Fieldbus Technology. The smart choice of Fluid Control Systems

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1 Fieldbus Technology The smart choice of Fluid Control Systems

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3 Contents Introduction Intelligent technology in the field Page 6 1. Technology 1.1. Function of field bus technology Page Automation with field bus technology Page Advantages of using field bus technology Page Industry requirements on a field bus Page PROFIBUS 2.1. Mode of operation Page PROFIBUS as a modular system Page Transmission systems Page Communication system: The PROFIBUS DP protocol Page Application profiles Page Integrationsystem Page FOUNDATION Fieldbus 3.1. Distributed intelligence Page The network is the control Page Link Active Scheduler Page The application is produced by function blocks Page Description and integration of field bus devices Page 29

4 4. Ethernet 4.1. Mode of operation Page 30 Topology Page Real-time capability Page Automation standards Page PROFInet Page Powerlink Page Ethernet/IP Page IDA Page High-Speed-Ethernet Page CAN (CANopen/DeviceNet) 5.1. Mode of operation Page Topology Page Bus access procedures Page International standardization: CANopen and DeviceNet Page 44 Features of CAN Page INTERBUS 6.1. INTERBUS topology Page INTERBUS LOOP Page Advantages of INTERBUS Page 47 4

5 7. AS-Interface 7.1. Mode of operation Page Topology Page Transmission reliability and interference immunity Page Safety at Work Page Basic data of AS-Interface Page HART 8.1. Cabling Page HART commands Page Communications-enabled field units from Bürkert Control units for pneumatically operated process valves Page 52 Valve couplers Page 52 Valve islands Page 52 Sensors Page 53 Mass flow controllers/meters (MFC/MFM) Page 53 Other field bus devices Page List of keywords Page 54 5

6 Intelligent technology in the field EtherNet The eighties was the decade in which automation technology made a fundamental leap in quality. The parallel wiring that was conventional until then was contrary to the need for complex communication with increasingly more digitized field units that ensured greater intelligence of the functional components in the field. Gradually, solutions with conventional wiring technology were displaced by advanced field bus technology. On the search for compatibility and universality As is the case with every genuine innovation, field bus technology also initially further developed in competition with differentiated solutions which were linked to company-own components. What was currently possible did not always coincide with the potential and dynamics of what was, in principle, an open technology. Catching the right bus became the essential question, and one which Bürkert responded to with a consistently customer-oriented approach towards practically-oriented standardization. The aim was just as simple as it is was elementary: units from different manufacturers should be able to be operated by the same bus system. The status quo: Application-specific standardization of systems As a key technology in the automation sector, field bus technology now offers a range of standardized bus systems that have been specialized and optimized for specific industries or specific applications. Opening up this intelligent technology with optimum efficiency for the customer is a welcome and sought-after challenge for our teams of consultants who, owing to their pioneering experience, possess the crucial knowledge for developing future-oriented solutions. And what would highly qualified engineers find more motivating than an unsolved problem? The fact that Bürkert has the tickets for futureoriented field bus technology worldwide makes the choice simple for our customers, but difficult for our experts who wish to be challenged by new tasks. 6

7 Catching the right bus The evolution of network technology has essentially developed from the principle of centralization through to distributed intelligence. Of course, this also necessitates components that comply with all aspects of the new "command structure. Maximum availability and minimum possible downtimes are but two key aspects of more efficient, i.e. advanced, operation of a system or installation which is based on future-proof field bus technology. It is certainly worth considering opting for a technology leader who has been involved right from the very start and who can therefore provide the appropriate solution to an individual problem as an integrated system. With Bürkert, you are riding the bus to the future. Central Open-loop control Data Program Distributed Communication relationships/data exchange Open-loop control c Data Program c Networking: information on the future of networking Various user associations track the ongoing development of individual bus systems. Visiting the following websites will fill you in on the latest: AS-International Association CANopen DeviceNet Ethernet FOUNDATION Fieldbus HART Communication Foundation INTERBUS Club PROFIBUS International (PI) Application 1, e.g. PLC Distributed network Application 2, e.g. CNC Application 3, e.g. visualization Distribution of variables in the network 7

8 1. Technology 1.1. Function of field bus technology Communication Field buses enable the digital networking of open-loop control systems, sensors and actuators. Data is exchanged both horizontally between the devices of a level and vertically to the systems in the next hierarchy level. Factory or coordinating level In order to achieve a practicallyoriented classification, we assign the communication structures in automation engineering to various application levels (Figure 1). The coordinating level monitors higherlevel plant control while the automation level controls the actual process. The focus is on the reliable transmission of even very long messages (file transfer). At the field level, data transmission of the measured values and manipulated variables is cyclic in many cases and necessitates as high an efficiency as possible so as not to impair real-time characteristics of the open-loop control application. In this connection, we usually speak of data-oriented communication. Moreover, field buses also support access to field units of the upper levels, e.g. engineering stations from the automation or coordinating level. Process data and status information can be read out, parameters can be polled and set and, in some cases, software can even be downloaded and program routines started for configuration, operation, monitoring and diagnosis, at the instigation of the user. This form of acyclic data exchange is referred to as messageoriented communication. Automation level Field level Plant installations Figure 1: Application levels in automation engineering 8

