Foundation Fieldbus -Seminar Report

INTRODUCTION

Fieldbus is a generic-term which describes a new digital communications network which will be used in industry to replace the existing 4 - 20mA analog signal. The network is a digital, bi-directional, multidrop, serial-bus, communications network used to link isolated field devices, such as controllers, transducers, actuators and sensors. So Fieldbus is a digital communication bus line connecting the field instruments with the control system components.

A field instrument is an instrument attached to a process, e.g.: measurement devices, valves, motor starters and alarm switches .A field instrument to be attached to a fieldbus must be equipped with some intelligence for the bus communication.

A bus is a number of wires for the transfer of digital signals. A fieldbus consists of two wires. The digital signal can be transfered as a voltage difference between the wires – or as a current value.The bus can also be an optical fiber.

Without fieldbus, each field instrument has to be connected to the I/O of the control system, two wires for each instrument.

  

FIELDBUS AND IEC61158 STANDARD

International standardization organizations have worked for many years in order to get an agreement on a common fieldbus standard. But it seems that it is still a long way to go to get something comparable to the old 4 – 20 mA standard. Among the existing fieldbuses some are  better in one system, some other in another system.

In year 2000 it came a document, the IEC 61158 standard, which includes 8 different fieldbuses in the same standard.

 The 8 fieldbuses included in this standard are:

•         ControlNet

•         FF – H1 (Foundation Fieldbus)

•         FF – HSE (Foundation Fieldbus)

•         Interbus

•         P-Net

•         PROFIBUS

•         SwiftNet

•         WorldFip

ADVANTAGES OF FIELDBUS

The fieldbus has a number of advantages that the end users will benefit from.The major advantage of the fieldbus, and the one that is most attractive to the end user is its reduction in capital costs. The various advantages are:

·         REDUCING CABLING

One of the main features of the fieldbus is its significant reduction in wiring. Each process cell requires only one wire to be run to the main cable, with a varying number of cells available. The cost of installing field equipment in a fieldbus system is thus significantly reduced. Installation costs are further reduced due to the fact that the fieldbus it is a multi-drop rather than point-to-point system and the multidrop network can offer a 5:1 reduction in field wiring expense.

·         HARDWARE REDUCTION

Since all Digital/Analogue or Analogue/Digital transformations have to be done in the instruments, we will also see that fieldbus systems have less hardware components. So, even though a fieldbus instrument usually is more expensive than a traditional one, the overall hardware cost may be lower.

·         SIMPLER CONFIGURATION

Less components and less cables mean a system that can easier be configured. Fieldbus systems also have special configuration and documentation tools that will help further.

·         SIMPLR MAINTENANCE

The fact that the fieldbus system is less complex than conventional bus systems implies that there will be less overall need for maintenance. With the fieldbus system, it is possible for the operators to easily see all of the devices included in the system and to also easily interpret the interaction between the individual devices. This will make discovering the source of any problems and carrying out maintenance much simpler.

·         RELIABILITY

The simplification of systems means that the long term reliability of the bus system is increased.

·         FLEXIBILITY

Fieldbus allows the user increased flexibility in the design of the bus system. Some algorithms and control procedures that with conventional bus systems must be contained in control programs can now reside in the individual field devices, reducing the overall size of the main control system. This reduces the overall systems cost and makes future expansion a simpler prospect.

OVERVIEW

FOUNDATION Fieldbus is the technological evolution to digital communication in instrumentation and process control. It differs from any other communication protocol, because it is designed to resolve process control applications instead of just transfer data in a digital mode.

FOUNDATION  Fieldbus is an all-digital, serial, two-way communication system, which interconnects “field” equipment such as sensors, actuators and controllers. Fieldbus is a Local

Area Network (LAN) for instruments used in both process and manufacturing automation with

built-in capability to distribute the control application across the network.

Two related implementations have been introduced to meet different needs within the process automation environment. These two implementations use different physical media and communication speeds.

·         H1 (31.25 kbit/s) interconnects “field” equipment such as sensors, actuators and I/O. H1 is currently the most common implementation.

·         HSE (100 Mbit/s) (High Speed Ethernet) provides integration of high speed controllers (such as PLCs), H1 subsystems (via a linking device), data servers and workstations.

  

WHO IS THE FIELDBUS FOUNDATION 

Driven by their customers needs, process control and manufacturing automation companies formed the Fieldbus Foundation to complete development of a single, open, international, and interoperable fieldbus. The Fieldbus Foundation is an independent, not-for profit organization based on the following principles:

• Fieldbus technology is an enabling technology not a differentiating technology.

• Fieldbus technology is open and available to all parties.

• Fieldbus technology is based on the work of the International Electrotechnical Commission               (I EC)  and  ISA  (the international society for measurement and control)

• Fieldbus Foundation members support and work  with the international and national standards

  committees.

FOUNDATION FIELDBUS TECHNOLOGY  

Foundation Fieldbus technology consists of three parts:

  1 – The Physical Layer.

  2 – The Communication Stack.

  3 – The User Application.

The Open Systems Interconnect (OSI) layered communication model is used to model these

components.

