of such control system. So the existed LAN protocols such as thetoken ring or the CSMA are not appropriate for the control applications. Forthe token ring case, the more modes added in circuit, the longer the time eachnode to wait till it can transmitted data.
Needless to say that the CSMAprotocols, which are contention based protocols, will add randomness to theoverall respond time of the node data. This came from the way that the CSMAallow only one node to transmit data if and only if no other node is seizingmedium. The randomness occurswhen a collision happens as the nodes which encountered the collision will haveto wait to for a random time before it start again transmitting. A number ofsolution have been proposed to extend the CSMA in order to enhance itsperformance. For example, the collision avoiding, collision detection, andcollision resolution are all extensions to the CSMA protocol. None of thissolutions can fit directly for the control network application. In fact,according to Raji in 1994, stated that, they need some other modifications likethe CAN fieldbus protocol.
III. historical aspect Although the term” fieldbus” appeared only about 20 years ago, the basic idea offield-level networks is much older. Still, the roots of modem fieldbustechnology are mixed 4. Both classical electrical engineering and computerscience has contributed their share to the evolution. This early stage isdepicted in Fig. 1.
One foundation of automation data transfer has to be seenin the classic telex networks, and also in standards for data transmission overtelephone lines. Large distances called for serial data transmission, and manyof these comparatively early standards still exist, like V.21 (datatransmission over telephone lines) and X.21 (data transmission over specialdata lines).
Various protocols have been defined, mostly described in statemachine diagrams and rather simple because of the limited computing power ofthe devices available. Talking about serial data communication, we shouldnotice that the engineers who defined the first protocolsoften had a differentunderstanding of the expression “serial” and “parallel” than we have today. Forexample, the “serial” Interface V.24 transmits the application data serially,but the control data in a parallel way over separate control lines.
In parallelto this development, hardware engineers defined interfaces for standalonecomputer systems: for memories and printers, but also for process control andinstrumentation equipment. The first application was to interconnectmeasurement devices, and therefore standards like CAMAC (in nuclear science)and GPIB (IEEE 488) were developed. To account for the limited data processingspeed and synchronization requirements, these bus systems had parallel data andcontrol lines. Later, the serial point-to-point connections of computerperipherals were extended to support longer distances and finally also multidrop arrangements. The capability of having a bus structure with more than justtwo connections together with an increased noise immunity due to differentialsignal coding eventually made the RS 485 a cornerstone of fieldbus technologyup to the present Then two big evolutions took place. First of all, computersystems became more and more common, and soon a need for interconnectionsbetween the computers emerged. Conventional telephone networks were no longersufficient to satisfy the requirements.
Second, the big communication systemsof the national telephone companies gradually changed from analog to digitalsystems. This opened the possibility to transfer large amounts of data from onepoint to another. Together with an improved physical layer, the first reallypower data transmission protocols were defined, such as X.25 or SS7. Figure 1 IV. fieldbus and the networkreferences model Since itis a network, it is important to know the relation with the famous OSI7 reference model.This section, will describe the fieldbus in terms of the layers of the OSImodel. The definition of the OSImodel from Tanenbaum 96 who said “The OSI model organizes the protocolsused and the services provided by a general communication system in a stack oflayers”.
In other words, the OSI is complete layerednetwork model in whicheach layer does certain communication service. One can see in Fig. 2-a.
the reference OSI model layers and itslayers. How it works? From the figurewe can see that if a node wants to send a data packet from the application itmust first call for the sending service of it Is application layer which inturn will call the sending functions in the next layer, and so on till the datais sent at the physical medium to the other node. This node will reverse the sequencetill the received data reaches the application layer of its node then to theapplication which will used this data.The list below the seven layers of the OSI modelwith their functions: – 1- Application layer is to provide the services thatare required by specific applications.
2- Presentation layer is responsible for the data interpretation, whichallows for interoperability among different equipment. 3- Session layer isconcerned with any execution of remote actions. 4- Transport layer is responsiblefor the end-to-end communication control. 5- Network layer is concerned withlogical addressing process of nodes and routing schemes. 6- Datalink layer isresponsible for the access to the communication medium, and for the logicaltransfer of the data. 7- Physical layer is concerned with the way that thecommunication is done physically. Anycommunication system that is based on the OSI seven layers will have bothmerits of higher flexibility and compatibility with products from differentvendors.
Nevertheless, the same OSI system (due to its complexity) has aconsiderable overhead in both, the communications, and the processing.There exist many protocols and services that arelaid in the 3-layerd hierarchy of the fieldbus network. This at the end willlead to a great difficulty in evaluating one and unique international fieldbusstandard.
In fact there are many different fieldbus protocols in the world.There are large differences that can be found in the three layers of anyfieldbus protocol and their similar layers in another fieldbus protocol. Thedesigner of the DCCS communications system has multi-option solutions tofulfill his system requirements.