9 Network topologies All field bus systems are based on the same idea of allowing addressable devices to use a common transmission medium. The network topology describes the spatial extent of a field bus network, but also the logical arrangement of the devices during communication. A B C Linear The bus or linear structure is clearly arranged and features little complexity. This is where all users communicate via a common line. The devices are linked either with or with-out very short stubs. Occasionally, this leads to untidy cabling in practice. D E A D Tree The tree structure is similar to the linear structure, the only difference being that several bus branches may converge at the nodes. The tree structure enables large areas to be networked more easily and more flexibly. F E B E A D Ring If a physical ring is constructed with several two-point connections, it is termed a ring structure. A message to be transferred is passed on from one user to the next. Since the signal can be amplified each time it is passed on, it is possible to span very large distances. C F B C F E A G D Star A central station is linked to all users with two-point connections in a star structure. This central station may either be responsible for network control as the Master, or as a star coupler, may simply establish the connection between the current sender and receiver. Complex network structures frequently consist of several independent subnetworks. Each of these sub-networks can work with differing topology and a different communication protocol. B C 9

10 Sender 7 Application layer 6 Presentation layer 5 Session layer 4 Transport layer 3 Network layer 2 Data link layer 1 Physical layer Figure 2: ISO/OSI model Physical transmission medium Standardization Rules must be defined for all communication partners so that communication between various users and across the network hierarchy levels can occur effectively and without misunderstandings. This is achieved with the ISO/ OSI model (Figure 2), which describes all elements required for communication, such as the cable type or physical mode of transmission of the messages. The model features seven layers that build upon each other, each of which describes a specific task. The ISO/OSI model has also become established as a virtual standard representation when implementing communication services outside of field bus technology, since it fundamentally describes the communication sequence. If specific services (layers) are no longer required within a communication system, these layers remain empty. Receiver 7 Application layer 6 Presentation layer 5 Session layer 4 Transport layer 3 Network layer 2 Data link layer 1 Physical layer Applicationoriented layers Transportoriented layers Generally, only layers one and two are fully defined when specifying field bus networks, while the application process itself or the subordinate layer seven handles all other services. Layer one defines the way in which data transmission is performed physically, i.e. electrically and mechanically. This includes, for example, the method of coding (e.g.: NRZ) and the transmission standard used (e.g.: RS-485). Layer two has the task of providing integral, i.e. error-free, infor mation. It must detect any errors which have occurred in layer one and remedy these errors via suitable error routines. Layer seven forms the interface to the application program and contains all functions with which the user, generally a computer program, can access the communication functions Automation with field bus technology Hierarchy levels in field bus networks Due to the various technical options and characteristics of the individual bus systems, system discontinuity may occur in the installations if the user uses different bus systems or variants of a bus system, for instance, in order to access the explosion-hazard area with the aid of PROFIBUS PA via PROFIBUS DP. Simple networking of the components used is just as important as linking them to lower-level and higher-level network structures. This may be achieved using gateways (protocol converters), which allow transition between the various bus systems. Utilizing a gateway, the AS-i system, for example, which is particularly suitable for data exchange within the I/O level (input/output level for generally simple sensor and actuator systems), can thus be integrated in a higherlevel field bus, such as INTERBUS or PROFIBUS, which possess broader technical capabilities in the area of the field and process level. In turn, field level buses offer interworking with Ethernet for communication with higher-level networks. The process and parameter data is forwarded this way and enables vertical integration of the application. 10

11 Figure 3: Hierarchy levels in automation engineering Even today, Ethernet already plays a crucial role in higher-level structures. Practical vertical integration allows universality of presentation and availability of process data and system statuses. If the machines and systems interwork with Ethernet (via subordinate bus structures), the resultant central system operation and telemaintenance provide the user with a great savings potential Dynamics of field bus communication If Ethernet is to be used in automation engineering instead of a classic field bus system, ( hard ) real-time capability is particularly important. Basically, the term real time is a question of definition. Thus, real time in the case of synchronization of drives or actuators may amount to microseconds, while times in terms of seconds are adequate in processengineering applications. If we compare the various field bus systems and Ethernet with regard to efficiency of data transmission, Ethernet achieves a poorer value. This results from the CSMA/CD procedure used (see also Section 4.1.), which must operate with a long minimum telegram length due to unconditional and secure collision detection. However, this disadvantage is compensated for by the high transmission speeds of up to 100 Mbit/s. Such high transmission speeds can be implemented only by a point-topoint connection between the units, which, besides Ethernet, is only offered by the INTERBUS system. On systems with a variable transmission speed, such as PROFIBUS or CAN, the maximum possible network extent is reduced with increasing transmission speed. The higher the transmission speed, the shorter the line length. This may lead to a situation in which the communication link to be laid is only a few meters long, which is not necessarily disadvantageous within closed systems or system sections. 11

