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Foundation fieldbus communication
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13.1 FIELDBUS TECHNOLOGY 13.1.1 INTRODUCTION In the early 1990s, foundation fieldbus (FF) emerged in the market. Two parallel supplier consortiums, Interoperable Systems Project (ISP) and WorldFIP North America, merged to form the Fieldbus Foundation Organization. The new organization immediately brought critical mass to achieve an internationally acceptable fieldbus protocol. The foundation organized development programs and conducted field trials for end users to drive FF technology in industrial applications. FF is a communication protocol, which is “all-digital, serial, two-way multidrop communication system that interconnects fieldbus devices with the control system”. FF is fully digital, serial, two-way, multidrop, communication system running at 31.25 Kbits/s, which will be used to connect intelligent field equipment such as sensors, actuators, and controllers. It serves as a local area network (LAN) for the instrumentation used within process plants and facilities with built-in capability to monitor and distribute control applications across the network.
13.1.2 OVERVIEW An FF system is a distributed system composed of field devices and control/monitoring equipment integrated into the physical environment of a plant or factory. FF devices work together to provide I/O and control for automated processes and operations. FF systems may operate in manufacturing and process control environments. Some environments require intrinsic safety where devices typically operate with limited memory and processing power and with networks that have low bandwidth.
13.1.2.1 Features The fieldbus retains the desirable features of the 4–20 mA analog system such as given below: • • • •
Single loop integrity Standardized physical interface to the wire Bus-powered devices on a single wire pair Intrinsic safety options In addition, FF enables:
• Enhanced capabilities owing to full digital communication, reduced wiring, and wire terminations due to multiple devices on one wire Industrial Process Automation Systems Copyright © 2015 Elsevier Inc. All rights reserved
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• Increased selection of suppliers due to interoperability • Reduced loading on control room equipment due to distribution of control and input/output functions to field devices • Connection to the HSE backbone for larger systems The advantages of the FF over the legacy systems are: • • • • • • • • • • • •
The signals are digital, therefore more immune to noise. The requirement for damping to filter out noise is eliminated. FF automatically detects all connected devices and includes them on a live list. Addresses are automatically assigned, eliminating any possibility of duplicate addressing. Traditional I/O use 16- or 32-channel cards; these are costly and a weak point. Module failures can generally cause all associated loops to crash. Accidental removal during faultfinding will affect all 16 or 32 loops. Minimizing the components reduces the failure probability. No requirement to manually configure alarms to detect transmitter failure or broken signal cable. FF builds this automatic safety function. FF uses engineering units, not scaled ranges; hence measuring actual process variable, not scaled or a % of 4–20 mA. This eliminates the need for range configuration. Conflict in the ranges is not possible. Analog to digital conversion is removed, improving accuracy and reliability. Dual measurement of parameters is possible from a single instrument. Failure prediction is possible to the data available.
13.1.2.2 Benefits FF enables increased capabilities due to full digital communications, reduced loading on control room equipment due to input and output functions being migrated to field devices, and reduced wiring and terminations due to multiple devices on one wire besides providing vast amount of additional information from each field device that can be utilized for asset management and health maintenance. In fieldbus, multiple process variables, as well as other information, can be transmitted along a single wire pair. This is quite different from the traditional approach of connecting 4–20 mA devices to a DCS system using dedicated pairs of wires for each device. Field devices and segments become a part of DCS. This shall require an integrated approach for configuration, data management, and system architecture approach to field network design. Also, some design activities have to be performed earlier in the project cycle. Complex functions can be achieved with the FF devices, which shall have significant cost designs and which appreciably reduce the commissioning and start up time. FF can be utilized for majority of the process control applications, including the field instruments connected to the DCS. This shall include control valves, motor-operated valves, transmitters, and local indicators.
13.1.2.3 Fieldbus architecture The fieldbus architecture is depicted in Figure 13.1. The characteristic of the foundation architecture is to ensure device interoperability with fully specified, standard user layer based on “blocks” and device descriptions (DDs).
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FIGURE 13.1 Fieldbus device architecture
The FF system architecture provides a framework for describing these systems as a collection of physical devices interconnected by a fieldbus network. The data communicated over the fieldbus is called object description. Object descriptions are collected together in a structure called object dictionary (OD). The object description is identified by its index in the OD. ODs and DDs are independent of the underlying environment, and therefore do not require adaptation. The FF system architecture defines a specific type of application process, the function block application process (FBAP), to address a variety of functional needs. The FBAP has been designed to support a range of functional models, each addressing a different need. The FF architecture uses the concept of a virtual field device (VFD). A VFD is used to remotely view local device data in the object dictionary. A device has at least two VFDs: • Network and system management VFD • User application VFD Network management is part of the network and system management application. Network anagement refers to managing various parameters to carry out fieldbus communication. The VFD m used for network management is also used for system management. It provides access to the network management information base (NMIB) and to the system management information base (SMIB). System management (SM) manages the parameters needed for the construction of a functional control system, rather than communication. SMIB data includes device tag and address information, and schedules function block execution.
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The system management kernel protocol (SMKP) communicates directly with the data link layer. The SMKP assigns end user defined names, called tags, and data link layer addresses to devices, as they are added to the fieldbus. It contains an OD that can be configured and interrogated using FMS operating over client/server virtual communication relationships. The use of EDDL allows the development of new devices while still maintaining compatibility. The second is to maintain distributed application time so that function block execution can be synchronized among devices. Electronic device descriptions (EDDs) created by device description language (EDDL) for a field device support the management of intelligent field devices. EDDs contain the description of all device parameters, parameter attributes, and device functions. EDDs also include a grouping of device parameters and functions for visualization and a description of transferable data records. The DD may be supplied with the device on a disk, or downloaded from the Fieldbus Foundation Web site, and loaded into the host system. The architecture of a fieldbus device is based on function blocks, which are responsible for performing the tasks required for the current applications, such as data acquisition, feedback and cascade loop control, calculations, and actuation. Every function block contains an algorithm, a database (inputs and outputs), and a user-defined name.