These requirements are varied from onesituation to another. In most cases the quality of services and the systemthroughput in addition to the overall system performance are all a commonrequirements any nearly all the DCCS systems. Also a fast response time isusually required by the real-time computer controlled networks designers. In addition, the different types of the fieldbuses that doexist in the international market and their national origins and standards. Modification to the MAP project was necessary asthe node implementation become more complex in order to support all theservices of the OSI reference model Almeida 99-1. The modification allowedthe short length control data packets, which occurs at high rates, to bedirectly transmitted through the application layer to the datalink layer. Whichmeans that we abbreviated the OSI hierarchy into 3-layer model as can be seenin Fig.2-b.
The resulting fieldbus is referred to as a 3-layered Architecture.These layers are: the Application layer, the Datalink layer, the Physicallayer. One may assume that the otherfour layers of the OSI model that are not available in the fieldbus hierarchyhave disappeared along with their own functions and services Almeida 99-1.This is absolutely wrong, as these functions are augmented into the existedlayers. For example, the main function of the presentation layer, which is tosupport the interoperability between different equipment, is done now by theapplication layer in the fieldbus. What is more, the assembling anddisassembling of data packets which was the function of the transport layer isdone now by the datalink layer in the fieldbus network.
If routers to be usedin some fieldbus networks, then the routing service, which was assigned to thenetwork layer, is done by the application layerin most cases in the fieldbus. b a Figure 2: The OSI 7-layers reference model (a),and the reduced fieldbus 3-layer structure (b). V. FUTURE FIELDBUS SYSTEM Likeall other technological products fieldbus is up to continuing process ofupdating.
The further developments are going on in the fieldbus technologyespecially in the vehicle system. A new era has been born which somespecialists called the “X-by-wire”. This technology tends to replace all themechanical linkage that are found in the vehicle with digital links and wireall these link into one network protocol that entirely run the vehicle. Risingnow in the horizon of this era are two protocols. The first is called “FlexRay”and secondly is called “TTP” .
This two are based on the older kin. Thecontrolled area network (CAN). Among the anticipated benefits :better fueleconomy, better vehicle performance in adverse conditions, and advances insafety features such as collision warning and even automatic collisionavoidance systems. Figure 3 shows one of the configuration topologies of theFlexRay protocol which called the active star.
The reason of the new protocolbased on the older CAN because these protocol try to resolve problem sencounter when designing the new X-by-wire automobiles using the old CANprotocol. These problem arise from the arbitration method that is used todivide the bus between the competing messages. The CAN used a priority schemeto assign the highest priority to the oldest que message regardless of thetransmitter making the attempt. FlexRay is a hybrid protocol that allocateportions of network time to both a time triggered protocol and to prioritizedmessage access. While the CAN prioritization scheme is based on dominants andrecessive bit values, FlexRay uses timing offset values proportional to priority.As its name suggest, FlexRay adds flexibility permitting coexistence of bothprioritized and time- triggered messages on the same network. There are otherproposed protocols such as TTP ( time-triggered protocol), FTT-CAN ( FlexibleTime-Triggered Communication on CAN) and TCN which is Train CommunicationNetwork. Figure 3: FlexRay Active Star Topology VI.
Fieldbus cabling Various types of cable are useable for fieldbus.Table 1 contains the types of cable indentified by the IEC/ISA Physical LayerStandard. Table 1:Fieldbus Cable Types & Maximum Lengths The most preferredfieldbus cable that being specified in the IEC/ISA Physical Layer Standard wasa Type A fieldbus cable.
(This cable will probably be used in newinstallations). Other types of cable can be used for fieldbus wiring. Thealternate preferred fieldbus cable is a Type B cable which is multiple, twistedpair cable with an overall shield. (This cable will probably be used in bothnew and retrofit installations where multiple fieldbuses are run in the samearea of the user’s plant). A less preferredfieldbus cable is a single or multiple, twisted pair cable without any shieldreferred to a Type C cable. The least preferred cable is Type D cable becauseit is a multiple conductor cable without twisted pairs but with overall shield.
Types C and D cables will mainly be used in retrofit applications. They willhave some limitations in fieldbus distance as compared to Type A and B. Thismay preclude the use of Type C and D cable in certain applications. The wiring rules offoundation fieldbus: 1) Building the NetworkTwo wires usually carry a signal voltage or current to or from thefield area.
In fieldbus, the wire pair is called a network. This definition ofa network is purposely narrow and includes only 31.25 kbit/s devices andsignaling. Notice that neither wire is grounded because this is one of theabsolute rules of fieldbus.
Figure 1: Simple Fieldbus Network 2) Adding to the NetworkWe can add to the network by tapping into the trunk at any point orby extending it. We can have a total of 32 devices on each segment of a networkwith some restrictions. One restriction is the total wire pair length in agiven segment. 3) Spurs Shorter spurs is better. The total spur length is limited according to the number of spursand number of devices per spur. A spur can be up to 120m in length if there arefew of them. 4) Repeaters If you need a lot more than 1900m of cable, it can be done by usingrepeater.