12 Status of system components with display of alarms Maintenance actions Automatic Database with Help function Error trend Figure 4: Asset management operating program Operational optimization by asset management The aim of asset management in automation is to effectively manage and optimize the use of equipment and systems. This includes, for instance, the ability to plan the required maintenance, minimization of faults occurring, enhancement of process diagnostics and process monitoring as well as both identification and utilization of function reserves. This requires complex information that must be obtained from the overall automation system. Asset management can function only by the interplay of intelligent field units, a highly developed communication structure and corresponding operating software. Thus, for example, diagnostic information from the field level is sent via the field bus to corresponding asset management stations where it is evaluated. System-oriented asset management is aimed not only at maintaining an existing system, but is already deployed in the engineering of components in process control engineering. It includ-es programming and configuring the field units and opens up access to system documentation and to the operating environment of the installation. Measured on the basis of the lifecycle of an installation, maintenance and, in particular, status detection of field units and other system components are very important. Measured values which characterize the status of equipment and machines and which are processed by the asset management system (AMS) on the basis of characteristics or models to form trend information or which combine these values centrally from other information systems are used for this purpose. 12

13 As the decision-making basis for maintenance measures, AMS must also offer access to documents such as shift logs, system documentation and CAE systems, in addition to current status information. With all asset management solutions, it is necessary to bear in mind the basic requirement that all technical operational support activities have to be performed from one workstation Central device management If a field bus network is constructed on the basis of devices from one single manufacturer and if the manufacturer s devices can all be operated in the same way, one manufacturerspecific software package suffices as the user interface. However, an average installation can certainly cover over 100 different field unit types from ten or more different manufacturers, which frequently also may involve ten or more different operating programs for configuring and programming the field units. In order to at least partially ameliorate this situation, proprietary descriptions (languages) based on the Standard Device Description Languages (DDLs) have been developed. However, each of these languages is tailored to the specific communication systems for which it was originally developed. Virtually every configuration tool and every field bus standard has implemented its own device description language or at least uses a dialect of the HART Device Description, which was developed at a very early point. The operating methods for PROFIBUS (GSD, EDD, DTM), HART devices (DD), DeviceNet (EDS) and FOUNDATION Fieldbus devices (DD, DTM) are examples that emerged from this. Thanks to the creation of an open and standardized communication platform, at least referring to the relevant system, field units are able to be easily integrated in the given control and instrumentation system structure and operated centrally via a common engineering tool Advantages of using field bus technology Comprehensive costing of an automation solution will cover the investment costs required for procuring the MCR equipment. Just as importantly, it is also necessary to allow for costs incurred during commissioning and subsequent expansion and conversion during system operation. The term Total Cost of Ownership has come into being for this cost analysis. Wiring system Using a field bus drastically reduces the cabling expenses, effort and complexity. While thick cable harnesses were run between MCR area and the field using conventional technology, field bus technology enables integrating the same I/Os by using one pair of conductors. This advantage is escalated in the savings on terminal blocks, control cubicles, lightning protection facilities and explosion barriers. Central engineering PLC Field bus Gateway Remote I/O 13 Field bus-enabled devices Figure 5: Device operation 4-20 ma devices (HART)