13.1.3 FF NETWORK 13.1.3.1 Fieldbus network classification The FF has two types of network as depicted in Figure 13.2. One is the FF H1 running on 31.25 Kbits/s, which is used to interconnect the field equipment like sensors, actuators, and I/Os. The other one is high speed Ethernet (HSE) running at 100 Mbits/s, which provides integration of high-speed controllers (PLCs), H1 subsystems (via a linking device), data servers, and workstations.
FIGURE 13.2 Fieldbus network classification
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13.1.3.2 Fieldbus topology FF network consists of fieldbus device, spur, trunk, device coupler, and fieldbus power supply, as illustrated in Figure 13.3. • Link: A link is the logical medium by which H1 fieldbus devices are interconnected. It consists of one or more physical segments interconnected by bus, repeaters, or couplers. • Segment: A section of a fieldbus that is terminated in its characteristic impedance, meaning a cable and devices installed between a pair of terminators. Repeaters are used to link segments to form a longer fieldbus. • Trunk: Trunk is the cable between the control room and the junction box in the field. Being the longest cable path on the fieldbus network, it is the main fieldbus communication cable. • Spur: Spur is a cable between the trunk cable and a fieldbus device, usually connected to the trunk via a device coupler. It is the cable that connects a device to the trunk. • Device coupler: FF device coupler is located where the trunk is connected to the various device spurs. The couplers have built-in short-circuit protection to minimize the impact of a short circuit at one device affecting the whole segment. • Fieldbus power supply: Fieldbus requires a special kind of power supply. If an ordinary power supply were to be used to power the fieldbus, the power supply would absorb signals on the cable because it would try to maintain a constant voltage level. For this reason, an ordinary power supply has to be conditioned for fieldbus. Putting an inductor between the power supply and the fieldbus wiring is a way to isolate the fieldbus signal from the low impedance of the bulk supply. The inductor lets the DC power on the wiring, but prevents signals from going into the power supply.
FIGURE 13.3 Fieldbus network overview
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• Fieldbus device: In a conventional 4–20 mA DCS, two wires are used to connect to a device, and in this case, the data acquisition and control lies with the controller. With the introduction of FF, data acquisition and control lies in the fieldbus device itself.
13.1.4 FIELDBUS CONNECTION TOPOLOGIES There are several possible topologies for fieldbus networks. This section illustrates some of the possible topologies and their characteristics.
13.1.4.1 Point-to-point topology This topology consists of a segment having only two devices. It could be a field device such as a transmitter connected to a host system for monitoring or a slave and host device operating independently such as a transmitter and valve with no connection beyond the two (Figure 13.4).
13.1.4.2 Tree topology This topology is also known as chicken foot topology. It consists of a single fieldbus segment connected to a common junction box to form a network (Figure 13.5).
13.1.4.3 Spur topology This topology consists of fieldbus devices connected to a multidrop bus segment through a length of cable called a spur. A spur can vary in length from 1 m to 120 m (Figure 13.6).
13.1.4.4 Daisy chain topology In this topology, the fieldbus cable is routed from device to device on this segment and is interconnected at the terminals of each fieldbus device (Figure 13.7).
13.1.5 FF TECHNOLOGY The FF technology is based on OSI (open system interconnect) model of layered communication. Figure 13.8 illustrates the comparison between a fieldbus model with an OSI model of network communications. Layers 3–6 are not present in the fieldbus model, since their services are not needed in a process control application. Moreover, application layer is divided into two sublayers, namely fieldbus message specification (FMS) and fieldbus access sublayer (FAS). An additional layer known as user application layer is present in the fieldbus model, which is used to define control tasks for process plants. The user application layer is not a part of the OSI model.
FIGURE 13.4
FIGURE 13.5
Point-to-point topology
Tree topology
13.1 Fieldbus technology
FIGURE 13.6
FIGURE 13.7
Spur topology
Daisy chain topology
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FIGURE 13.8 Communication model
13.1.5.1 Physical layer The physical layer or the H1 bus interconnects field devices such as sensors, actuators, and I/Os with the host in the control system. Communication signals and current signals are conveyed along the H1 bus at 31.25 Kbps. The equivalent electrical circuit of physical layer is shown in Figure 13.9. Voltage is applied through impedance conditioner. DC through the impedance is then applied to the devices present in the fieldbus network.
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FIGURE 13.9 Electrical equivvalent circuit of fieldbus physical layer
13.1.5.2 Device power requirements The DC supply voltage across the H1 bus can range from 9 to 32 V. All the fieldbus devices can be powered directly from the H1 bus. The DC supply voltage can range from 9 to 32 VDC. At each end of the cable terminators of 10 Ω, impedances are provided to maintain a balanced transmission line so that high-frequency signals can be transmitted with less distortion. The transmitting device delivers ±10 mA at 31.25 Kbit/s into a 50 Ω equivalent load to create a 1.0 V peak-to-peak voltage modulated on top of the direct current (DC) supply voltage. Loss of power or disturbance to one power supply module shall not result in loss of field device in any circumstances. The function of the physical layer is to convert the messages received from the communication stacks by adding and removing preambles, start delimiters, and end delimiters into physical signals. The physical signals are then transmitted on the H1 bus. A typical waveform of the physical layer is shown in Figure 13.10. The start delimiter and end delimiter indicate the start and end of the physical layer signal (fieldbus message). The receiver uses start delimiter to find the beginning of message and accepts the message until end delimiter is used. The function of the preamble at the start is to synchronize the internal clock with incoming fieldbus signal by the receiver. Fieldbus signals are encoded using Manchester biphaseL technique Figure 13.11. Data signal are combined with the clock signal to create fieldbus signal. Since clock information is embedded into the serial data, the signal is called synchronous serial. The number of devices possible on fieldbus link depends on many factors such as power consumption of each device, type of cable used, and whether devices are intrinsically safe. Without the use of repeaters, the length of H1 cable can be as long as 1900 m.