The repeater takes the place of one of the field devices. Toincreasing the length of network, repeaters can be used to increase the numberof devices in a network beyond the limit of 32 on one segment. 5) Mixing CablesOccasionally you might need to mix cable types. For communicationspurposes, the combined lengths of cable should work OK but in fieldbus, it maynot be possible to supply devices at the opposite end of the bus from the powersupply with the operating voltage current they require (due to effects of OhmsLaw and cable resistance). 6) Shielding For best performance, fiedlbus cables should be shielded. Commonmulti-conductor (mutli-core) “instrument” cable can be used.
When used shieldedcable, connect each spur’s shield to the trunk shield and connect the overallshield to ground at one point. 7) Polarity The Manchester signal used by fieldbus is an alternating voltagethat changes polarity once or twice per bit. The fieldbus receive circuits lookat only the alternating voltage.
Field devices must be connected to see thesignal in correct polarity. 8) DC Power for Two-Wire Field DevicesWe can’t use just any off-the-shelf power supply, because it wouldshort-circuit the fieldbus signals. Ifyou have 2-wire field devices in your network, you have to make sure they haveenough voltage to operate. Each device should have at least 9 volts. 9) Intrinsic SafetyThe number of field devices may be limited due to power limitations.A special fieldbus barrier and special terminators may be required.
The amountof cable may be limited due its capacitance or inductance per length. 10) Live WireFieldbus devices are designed for connection to a live network. Thisis done so that the network needs not to be shut down to service a device. 11) What Not to ConnectYou can’t connect non-fieldbus device such as light bulbs, analog4-20 mA field devices to the network. 12) Connecting to Higher SpeedFieldbus Networks These networks may be part a larger network operating at speeds of1.0 or 2.5 Mbit/s.
the 31.25 kbit/s network must never be connected directly toa higher speed network. A special device called a bridge must be placed betweenthem. 13) What If It Is Not WiredCorrectly?The nature of digital communication systems (including fieldbus) isthat they slow down if you don’t have things quite right. If a master device orbus analyzer tells you that there are numerous retries, this is a clue thatsomething’s wrong.
VII. Fieldbus Power Supply Power is one of theareas in which Foundation Fieldbus is quite similar to conventional analognetworks.Foundation fieldbuspower is:· Shared across many devices andsegments· Quite iften redundant· Multiplexed either through aseparate power multiplexer or through multiplexers integrated with thesupplies.
· 24 volts is the most commonsupply.Most existing analogbulk power supplies will work with FOUNDATION fieldbus and are good sources forFOUNDATION fieldbus bulk power. Power SourceConfiguration For Multiple Segments. Practical PointerMake sure the voltage atthe farthest point of the segments powered is at least 9 vdc when the batteriesare at their lowest expected operating voltage.
To ensure this, higher voltageis required at the power supply. Some plants have backup batteries that floaton a 24 vdc bus. These batteries take over if the AC/DC power supplies are lost.A margin of several volts is recommended. Power ConditionersIt’s critical thatcommunications on a segment not cross to other segments through the powersupplies.
Power conditioners prevent this “cross-talk” betweenmultiple segments using the same power supply. The power conditioner limitsmaximum segment power. A typical value is 400 mA.. If a typical device draws15-20 mA, a power conditioner could supply about 20 devices and still have somereserve capacity.
Power conditioners also current-limit the segment, so thatgrounding of one segment won’t affect other segments attached to the same bulkpower source. Practical PointerCable shields can begrounded to the power conditioner. But make sure the shield doesn’t touch afield device housing; letting it do so can create a second grounding point andthus cause a ground loop. Also, FOUNDATION fieldbus signaling uses a balancedline to provide more robust communications than a signal and ground. Balancedlines require that the individual signal wires NOT be grounded.
TerminatorsTerminators are simpleresistor-capacitor circuits used to prevent problems like signal reflectionfrom the end of the wires. They’re installed in pairs, with one terminator asclose as practical to each end of a fieldbus segment. Fieldbus segments havebeen known to work with terminators incorrectly installed or missing, but thissituation dramatically increases the chances of segment problems. Powerconditioners frequently include a terminator, eliminating the need for a separateexternal terminator on that end of the segment. However, terminators areusually NOT built into or installed in field devices. That’s because thesegment could be left without proper termination if the device that containsthe terminator is removed from service. Practical PointerPut a terminator in thejunction box that’s closest to the far end of a segment. Even if there areindividual devices farther out, the junction box is usually close enough to theend of the segment for the terminator to function properly.
Because terminatorsare very simple circuits, it’s tempting to make your own. But homemade unitsfrequently fail in installation, checkout, or service. Whatever you saved bymaking the terminators will be spent many times over fixing or retrofittingthem. RepeaterRepeaters are optionalcomponents used either to extend the length of a fieldbus segment or toincrease the number of devices on a segment. They provide power and a cleancommunication signal for the extended part of the segment. A segment can haveas many as four repeaters dividing the segment into five pieces.
Electrically,each piece acts as a separate segment — but devices can communicate with eachother as though they were on the same segment, even if there are up to two repeatersbetween the devices. Although a fieldbus segment can have up to 32 deviceswithout repeaters, H1 segments typically don’t have more than 12-16 deviceseven if repeaters are used.