14 14 For the user, this brings a tremendous savings potential in terms of space requirement and costs for the entire MCR wiring. One other tremendous advantage is as follows: The reduction in wiring effort and expense leads to the same reduction in documentation expenditures related to all electrical wiring diagrams and ladder diagrams. Flexibility A new field unit can be added to a field bus at any point without having to separately lay a complete cable raceway. Subsequent modifications and extensions thus pose no problem. This applies in particular to wiring with the two-wire system in which data and power are transferred in one cable. Commissioning Significant advantages can be anticipated in relation to the duration of the commissioning phase. Modern technologies mean faster integration of the field components (loop check and calibration) in the process control system. Wiring errors are prevented thanks to the simple cabling. If difficulties within the network structure occur nevertheless, these are rapidly diagnosed using bus testers and bus monitors. Maintenance Warning and error signals of the process devices constantly inform the system operator of the current operating state of the system. This allows the system operator to precisely assess the situation and take suitable measures based on it. For example, if a measuring circuit malfunction occurs on a control valve, maintenance personnel are informed of the error or fault detected. Owing to the access to the field unit via operating programs, the maintenance technician receives detailed information on the fault or error which has occurred and can remedy the situation in targeted manner and within a very short time. System availability One other substantial cost advantage results from the reduction in downtimes as a result of unequivocal and detailed diagnosis from the field units and an associated enhanced machine and system availability. Intelligent field units issue precise fault or error descriptions to the system operator or even signal failures before they occur (predictive maintenance). Universality All process data, device data or business management data is available via a universal communication structure from all locations, and even outside of the system via the Internet. This allows central and distributed operation and engineering. Comprehensive and central data management forms the basis for operational optimization in every system Industry requirements of a field bus Various factors are crucial to selecting the field bus system to be used. Essentially however, the field bus requirements necessitated by the application play a crucial role. Since every system is able to meet specific requirements particularly well due to its technical characteristics, various field buses have a high share of the market in individual industries. Production industry Production based on lot sizes and execution of repetitive, frequently mutually independent work steps are characteristic of the production industry. The level of decentralization within a production plant is low. The requirements applicable to communication between programmable controller (PLC) and field units are very stringent. In many sectors, such as robotics, measuring technology and test and inspection technology, stringent real-time requirements arise with cycle times less than 20 milli-seconds. In many cases, equidistant data transmission is required in drive engineering, e.g. for axis interpolation. The requirement regarding safety against system failure is only moderate. In many cases, halting production in the event of device failure costs less than designing the entire system fully redundantly. More stringent requirements related to fail-safe design exist in areas in which persons may be placed at risk, e.g. burner controls, press controls and lathes, etc.

15 Process industry Production is generally batch-oriented in the food industry, chemical industry and pharmaceuticals industry as well as within the process industry and process engineering. Typically, the process industry utilizes very complex installations that are greatly decentralized and implemented over a very large area in the form of distributed systems. The volume of project data in such installations may comprise several hundred thousand data points. This means that the requirements for process control systems are very stringent as regards handling the data volume. However, the time aspect is less critical and is within the range of seconds in many cases. One fundamentally important criterion in the process industry is high availability. The systems are not switched off due to the complex and long, drawn-out starting procedures for continuous processes, which often last several hours. Expensive redundancy concepts with hot standby prevent fault or error-related interruption in the process. The requirements in the areas of maintenance and commissioning are correspondingly stringent. It must be possible to convert or extend the system during ongoing operation. Additional safety requirements apply to explosion-hazard environments, such as in the petrochemical and gas industries. The entire MCR system in the field must ensure that the legally required safety regulations, such as ATEX Directives, are complied with. Depending on the level of danger or hazard, there is a classification into zones from 0 to 2 which, in turn, allow only specific automation concepts, including the communication method to be used. In the illustration below, widespread field bus systems are shown on the basis of their main applications. The types on the left focus on use in the production industry. A special role is assigned to Ethernet, which connects industrial networking to office communication. The buses for process automation, which also cover the requirements of the explosionprotected area, are shown on the right. The cross-industry characteristics can also be seen with the PROFIBUS and AS-Interface. Zone 2 Zone 1 Ethernet FOUNDATION Fieldbus DP focus PROFIBUS PA focus Interbus CANopen DeviceNet HART (no fieldbus) AS-Interface Production industry Process industry 15 Figure 6: Major industrial focuses of field bus types

16 2. PROFIBUS PROFIBUS (PROcess FIeld BUS) is a universal, open, digital communication system. It opens up diverse applications from production automation through to process automation. PROFIBUS is suitable for fast, timecritical and complex communication tasks Mode of operation PROFIBUS uses cyclic data exchange for communication. Each field unit (slave) exchanges its measured values and set-point values with the programmable controller, the Class-1- Master (PLC, controller), in a stipulated cycle time (deterministic). This master-slave communication on which the field units are served centrally and consecutively is referred to as polling. Besides the programmable controller, a visualization system (Class-2 Master) is also necessary for system monitoring and operation. The Class-2- Master is responsible for the diverse commissioning, programming and monitoring functions of modern field units. Related data exchange occurs if required, thus acyclic communication services are used on the Class- 2-Master. The master function is allotted in this multi-master system in a fixed sequence: the token-passing procedure. For this purpose, a special message, the token, is passed from one active master to the next within a logical ring. This bus access procedure comprising the master-slave and token-passing procedures is referred to as a hybrid access procedure. Logical token ring between master devices Active stations, master devices Class-1 Master Class-2 Master PLC PROFIBUS DP 16 Passive stations (slave devices) are polled Figure 7: PROFIBUS network with master and slaves