FIGURE 13.10 Fieldbus signal
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FIGURE 13.11 Manchester biphase-L encoding
13.1.6 DATA LINK LAYER Data link layer controls the flow of messages onto the fieldbus physical layer. It transfers the data from one device to other devices on the network. FF devices are classified as given below: • Basic devices: Basic devices are not capable of functioning as link masters. They do not have link active scheduler (LAS) functionality. A basic device performs all the basic communications required for a field device, except scheduling communication. • Link master device: Link master devices are capable of functioning as link master, with LAS capability. They are capable of scheduling communication in an H1 network. Any H1 network should have one link master device. It can be DCS or any other device such as an actuator or a transmitter. Once the device configuration is downloaded in the device, the control strategy can work even when the computer is disconnected. The link can have many devices configured as link master, but only one device actively controls the bus at any given time. The bus that controls the bus is called link active scheduler. • Bridge: Bridge devices connect H1 link together. They are link master devices and must act as link active scheduler.
13.1.6.1 Link active scheduler All communications on the fieldbus are controlled by a single device called link master. Every host has the capability to act as a link master. The link master has a function known as LAS. LAS decides which device has to transmit data and at what time. This helps in avoiding collision of messages. Following are some of the functions that are performed by LAS.
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When it is time for the device to send data on fieldbus layer, LAS issues a compel data (CD) message to the device. On receipt of the CD, the device broadcasts or publishes data on the bus for all other devices on the H1 network. This is the highest priority function that is performed by the LAS. The devices that are configured to read the data published on the bus are known as subscriber devices, and the device that publishes the data is called publisher. The following are the main functions of the LAS: • Live list maintenance: Live list contains the list of all the devices that properly respond to pass token (PT). Whenever devices are added or deleted, LAS broadcasts changes to the live list. All link master devices maintain a copy of the live list. • Data link time synchronization: Time distribution across all the devices is achieved through LAS. All devices are time stamped with the host time by LAS. The LAS sends time distribution messages periodically to all the devices, across the fieldbus network, to synchronize time stamp of all devices with time stamp of LAS. • Redundancy in LAS: Fieldbus can have many link master devices, which are capable of functioning as LAS. Suppose if current LAS link master device fails, other devices configured as link master take over the control. The link operation does not get failed in any condition. • Token passing: The LAS sends PT messages to all devices on its network. The device transmits its data when it receives the PT.
13.1.6.2 Types of communication There are two types of communication in FF. • Scheduled communication: Scheduled communications are also called cyclic communications. The transfer of regular, cyclic control loop data between devices on the fieldbus network is known as scheduled communication. Function block execution and communication between function blocks – publish/subscribe virtual communication relationship (VCR). • Unscheduled communication: All the devices on fieldbus network send unscheduled messages between scheduled communications. Unscheduled communication takes place when LAS grants permission by issuing PT. Examples of this type of communication include events and user operation. Unscheduled communication (for event-driven or on-demand communication) – client/ server VCR. The LAS is responsible for scheduled communication that takes place between the various function blocks, which is also known as the publisher/subscriber model of communication. The H1 card contains an LAS algorithm that automatically determines the macrocycle and schedules all connected devices. No manual intervention is required. During macrocycle time, time other than scheduled communication time is used for nondeterministic bus communications known as unscheduled communication. During unscheduled communication, alarm transmission and set-point changes take place. There should be sufficient spare time in the macrocycle to satisfy the general sparing conditions of this project. The recommended unscheduled (free asynchronous) time will be 55% of the macro cycle. (i.e., scheduled time shall be maximum 50% of the macrocycle). Of the available unscheduled time, 25% should be unallocated to defined tasks (i.e., function block views and DCI data).
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13.1.6.2.1 Device addressing concept The devices in FF are represented by an address that is an 8-bit primary number. In decimal, address ranges from 0 to 255. The following tabular column shows the distribution of addresses in FF. Address in Decimal
Address in Hexadecimal
Allocated for
0–15
00–0F
Reserved
16–247
10–F7
Permanent address
248–251
F8–FB
Temporary address
252–255
FC–FF
Visitor address
The devices are addressed by the host system, usually DCS in the network. A maximum of 32 devices can be connected in an H1 network.
13.1.6.2.2 Fieldbus access sublayer (FAS) The FAS uses the scheduled and unscheduled features of the data link layer to provide communication service for the FMS. VCR describes the types of FAS services. The VCR is the communication channel between fieldbus devices. All data has to pass through their own VCRs. There are three types of VCRs: • Client–server: The client/server VCR type is used for unscheduled, operator-initiated communication between devices on the fieldbus. When a device receives a PT from the LAS, it can send a request message to another device on the fieldbus. The requester is called the Client, and the device that received the request is called the Server. The Server sends the response when it receives a PT from the LAS. • Publisher–subscriber: The publisher/subscriber VCR type is used by the field devices for cyclic, scheduled publishing of user application function block input and outputs such as process variable (PV) and primary output (OUT) on the fieldbus. When a device receives the CD, the device “publishes” or broadcasts its message to all devices on the fieldbus. Devices that wish to receive the published message are called Subscribers (Figure 13.12). • Source–sink: The source–sink VCR type is used for unscheduled, operator-initiated, one-to-many communications. It is typically used by fieldbus devices to send alarm notifications to the operator consoles.
13.1.6.3 Fieldbus message specifications FMS services enable user applications to send messages to each other across the fieldbus using a standard set of message formats. FMS describes the communication services, message formats, and protocol behavior needed to build messages for the user application.
13.1.6.3.1 FMS services • • • •
Establish and release VCRs and determine the status of a VFD. Enable user application to access and change variables associated with an object description. Enable user application to report events and manage event processing. Remotely upload or download data and programs over the fieldbus.
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FIGURE 13.12 Scheduled data transfer
13.1.6.3.2 User application layer The user application layer defines the way of accessing information within fieldbus devices so that such information may be distributed to other devices or nodes in the fieldbus network. This is a fundamental attribute for process-control applications. User application layer defines an FBAP using resource blocks, function blocks, transducer blocks, system management, network management, and DD technology (Figure 13.13). • Resource block: A resource block shows the contents of a VFD by providing the manufacturer’s name, device name, DD, and so on. The resource block controls the overall device hardware and function blocks within the VFD, including hardware status. • Transducer block: A transducer block converts the signal from physical to digital form. A transducer block is important because it is also used to capture and store all the diagnostic and maintenance-related data for a device.
FIGURE 13.13 Fieldbus architecture
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• Transducer specifications: The device developers generally define transducer specifications. The transducer specifications establish the base scope of transducer functions. A device may have additional functions, but it must contain the functions specified in the specification to be interoperable within the given specification. • Function block: The function blocks are responsible for performing the tasks required by the current applications, such as data acquisition, feedback and cascade loop control, calculations, and actuation. Every function block contains an algorithm, a database (inputs and outputs), and a userdefined name. The three function block classes are as follows: • Standard block • Enhanced block • Vendor-specific block
13.1.6.4 Function block application process The FBAP is composed of a set of function blocks configured to communicate with each other. The outputs from one function block can be linked to the inputs of another function block. Function blocks may be linked within a device, or across the network. The field data are acquired by the function block through the transducer block.
13.1.6.5 Applications of FF technology The important feature of FF in control system is “control in field application.” This is explained in the following examples.
13.1.6.5.1 Example of simple control loop The analog input (AI) value from device A provides the process value to the PID of the device B, which controls the output depending upon the set point given to the PID block. The processed output is fed into the final control element through the AO block (Figure 13.14). Device A in this case could be an input device used to measure process parameters such as temperature, pressure, flow, or level transmitter, which provides process value to the PID of Device B from which the output goes to AO block of the same device, which in turn actuates the final control element.
13.1.6.5.2 Example of cascade control loop Consider the following example (Figure 13.15) of a flow control loop involving a temperature transmitter (device A) and flow transmitter (device B), along with a control valve (device C) that is link master
FIGURE 13.14 Example of simple fieldbus control loop
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FIGURE 13.15 Application of fieldbus – cascade loop
device. The loop illustrated in the following figure is a cascade loop, where the primary element is temperature and secondary element is flow. The temperature transmitter senses and gives the output to primary PID (device B), which is controlled using the operator set point. The output of primary PID is given as set point to secondary controller, which is nothing but PID (device C) of final control element, which in turn controls the final flow. In the below example, the PIDs (controllers) are residing inside the transmitter and final control valve (Figure 13.16). Advantages of cascade control loop are as follows: • Reduced wiring • Reduced hardware and panels • Host supports different fieldbus devices irrespective of vendor, that is, seamless interoperability between devices and hosts • Easy installation and maintenance • Easy calibration and troubleshooting • Advance diagnostics • Predictive maintenance • Applicable for almost all applications including intrinsic safety
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FIGURE 13.16 Loop representation of Figure 13.15
Disadvantages of cascade control loop are the following: • • • •
More expensive Needs training and expertise to handle fieldbus devices Designing fieldbus segments requires experience and training Fieldbus cannot handle faster loops that require fast response, such as antisurge control
FURTHER READING ISA-50.02, Part 2-1992 Fieldbus Standard for Use in Industrial Control Systems, Part 2 Physical Layer Specifications. Research Triangle Park NC: ISA, ISBN 1-55617-317-2, 1992. Fieldbus Foundation, 31.25 Kbits/s Intrinsically Safe Systems, Applications Guide AG-163, Fieldbus Foundation, Austin, TX, 1996. Fieldbus Foundation, Wiring and Installation 31.25 kbit/s, Voltage mode, Wire Medium, Applications Guide AG-140, Fieldbus Foundation, Austin, TX, 1996. Foundation Specifications. Transducer Block Application Process Part 1, FF-902, revision PS 3.0, April 21, 1998. Technical Overview, FD-043, Revision 2.0. Fieldbus Foundation, 1998. Foundation Specifications. Transducer Block Application Process Part 2, FF-903, revision PS 3.0, April 21, 1998. Foundation Specifications. Function Block Application Process Part 1, FF-890, revision 1.4, June 29, 1999. Foundation Specifications. Function Block Application Process Part 2, FF-891, revision 1.4, June 29, 1999. Foundation Specifications. Function Block Application Process Part 3, FF-892, revision 1.4, June 29, 1999.
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Verhappen, I., Pereira, A., Foundation Fieldbus: A Pocket Guide. ISBN: 1-55617-775-5 (note – ISA publication). Also available in Portuguese. Berge, J., Fieldbuses for Process Control: Engineering, Operation and Maintenance. ISBN: 1-55617-760-7 (note – ISA publication). Also available in Chinese. Fieldbus Technical Overview – FF.org. Relcom, Inc., Foundation Fieldbus Wiring Design & Installation Guidelines. Download @ http://www.relcominc. com/download/fbguide.pdf. http://www.pacontrol.com/download/Fieldbus-System-Engineering-Guidelines.pdf. Fieldbus System Engineering Guide – http://www.Fieldbus.org/images/stories/enduserresources/technicalreferences/ documents/system_engineering_guidelines_version_3.pdf.