17 2.2. PROFIBUS AS A MODULAR SYSTEM PROFIBUS is designed on the basis of a modular principle owing to the provision of various transmission technologies, its versatile communication protocol and numerous application profiles. The PROFIBUS modular system describes the technological capabilities of this field bus type as a whole and thus covers diverse and application-specific requirements: Horizontal universality: standard automation technology for differing applications and industries within one system (upstream, mainstream and downstream) Vertical universality: from the field level through to the company level. From a technological point of view, the PROFIBUS system structure is based on the ISO/OSI Reference Model (see also Section , Standardization) and consists of specifications of the following basic elements: Transmission technology Definition and description of hardware (physical transmission system) Transmission medium: copper, fiber-optic cable or waveguide, radio Signal level: RS-485, MBP Topology: linear, stubs, star Transmission speed: baud rate (variable and fixed). Communication technology Definition of the PROFIBUS DP protocol via which communication occurs between the bus users. Three rating classes are available for PROFIBUS DP: DP V0 cyclic data exchange (process data) DP V1 (incl. V0) acyclic data exchange (useful data) DP V2 (incl. V1) additional services (with the focus on drive engineering). Application profiles Cross-manufacturer stipulations of characteristics, performance features and behaviors of the devices, e.g.: PA Devices Definition of functions and parameters for process devices in process engineering PROFIsave Profile for safety-oriented applications (SIL) PROFIdrive Definition of the device behavior and access procedure for drives and actuators. PROFIBUS Specific application profiles Device behavior, operation, configuration, programming Integration technoloy Integration and operation of the devices General application profiles Cross-application behavior of functions Communication technology Cross-application behavior of functions Transmission technology Transmission medium, connection system, interfaces, network topology 17 Figure 8: PROFIBUS modular system

18 18 Integration technology Description of the integration of field units in process control systems and configuration tools: GSD (necessary) Electronic Data Sheet (cyclic communication) EDD (optional) Textual Device Description (acyclic communication) DTM/FDT (optional) Device Operating Program (acyclic communication via the standardized FDT interface). Modular elements establish the PROFIBUS From the user s point of view, only the elements required for the tasks to be automated are taken from the PROFIBUS modular system. This means the following are selected: The appropriate transmission medium/topology: transmission system The required protocol rating class: communication system The profile (optional): application profiles The required and optional device integration: integration system. PROFIBUS is thus presented in the form of various, application-specific focal points which have not been permanently defined, but which have proven to be practical in frequent applications. Each focal point is produced by a typical (but not mandatory) stipulated combination of the modular elements of these specified groups. The following examples explain this principle. PROFIBUS DP Focus PROFIBUS DP is the variant for production automation; it typically deploys: RS-485 as the transmission system; the DP communication protocol in its rating classes; however, generally DP V0 one or more optional application profiles typical for production automation, e.g. identification systems or PROFIdrive GSD as integration for purely cyclic communication. Application Integration Communication Transmission PROFIBUS PROFIBUS DP Focus e.g. Ident profile GSD DP protocol RS-485 PROFIBUS PA Focus PROFIBUS PA is the variant for process automation, typically with: the MBP transmission system DP V1 communication protocol rating class PA Devices application profile GSD for cyclic data transmission and, e.g., EDD technology for acyclic data transmission. Application Integration Communication Transmission PROFIBUS PROFIBUS PA Focus PA Devices profile GSD/EDD DP protocol MBP-IS 2.3. Transmission systems RS-485 The simple and economical RS-485 transmission system is mainly used for tasks necessitating a high transmission speed without intrinsic safety. A twisted, shielded copper cable with one pair of conductors is used. The bus structure allows non-retroactive coupling and decoupling of stations or step-by-step commissioning of the system. Subsequent extensions do not affect stations which are in operation within defined limits. Network topology RS-485 All devices are connected in a bus structure (linear). The transmission speed can be selected in the range between 9.6 kbit/s and 12 Mbit/s. It is defined uniformly for all devices on the bus when commissioning the system. Up to 32 bus users may be connected per segment and the maximum permitted line length is dependent on the transmission speed. It is shown in Table 1. Transmission Range speed per segment [KBit/s] [m] 9.6; 19.2; 45.45; 1, , , ,000; 6,000; ,000 Table 1: RS-485