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13.1 FIELDBUS TECHNOLOGY 13.1.1 INTRODUCTION In the early 1990s, foundation fieldbus (FF) emerged in the market. Two parallel supplier consortiums, Interoperable Systems Project (ISP) and WorldFIP North America, merged to form the Fieldbus Foundation Organization. The new organization immediately brought critical mass to achieve an internationally acceptable fieldbus protocol. The foundation organized development programs and conducted field trials for end users to drive FF technology in industrial applications. FF is a communication protocol, which is “all-digital, serial, two-way multidrop communication system that interconnects fieldbus devices with the control system”. FF is fully digital, serial, two-way, multidrop, communication system running at 31.25 Kbits/s, which will be used to connect intelligent field equipment such as sensors, actuators, and controllers. It serves as a local area network (LAN) for the instrumentation used within process plants and facilities with built-in capability to monitor and distribute control applications across the network.
13.1.2 OVERVIEW An FF system is a distributed system composed of field devices and control/monitoring equipment integrated into the physical environment of a plant or factory. FF devices work together to provide I/O and control for automated processes and operations. FF systems may operate in manufacturing and process control environments. Some environments require intrinsic safety where devices typically operate with limited memory and processing power and with networks that have low bandwidth.
13.1.2.1 Features The fieldbus retains the desirable features of the 4–20 mA analog system such as given below: • • • •
Single loop integrity Standardized physical interface to the wire Bus-powered devices on a single wire pair Intrinsic safety options In addition, FF enables:
• Enhanced capabilities owing to full digital communication, reduced wiring, and wire terminations due to multiple devices on one wire Industrial Process Automation Systems Copyright © 2015 Elsevier Inc. All rights reserved
401
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• Increased selection of suppliers due to interoperability • Reduced loading on control room equipment due to distribution of control and input/output functions to field devices • Connection to the HSE backbone for larger systems The advantages of the FF over the legacy systems are: • • • • • • • • • • • •
The signals are digital, therefore more immune to noise. The requirement for damping to filter out noise is eliminated. FF automatically detects all connected devices and includes them on a live list. Addresses are automatically assigned, eliminating any possibility of duplicate addressing. Traditional I/O use 16- or 32-channel cards; these are costly and a weak point. Module failures can generally cause all associated loops to crash. Accidental removal during faultfinding will affect all 16 or 32 loops. Minimizing the components reduces the failure probability. No requirement to manually configure alarms to detect transmitter failure or broken signal cable. FF builds this automatic safety function. FF uses engineering units, not scaled ranges; hence measuring actual process variable, not scaled or a % of 4–20 mA. This eliminates the need for range configuration. Conflict in the ranges is not possible. Analog to digital conversion is removed, improving accuracy and reliability. Dual measurement of parameters is possible from a single instrument. Failure prediction is possible to the data available.
13.1.2.2 Benefits FF enables increased capabilities due to full digital communications, reduced loading on control room equipment due to input and output functions being migrated to field devices, and reduced wiring and terminations due to multiple devices on one wire besides providing vast amount of additional information from each field device that can be utilized for asset management and health maintenance. In fieldbus, multiple process variables, as well as other information, can be transmitted along a single wire pair. This is quite different from the traditional approach of connecting 4–20 mA devices to a DCS system using dedicated pairs of wires for each device. Field devices and segments become a part of DCS. This shall require an integrated approach for configuration, data management, and system architecture approach to field network design. Also, some design activities have to be performed earlier in the project cycle. Complex functions can be achieved with the FF devices, which shall have significant cost designs and which appreciably reduce the commissioning and start up time. FF can be utilized for majority of the process control applications, including the field instruments connected to the DCS. This shall include control valves, motor-operated valves, transmitters, and local indicators.
13.1.2.3 Fieldbus architecture The fieldbus architecture is depicted in Figure 13.1. The characteristic of the foundation architecture is to ensure device interoperability with fully specified, standard user layer based on “blocks” and device descriptions (DDs).
13.1 Fieldbus technology
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FIGURE 13.1 Fieldbus device architecture
The FF system architecture provides a framework for describing these systems as a collection of physical devices interconnected by a fieldbus network. The data communicated over the fieldbus is called object description. Object descriptions are collected together in a structure called object dictionary (OD). The object description is identified by its index in the OD. ODs and DDs are independent of the underlying environment, and therefore do not require adaptation. The FF system architecture defines a specific type of application process, the function block application process (FBAP), to address a variety of functional needs. The FBAP has been designed to support a range of functional models, each addressing a different need. The FF architecture uses the concept of a virtual field device (VFD). A VFD is used to remotely view local device data in the object dictionary. A device has at least two VFDs: • Network and system management VFD • User application VFD Network management is part of the network and system management application. Network anagement refers to managing various parameters to carry out fieldbus communication. The VFD m used for network management is also used for system management. It provides access to the network management information base (NMIB) and to the system management information base (SMIB). System management (SM) manages the parameters needed for the construction of a functional control system, rather than communication. SMIB data includes device tag and address information, and schedules function block execution.
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The system management kernel protocol (SMKP) communicates directly with the data link layer. The SMKP assigns end user defined names, called tags, and data link layer addresses to devices, as they are added to the fieldbus. It contains an OD that can be configured and interrogated using FMS operating over client/server virtual communication relationships. The use of EDDL allows the development of new devices while still maintaining compatibility. The second is to maintain distributed application time so that function block execution can be synchronized among devices. Electronic device descriptions (EDDs) created by device description language (EDDL) for a field device support the management of intelligent field devices. EDDs contain the description of all device parameters, parameter attributes, and device functions. EDDs also include a grouping of device parameters and functions for visualization and a description of transferable data records. The DD may be supplied with the device on a disk, or downloaded from the Fieldbus Foundation Web site, and loaded into the host system. The architecture of a fieldbus device is based on function blocks, which are responsible for performing the tasks required for the current applications, such as data acquisition, feedback and cascade loop control, calculations, and actuation. Every function block contains an algorithm, a database (inputs and outputs), and a user-defined name.
13.1.3 FF NETWORK 13.1.3.1 Fieldbus network classification The FF has two types of network as depicted in Figure 13.2. One is the FF H1 running on 31.25 Kbits/s, which is used to interconnect the field equipment like sensors, actuators, and I/Os. The other one is high speed Ethernet (HSE) running at 100 Mbits/s, which provides integration of high-speed controllers (PLCs), H1 subsystems (via a linking device), data servers, and workstations.
FIGURE 13.2 Fieldbus network classification
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13.1.3.2 Fieldbus topology FF network consists of fieldbus device, spur, trunk, device coupler, and fieldbus power supply, as illustrated in Figure 13.3. • Link: A link is the logical medium by which H1 fieldbus devices are interconnected. It consists of one or more physical segments interconnected by bus, repeaters, or couplers. • Segment: A section of a fieldbus that is terminated in its characteristic impedance, meaning a cable and devices installed between a pair of terminators. Repeaters are used to link segments to form a longer fieldbus. • Trunk: Trunk is the cable between the control room and the junction box in the field. Being the longest cable path on the fieldbus network, it is the main fieldbus communication cable. • Spur: Spur is a cable between the trunk cable and a fieldbus device, usually connected to the trunk via a device coupler. It is the cable that connects a device to the trunk. • Device coupler: FF device coupler is located where the trunk is connected to the various device spurs. The couplers have built-in short-circuit protection to minimize the impact of a short circuit at one device affecting the whole segment. • Fieldbus power supply: Fieldbus requires a special kind of power supply. If an ordinary power supply were to be used to power the fieldbus, the power supply would absorb signals on the cable because it would try to maintain a constant voltage level. For this reason, an ordinary power supply has to be conditioned for fieldbus. Putting an inductor between the power supply and the fieldbus wiring is a way to isolate the fieldbus signal from the low impedance of the bulk supply. The inductor lets the DC power on the wiring, but prevents signals from going into the power supply.
FIGURE 13.3 Fieldbus network overview
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• Fieldbus device: In a conventional 4–20 mA DCS, two wires are used to connect to a device, and in this case, the data acquisition and control lies with the controller. With the introduction of FF, data acquisition and control lies in the fieldbus device itself.
13.1.4 FIELDBUS CONNECTION TOPOLOGIES There are several possible topologies for fieldbus networks. This section illustrates some of the possible topologies and their characteristics.
13.1.4.1 Point-to-point topology This topology consists of a segment having only two devices. It could be a field device such as a transmitter connected to a host system for monitoring or a slave and host device operating independently such as a transmitter and valve with no connection beyond the two (Figure 13.4).
13.1.4.2 Tree topology This topology is also known as chicken foot topology. It consists of a single fieldbus segment connected to a common junction box to form a network (Figure 13.5).
13.1.4.3 Spur topology This topology consists of fieldbus devices connected to a multidrop bus segment through a length of cable called a spur. A spur can vary in length from 1 m to 120 m (Figure 13.6).
13.1.4.4 Daisy chain topology In this topology, the fieldbus cable is routed from device to device on this segment and is interconnected at the terminals of each fieldbus device (Figure 13.7).
13.1.5 FF TECHNOLOGY The FF technology is based on OSI (open system interconnect) model of layered communication. Figure 13.8 illustrates the comparison between a fieldbus model with an OSI model of network communications. Layers 3–6 are not present in the fieldbus model, since their services are not needed in a process control application. Moreover, application layer is divided into two sublayers, namely fieldbus message specification (FMS) and fieldbus access sublayer (FAS). An additional layer known as user application layer is present in the fieldbus model, which is used to define control tasks for process plants. The user application layer is not a part of the OSI model.
FIGURE 13.4
FIGURE 13.5
Point-to-point topology
Tree topology
13.1 Fieldbus technology
FIGURE 13.6
FIGURE 13.7
Spur topology
Daisy chain topology
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FIGURE 13.8 Communication model
13.1.5.1 Physical layer The physical layer or the H1 bus interconnects field devices such as sensors, actuators, and I/Os with the host in the control system. Communication signals and current signals are conveyed along the H1 bus at 31.25 Kbps. The equivalent electrical circuit of physical layer is shown in Figure 13.9. Voltage is applied through impedance conditioner. DC through the impedance is then applied to the devices present in the fieldbus network.
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FIGURE 13.9 Electrical equivvalent circuit of fieldbus physical layer
13.1.5.2 Device power requirements The DC supply voltage across the H1 bus can range from 9 to 32 V. All the fieldbus devices can be powered directly from the H1 bus. The DC supply voltage can range from 9 to 32 VDC. At each end of the cable terminators of 10 Ω, impedances are provided to maintain a balanced transmission line so that high-frequency signals can be transmitted with less distortion. The transmitting device delivers ±10 mA at 31.25 Kbit/s into a 50 Ω equivalent load to create a 1.0 V peak-to-peak voltage modulated on top of the direct current (DC) supply voltage. Loss of power or disturbance to one power supply module shall not result in loss of field device in any circumstances. The function of the physical layer is to convert the messages received from the communication stacks by adding and removing preambles, start delimiters, and end delimiters into physical signals. The physical signals are then transmitted on the H1 bus. A typical waveform of the physical layer is shown in Figure 13.10. The start delimiter and end delimiter indicate the start and end of the physical layer signal (fieldbus message). The receiver uses start delimiter to find the beginning of message and accepts the message until end delimiter is used. The function of the preamble at the start is to synchronize the internal clock with incoming fieldbus signal by the receiver. Fieldbus signals are encoded using Manchester biphaseL technique Figure 13.11. Data signal are combined with the clock signal to create fieldbus signal. Since clock information is embedded into the serial data, the signal is called synchronous serial. The number of devices possible on fieldbus link depends on many factors such as power consumption of each device, type of cable used, and whether devices are intrinsically safe. Without the use of repeaters, the length of H1 cable can be as long as 1900 m.