19 The start and end of each segment are provided with an active bus termination, and it must be ensured that both bus terminations are constantly powered in order to achieve disturbance-free operation. Repeaters which connect the individual bus segments must be used in the case of more than 32 stations or to expand the network extent. However, a maximum of 126 devices (masters or slaves) can be connected to a bus (specified address space: 0-125). MBP The MBP transmission system (Manchester Coded, Bus Powered, previously IEC Physical Layer ) is available for process automation applications involving the requirement for bus powering and intrinsic safety of the devices. Wiring comprises the twowire system, which means that both bus communication and the power supply of the field units use one twisted pair of wires, the bus cable. The Fieldbus Intrinsically Safe Concept (FI- SCO, see following section), developed specifically for interconnection of in-trinsically safe field bus devices, substantially simplifies the design and installation as compared to the procedure conventionally used previously. Network topology MBP MBP uses synchronous transmission with a fixed transmission speed of kbit/s and Manchester-II coding. In general, line topologies, lines with stubs or star topologies are possible. These may also be combined. It must be noted that a stub may have a maximum length of 30 m in intrinsically safe applications. The maximum extent per segment is 1,900 m, but is dependent on the application area (explosion group and category) and the line cross-section. One example of a typical application: with instrumentation having EEx ia/ib IIC type of protection, the maximum cable length is approx. 1,000 m. The number of users connectable to a segment is limited to 32. However, it is determined by the selected type of protection and is typically between 6 and 9 devices in the case of intrinsically safe applications. A two-core, screened cable is used as the transmission medium. The main bus cable is provided with a passive line terminator at both ends. The bus terminator is already permanently integrated on the segment coupler or on the link. Reverse-polarity connection of a field unit with the MBP system does not affect operability of the bus since these devices normally feature automatic polarity detection. Wiring information for MBP The intrinsically safe transmission system MBP is generally restricted to specific sub-segments (field units in the explosion-hazard area) of a system, which are then connected to the RS-485 segment via segment couplers or links (Figure 8). Segment couplers are signal converters which adapt RS-485 signals to the MBP signal levels and vice versa. They are transparent from the standpoint of the bus protocol. By contrast, links have their own intelligence. They map all field units connected in the MBP segment upwards as a single slave in the RS-485 segment; it acts as master downwards. Process controlsystem Visualization/ engineering PROFIBUS RS 485 up to 12 Mbit C MBP kbits PROFIBUS Segment coupler/link 19 Figure 9: PROFIBUS topology

20 MBP RS-485 RS-485-IS Fiber-optic Data transmission Digital, bit-synchronous Digital, differential Digital, differential- optical Manchester coding signals to RS-485 signals to RS-485 digital NRZ NRZ NRZ Transmission speed kbit/s 9.6 to 12,000 kbit/s 9.5 to 1,500 kbit/s 9.5 to 12,000 kbit/s Data integrity Preamble HD = 4, parity bit HD = 4, parity bit HD = 4, parity bit error fail-safe start start and end delimiters start and end delimiters start and end delimiters and end delimiters Cable Twisted, shielded Twisted, shielded Twisted, shielded Multimode and singletwo-wire line two-wire line two-wire line mode glass fiber, PVC cable type A cable type A plastic fiber Remote powering Optionally via the Possible via Possible via Possible via hybrid signal wires signal wires signal wires line Types of protection Intrinsic safety None Intrinsic safety None (EEx ia/ib) (EEx ia/ib) Topology Linear and tree Linear topology with Linear topology with Star and ring topology topology, also termination termination typical, linear topology combined with possible termination Number of users Up to 32 users per Up to 32 users per Up to 32 users per Up to 26 users per segment; total of segment without repeater segment; total of per network max. 126 per network up to 126 per network max. 126 per network Number of Max. 4 Max. 4 with signal Max. 9 with signal Unlimited with signal repeaters refreshing refreshing refreshing (note signal propagation time) Table 2: PROFIBUS transmission systems 20 The FISCO model The FISCO model (Fieldbus Intrinsically Safe Concept) offers substantial simplification when planning, wiring and extending PROFIBUS networks in explosion-hazard areas. This model was developed in Germany by the Physikalisch Technische Bundesanstalt (National Standards Laboratory PTB) and today is acknowledged, even internationally, as the basic model for operation of field buses in explosion-hazard areas. If FISCO-approved devices are used, not only is it possible to operate several devices on one line, but the devices can also be replaced, even during operation, by other manufacturers devices or the line can be extended during operation. All this is possible without complex calculation and without a system certification. That means Plug & Play in the explosion-hazard area. All that needs to be noted are the rules for selecting supply units, line length and bus terminations. Transmission in accordance with the MBP and FISCO model is performed in accordance with the following principles: All bus devices must be approved in accordance with FISCO. In each segment, there is only one infeed source: the segment coupler/link. Each field unit consumes a constant basic current of at least 10 ma. The cable length may not exceed 1,000 m (type of protection i, Category a) or 1,900 m (type of protection i, Category b). For all combinations between supply unit and field units, it must be ensured that the permissible input variables of each field unit (Ui, Ii, and Pi) are greater than the maximum output variables (U0, I0 and P0) of the related supply unit which are possible and permitted in the event of a fault. In addition, for reasons relating to operational reliability, it must be ensured that all field units are adequately powered. The sum of the current consumption of all field units and the FDE value must thus lie below the maximum supply current of the supply unit (coupler or link), whereby, in the case of many supply units, a further 9 ma must be taken into account for modulation of the data signal. The FDE (= Fault Disconnection Equipment) ensures that, even in the event of a short-circuit in one unit, the communication of the entire segment does not fail. When calculating, the value of the field unit with the highest FDE value must be taken into account. RS-485-IS There is major interest among users in also being able to use RS-485 with its high transmission speed in the explosion-hazard area as well.