FIGURE 13.10 Fieldbus signal
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FIGURE 13.11 Manchester biphase-L encoding
13.1.6 DATA LINK LAYER Data link layer controls the flow of messages onto the fieldbus physical layer. It transfers the data from one device to other devices on the network. FF devices are classified as given below: • Basic devices: Basic devices are not capable of functioning as link masters. They do not have link active scheduler (LAS) functionality. A basic device performs all the basic communications required for a field device, except scheduling communication. • Link master device: Link master devices are capable of functioning as link master, with LAS capability. They are capable of scheduling communication in an H1 network. Any H1 network should have one link master device. It can be DCS or any other device such as an actuator or a transmitter. Once the device configuration is downloaded in the device, the control strategy can work even when the computer is disconnected. The link can have many devices configured as link master, but only one device actively controls the bus at any given time. The bus that controls the bus is called link active scheduler. • Bridge: Bridge devices connect H1 link together. They are link master devices and must act as link active scheduler.
13.1.6.1 Link active scheduler All communications on the fieldbus are controlled by a single device called link master. Every host has the capability to act as a link master. The link master has a function known as LAS. LAS decides which device has to transmit data and at what time. This helps in avoiding collision of messages. Following are some of the functions that are performed by LAS.
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When it is time for the device to send data on fieldbus layer, LAS issues a compel data (CD) message to the device. On receipt of the CD, the device broadcasts or publishes data on the bus for all other devices on the H1 network. This is the highest priority function that is performed by the LAS. The devices that are configured to read the data published on the bus are known as subscriber devices, and the device that publishes the data is called publisher. The following are the main functions of the LAS: • Live list maintenance: Live list contains the list of all the devices that properly respond to pass token (PT). Whenever devices are added or deleted, LAS broadcasts changes to the live list. All link master devices maintain a copy of the live list. • Data link time synchronization: Time distribution across all the devices is achieved through LAS. All devices are time stamped with the host time by LAS. The LAS sends time distribution messages periodically to all the devices, across the fieldbus network, to synchronize time stamp of all devices with time stamp of LAS. • Redundancy in LAS: Fieldbus can have many link master devices, which are capable of functioning as LAS. Suppose if current LAS link master device fails, other devices configured as link master take over the control. The link operation does not get failed in any condition. • Token passing: The LAS sends PT messages to all devices on its network. The device transmits its data when it receives the PT.
13.1.6.2 Types of communication There are two types of communication in FF. • Scheduled communication: Scheduled communications are also called cyclic communications. The transfer of regular, cyclic control loop data between devices on the fieldbus network is known as scheduled communication. Function block execution and communication between function blocks – publish/subscribe virtual communication relationship (VCR). • Unscheduled communication: All the devices on fieldbus network send unscheduled messages between scheduled communications. Unscheduled communication takes place when LAS grants permission by issuing PT. Examples of this type of communication include events and user operation. Unscheduled communication (for event-driven or on-demand communication) – client/ server VCR. The LAS is responsible for scheduled communication that takes place between the various function blocks, which is also known as the publisher/subscriber model of communication. The H1 card contains an LAS algorithm that automatically determines the macrocycle and schedules all connected devices. No manual intervention is required. During macrocycle time, time other than scheduled communication time is used for nondeterministic bus communications known as unscheduled communication. During unscheduled communication, alarm transmission and set-point changes take place. There should be sufficient spare time in the macrocycle to satisfy the general sparing conditions of this project. The recommended unscheduled (free asynchronous) time will be 55% of the macro cycle. (i.e., scheduled time shall be maximum 50% of the macrocycle). Of the available unscheduled time, 25% should be unallocated to defined tasks (i.e., function block views and DCI data).
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13.1.6.2.1 Device addressing concept The devices in FF are represented by an address that is an 8-bit primary number. In decimal, address ranges from 0 to 255. The following tabular column shows the distribution of addresses in FF. Address in Decimal
Address in Hexadecimal
Allocated for
0–15
00–0F
Reserved
16–247
10–F7
Permanent address
248–251
F8–FB
Temporary address
252–255
FC–FF
Visitor address
The devices are addressed by the host system, usually DCS in the network. A maximum of 32 devices can be connected in an H1 network.
13.1.6.2.2 Fieldbus access sublayer (FAS) The FAS uses the scheduled and unscheduled features of the data link layer to provide communication service for the FMS. VCR describes the types of FAS services. The VCR is the communication channel between fieldbus devices. All data has to pass through their own VCRs. There are three types of VCRs: • Client–server: The client/server VCR type is used for unscheduled, operator-initiated communication between devices on the fieldbus. When a device receives a PT from the LAS, it can send a request message to another device on the fieldbus. The requester is called the Client, and the device that received the request is called the Server. The Server sends the response when it receives a PT from the LAS. • Publisher–subscriber: The publisher/subscriber VCR type is used by the field devices for cyclic, scheduled publishing of user application function block input and outputs such as process variable (PV) and primary output (OUT) on the fieldbus. When a device receives the CD, the device “publishes” or broadcasts its message to all devices on the fieldbus. Devices that wish to receive the published message are called Subscribers (Figure 13.12). • Source–sink: The source–sink VCR type is used for unscheduled, operator-initiated, one-to-many communications. It is typically used by fieldbus devices to send alarm notifications to the operator consoles.
13.1.6.3 Fieldbus message specifications FMS services enable user applications to send messages to each other across the fieldbus using a standard set of message formats. FMS describes the communication services, message formats, and protocol behavior needed to build messages for the user application.
13.1.6.3.1 FMS services • • • •
Establish and release VCRs and determine the status of a VFD. Enable user application to access and change variables associated with an object description. Enable user application to report events and manage event processing. Remotely upload or download data and programs over the fieldbus.
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FIGURE 13.12 Scheduled data transfer
13.1.6.3.2 User application layer The user application layer defines the way of accessing information within fieldbus devices so that such information may be distributed to other devices or nodes in the fieldbus network. This is a fundamental attribute for process-control applications. User application layer defines an FBAP using resource blocks, function blocks, transducer blocks, system management, network management, and DD technology (Figure 13.13). • Resource block: A resource block shows the contents of a VFD by providing the manufacturer’s name, device name, DD, and so on. The resource block controls the overall device hardware and function blocks within the VFD, including hardware status. • Transducer block: A transducer block converts the signal from physical to digital form. A transducer block is important because it is also used to capture and store all the diagnostic and maintenance-related data for a device.
FIGURE 13.13 Fieldbus architecture
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• Transducer specifications: The device developers generally define transducer specifications. The transducer specifications establish the base scope of transducer functions. A device may have additional functions, but it must contain the functions specified in the specification to be interoperable within the given specification. • Function block: The function blocks are responsible for performing the tasks required by the current applications, such as data acquisition, feedback and cascade loop control, calculations, and actuation. Every function block contains an algorithm, a database (inputs and outputs), and a userdefined name. The three function block classes are as follows: • Standard block • Enhanced block • Vendor-specific block
13.1.6.4 Function block application process The FBAP is composed of a set of function blocks configured to communicate with each other. The outputs from one function block can be linked to the inputs of another function block. Function blocks may be linked within a device, or across the network. The field data are acquired by the function block through the transducer block.
13.1.6.5 Applications of FF technology The important feature of FF in control system is “control in field application.” This is explained in the following examples.
13.1.6.5.1 Example of simple control loop The analog input (AI) value from device A provides the process value to the PID of the device B, which controls the output depending upon the set point given to the PID block. The processed output is fed into the final control element through the AO block (Figure 13.14). Device A in this case could be an input device used to measure process parameters such as temperature, pressure, flow, or level transmitter, which provides process value to the PID of Device B from which the output goes to AO block of the same device, which in turn actuates the final control element.
13.1.6.5.2 Example of cascade control loop Consider the following example (Figure 13.15) of a flow control loop involving a temperature transmitter (device A) and flow transmitter (device B), along with a control valve (device C) that is link master
FIGURE 13.14 Example of simple fieldbus control loop
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FIGURE 13.15 Application of fieldbus – cascade loop
device. The loop illustrated in the following figure is a cascade loop, where the primary element is temperature and secondary element is flow. The temperature transmitter senses and gives the output to primary PID (device B), which is controlled using the operator set point. The output of primary PID is given as set point to secondary controller, which is nothing but PID (device C) of final control element, which in turn controls the final flow. In the below example, the PIDs (controllers) are residing inside the transmitter and final control valve (Figure 13.16). Advantages of cascade control loop are as follows: • Reduced wiring • Reduced hardware and panels • Host supports different fieldbus devices irrespective of vendor, that is, seamless interoperability between devices and hosts • Easy installation and maintenance • Easy calibration and troubleshooting • Advance diagnostics • Predictive maintenance • Applicable for almost all applications including intrinsic safety
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FIGURE 13.16 Loop representation of Figure 13.15
Disadvantages of cascade control loop are the following: • • • •
More expensive Needs training and expertise to handle fieldbus devices Designing fieldbus segments requires experience and training Fieldbus cannot handle faster loops that require fast response, such as antisurge control
FURTHER READING ISA-50.02, Part 2-1992 Fieldbus Standard for Use in Industrial Control Systems, Part 2 Physical Layer Specifications. Research Triangle Park NC: ISA, ISBN 1-55617-317-2, 1992. Fieldbus Foundation, 31.25 Kbits/s Intrinsically Safe Systems, Applications Guide AG-163, Fieldbus Foundation, Austin, TX, 1996. Fieldbus Foundation, Wiring and Installation 31.25 kbit/s, Voltage mode, Wire Medium, Applications Guide AG-140, Fieldbus Foundation, Austin, TX, 1996. Foundation Specifications. Transducer Block Application Process Part 1, FF-902, revision PS 3.0, April 21, 1998. Technical Overview, FD-043, Revision 2.0. Fieldbus Foundation, 1998. Foundation Specifications. Transducer Block Application Process Part 2, FF-903, revision PS 3.0, April 21, 1998. Foundation Specifications. Function Block Application Process Part 1, FF-890, revision 1.4, June 29, 1999. Foundation Specifications. Function Block Application Process Part 2, FF-891, revision 1.4, June 29, 1999. Foundation Specifications. Function Block Application Process Part 3, FF-892, revision 1.4, June 29, 1999.
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Verhappen, I., Pereira, A., Foundation Fieldbus: A Pocket Guide. ISBN: 1-55617-775-5 (note – ISA publication). Also available in Portuguese. Berge, J., Fieldbuses for Process Control: Engineering, Operation and Maintenance. ISBN: 1-55617-760-7 (note – ISA publication). Also available in Chinese. Fieldbus Technical Overview – FF.org. Relcom, Inc., Foundation Fieldbus Wiring Design & Installation Guidelines. Download @ http://www.relcominc. com/download/fbguide.pdf. http://www.pacontrol.com/download/Fieldbus-System-Engineering-Guidelines.pdf. Fieldbus System Engineering Guide – http://www.Fieldbus.org/images/stories/enduserresources/technicalreferences/ documents/system_engineering_guidelines_version_3.pdf.