21 Function levels The PROFIBUS International has taken up this task and has elaborated a guide on project planning of intrinsically safe RS-485 solutions with easy interchangeability of the devices. The ongoing investigations by the testing facility would lead one to anticipate that, as is the case with the standard version, up to 32 users can be connected to the intrinsically safe bus circuit. Optical waveguides There are field bus operating conditions in which wire-bound transmission systems have their limits, for example in the case of environments subject to strong interference or when spanning particularly long distances. In such cases, optical transmission with optical waveguides is available. On account of the transmission characteristics, star and ring are the typical topology structures, linear structures are also possible, however. Implementing an optical waveguide network, in the simplest case, is done via the use of electro-optical transducers that are connected to the device via an RS-485 interface as well as to the optical waveguide. This also enables switching between RS-485 and optical waveguide transmission within a system, depending on the situation Communication system: The PROFIBUS DP protocol The PROFIBUS DP (Decentralized Peripherals) communication protocol is designed for fast data exchange at the field level. This is where central programmable controllers, such as PLCs, PCs or process control systems, communicate via a fast serial connection with distributed field units DP V2 Data Exchange Broadcast (Publisher/Subscriber) Isochronous mode (equidistance) additional extensions: Time synchronization and time stamping HART on PROFIBUS DP Up/download (segmentation) DP V1 Acyclic data exchange between PLC and field devices additional extensions: Device integration: EDD and FDT Portable PLC software function blocks (IEC ) Fail-safe communication (PROFIsafe) Alarms DP V0 Cyclic data exchange between PLC and field devices additional extensions: GSD configuration Diagnosis Figure 10: Functionalities of PROFIBUS DP rating classes such as I/O, drives or actuators, valves, transducers or analyzers. Data is exchanged mainly cyclically between these units. The communication functions required for this are defined by the DP basic functions (rating class DP V0). Beyond these basic functions, DP has been gradually expanded with special functions, aimed at the specific requirements of the various application areas, so that DP is available today in three rating classes: DP V0, DP V1 and DP V2, with each class featuring a specific focus. This classification primarily reflects the temporal sequence of the specification work, as a consequence of the extended requirements of the applications. Rating classes V0 and V1 contain both characteristics (these are mandatory for implementation) and also options, while class V2 specifies only options. The most important contents of the three classes are as follows: DP V0 Makes available the basic functionalities of DP. These include cyclic data exchange and station, module and channel-specific diagnosis. Device characteristicsm Time DP V1 Contains supplements aimed at process automation, primarily acyclic data exchange for programming, operation, observation and alarm recovery of intelligent field units, in parallel with cyclic useful data exchange. This enables online access to bus users via engineering tools. In addition, DP V1 contains alarms. This includes, among others, the status alarm, update alarm and a manufacturer-specific alarm. DP V2 Contains further supplements and is primarily aimed at the requirements of drive engineering. Owing to additional functionalities, DP V2 can thus also be used as a drive bus for controlling fast sequences of motion on drive axes. Among others, services include the following: Slave-to-slave communication (DXB) This function allows direct and, thus, time-saving communication between slaves via broadcast, without having to take the roundabout route via a master. Isochronous mode This function allows clocksynchronous control in master and slaves, regardless of the loading of the bus. Clock control This function synchronizes all bus users to a system time. 21

22 2.5. Application profiles Profiles are specifications made by manufacturers and users on specific characteristics, performance features and behavior of devices and systems. Profile specifications are aimed at being able to operate devices and systems which belong to a profile family on the basis of a profile-compliant development, interoperably on one bus as well as exchangeably, up to a certain degree. Profiles allow for application and type-specific special aspects of the field units, controls and integration resources (engineering). The most important of these are: PA Devices The PA Devices profile defines parameters and function blocks of field units in process automation, e.g. digital positioners, transmitters and I/O boxes. These enable interoperability and exchange of one field unit with that of another manufacturer (= interchangeability). The PA Devices profile is available in version 3.0. PROFIsafe PROFIsafe defines how safety-related devices (emergency stop buttons, light grids, overfilling safeguards, etc.) communicate reliably via PROFIBUS with safety controls, enabling them to be used in safety-related automation tasks up to CAT4 in accordance with EN954, AK6 or SIL3 (Safety Integrity Level). It implements safe communication via a profile, i.e. via a special useful data format and a special, higher-level protocol. Designation Profile content Current status PUO Directive PROFIdrive The profile specifies the behavior of V devices and the procedures for access V to data for variable-speed electrical drives and actuators on PROFIBUS. PA Devices The profile specifies the characteristics V of process-engineering devices in process automation on PROFIBUS. Robots/NC The profile describes how manipulator V and assembly robots are controlled via PROFIBUS. Panel Devices The profile describes coupling of V1.0D simple operating devices and observation devices (HMI) to higher-level automation components. Encoder The profile describes coupling of rotary V encoders, angle encoders and linear encoders with single-turn or multi-turn resolution. Fluid Power The profile describes control of hydraulic V drives and actuators via PROFIBUS. Cooperation with VDMA. SEMI The profile describes characteristics of the devices for semiconductor manufacture on PROFIBUS (SEMI Standard). Low Voltage The profile defines data exchange for low Switch Gear voltage switchgear devices (switch-disconnectors, motor starters, etc.) on PROFIBUS. Dosage/Weighing The profile describes the usage of weighing and dosing systems on PROFIBUS DP. Ident Systems The profile describes communication between devices for identification (barcode and transponders). Liquid Pumps The profile defines the use of liquid pumps on PROFIBUS DP. Cooperation with VDMA. Remote I/O for Owing to their special position with regards PA Devices to bus operation, the remote I/Os are provided with a different device model and different data types as compared to PROFIBUS PA Devices. 22 Table 3: Application profiles (specific)

23 Cyclic process I/O Engineering System Acyclic device operation Acyclic device operation Application Acyclic integration in communication M M M M M Integration Cyclic integration EDD EDS EDS EDS DTM Field units Figure 11: PROFIBUS integration systems HART on PROFIBUDS DP In view of the very large number of HART devices installed in the field, integrating them in existing or new PROFIBUS systems is an urgent task for most users. The HART on PROFIBUS DP profile offers an open solution for this. PROFIdrive The PROFIdrive profile defines the device behavior and the access procedures to drive/actuator data for electrical drives/actuators on PROFIBUS, from simple frequency converters to highly dynamic servo-controllers Integration system Modern field units provide a diverse range of information and perform functions which were previously the domain of PLCs and process control systems. Accordingly, in order to allow open-loop controls or the process control system to achieve smooth cyclic data exchange with field units, it is necessary to declare ( integrate ) the specific parameters and data formats to the field units. The operating programs for commissioning, maintaining, engineering and programming these devices require a precise and complete description of the device characteristics. These are the functions and data of the devices, such as the type of application function, configuration parameters, units of measure, value ranges, limit values, default values, etc. Methods with which device management can be standardized have been developed by PROFIBUS for such a device description. The scope of services of these methods are optimized for specific tasks, thus the term structured device integration is commonly used for this. 23

24 Electronic Data Sheet (GSD) The GSD is the obligatory passport of every PROFIBUS device. It contains the characteristic data of the device, information on its communication capabilities and further information on diagnosis values, for example. The GSD suffices alone for device integration for cyclic exchange of variables and manipulated variables between field unit and programmable controller. The GSD is an electronic data sheet provided by the device manufacturer a simple text description of the device characteristics for PROFIBUS communication the basic description for every PROFIBUS device which needs to be integrated by the engineering system for configuration of the PROFIBUS network for cyclic communication with the PROFIBUS master. Electronic Device Description (EDD) The GSD alone does not suffice to describe application-specific functions and parameters of complex field units. A more powerful language is required for configuration, programming, commissioning, maintenance and diagnosis of the devices from the engineering system. For this, PROFIBUS has further developed the Electronic Device Description Language (EDDL), which was standardized in IEC and with which EDDs are generated. An EDD is a text device description independent of the engineering system s operating system the description of the acyclically communicated device functions, including graphic capabilities; it also describes device information such as ordering data, materials and maintenance information, etc. a file developed and provided by the device manufacturer which is used in addition to the GSD the basis for execution and presentation by an EDD interpreter. The EDD interpreter supplies the data for visualization with a standard look and feel extending beyond devices and manufacturers for the operating program. We can compare it to an Internet browser which interprets the source code of an HTML page and displays it on the screen. Siemens currently offers such an interpreter with the Process Device Manager (PDM). EDD 24 Figure 12: EDD interpreter

25 Device Type Manager (DTM) and Field Device Tool (FDT) interface In comparison to the GSD and EDD technologies which are based on descriptions, FDT/DTM technology is a software-based method for device integration. DTM is a software component for device operation and communicates with the engineering system via the FDT interface. FDT/DTM means that flexibility and degrees of freedom of software programs can be used for device integration throughout the entire lifecycle. A DTM is a device operating program which makes the device functionality (Device DTM) or the communication capabilities (Communication DTM) useable features the standardized FDT interface (Field Device Tool) to a frame application (engineering system); is comparable to a printer driver, able to run in all FDT frame applications and is programmed on a device-specific basis by the manufacturer contains an individual user interface for each device is used in addition to the GSD. The FDT interface is an open interface specification developed on a cross-manufacturer basis (not a tool as the name suggests) serves the purpose of open integration of field units of various manufacturers via DTMs in operating programs and, further on, in process control systems defines the interplay between the DTMs and an FDT frame application in the operating tool or engineering system. Note: Some of the content included in the above-mentioned information on PROFIBUS originates from publications of the PUO, the PROFIBUS User Organization. More extensive information can be found at DTM DTM DTM 25 Figure 13: FDT frame application