The Physical Layer is OSI layer 1. The Data Link Layer (DLL) is OSI layer 2. The Fieldbus Message Specification (FMS) is OSI layer 7. The Communication Stack is comprised of layers 2 and 7 in the OSI model. The fieldbus does not use OSI layers 3, 4, 5 and 6.

The Fieldbus Access Sublayer (FAS) maps the FMS onto the DLL.

PHYSICAL LAYER

The Physical Layer is defined by approved standards from the International Electrotechnical Commission (IEC) and The International Society of Measurement and Control (ISA).

The Physical Layer receives messages from the communication stack and converts the messages

into physical signals on the fieldbus transmission medium and vice-versa.

Conversion tasks include adding and removing preambles, start delimiters, and end delimiters.

Fieldbus signals are encoded using the well-known Manchester Biphase-L technique. The signal is called “synchronous serial” because the clock information is embedded in the serial data stream. Data is combined with the clock signal to create the fieldbus signal as shown in the figure below. The receiver of the fieldbus signal as shown in the figure below. The receiver of  the fieldbus signal interprets a positive transition in the middle of a bit time as a logical “O” and a negative transition as logical “1”. 

The preamble is used by the receiver to synchronize its internal clock with the incoming fieldbus

signal. Special N+ and N- codes are in the start delimiter and end delimiter. The N+ and N- signals do not transition in the middle of a bit time. The receiver uses the start delimiter to find the beginning of a fieldbus message. After it finds the start delimiter, the receiver accepts data until the end delimiter is received.

 ( 31.25 kbit/s) Fieldbus Signaling

The transmitting device delivers + 10 mA at 31.25 kbit/s into a 50 ohm equivalent load to create a 1.0 volt peak-to-peak voltage modulated on top of the direct current (DC) supply voltage.

The DC supply voltage can range from 9 to 32 volts, however for I.S. applications, the allowed

power supply voltage depends on the barrier rating. 

31.25 kbit/s devices can be powered directly from the fieldbus and can operate on wiring that was previously used for 4-20 mA devices. The 31.25 kbit/s fieldbus also supports intrinsically safe (I.S.) fieldbuses with bus powered devices. To accomplish this, an I.S. barrier is placed between the power supply in the safe area and the I.S. device in the hazardous area.

31.25 kbit/s Fieldbus Wiring

Fieldbus allows stubs or “spurs”. The length of the fieldbus is determined by the communication rate, cable type, wire size, bus power option, and I.S. option.

HIGH SPEED ETHERNET(HSE )

A Linking Device is used to interconnect 31.25 kbit/s fieldbuses and make them accessible to a

High Speed Ethernet (HSE) backbone running at 100 Mbit/s or 1 Gbit/s. The I/O Subsystem

Interface shown in the figure allows other networks such as DeviceNetâ and Profibusâ to be

mapped into standard FOUDATION Fieldbus function blocks. The I/O Subsystem Interface can be connected to the 31.25 Kbit/s fieldbus or HSE. 

Since all of the 31.25 kbit/s FOUDATION Fieldbus messages are communicated on the HSE using standard Ethernet protocols (e.g., TCP/IP, SNTP, SNMP, etc.), commercial off-the-shelf HSE equipment such as Switches and Routers are used to create larger networks. OF course all or part of the HSE network can be made redundant to achieve the level fault tolerance needed by the application.

COMMUNICATION STACK

The Data Link Layer (DLL)

Layer 2, the Data Link Layer (DLL), controls transmission of messages onto the fieldbus. The DLL manages access to the fieldbus through a deterministic centralized bus scheduler called the Link Active Scheduler (LAS). The DLL is a subset of the emerging IEC/ISA DLL standard.

Device Types

Two types of devices are defined in the DLL specification:

• Basic Device

• Link Master

Link Master devices are capable of becoming the Link Active Scheduler (LAS). Basic devices do not have the capability to become the LAS.

 Scheduled Communication

The Link Active Scheduler (LAS) has a list of transmit times for all data buffers in all devices that need to be cyclically transmitted. When it is time for a device to send a buffer, the LAS issues a Compel Data (CD) message to the device. Upon receipt of the CD, the device broadcasts or “publishes” the data in the buffer to all devices on the fieldbus. Any device that is configured to receive the data is called a “subscriber”.

Scheduled data transfers are typically used for the regular, cyclic transfer of control loop data

between devices on the fieldbus.

All of the devices on the fieldbus are given a chance to send “unscheduled” messages between

transmissions of scheduled messages. The LAS grants permission to a device to use the fieldbus by issuing a pass token (PT) message to the device. When the device receives the PT, it is allowed to send messages until it has finished or until the “maximum token hold time” has expired, whichever is the shorter time.

Unscheduled Communication

All of the devices on the fieldbus are given a chance to send “unscheduled” messages between

transmissions of scheduled messages. The LAS grants permission to a device to use the fieldbus by issuing a pass token (PT) message to the device. When the device receives the PT, it is allowed to send messages until it has finished or until the “maximum token hold time” has expired, whichever is the shorter time.

CD Schedule

The CD Schedule contains a list of activities that are scheduled to occur on a cyclic basis. At

precisely the scheduled time, the LAS sends a Compel Data (CD) message to a specific data buffer in a fieldbus device. The device immediately broadcasts or “publishes” a message to all devices on the fieldbus. This is the highest priority activity performed by the LAS. The remaining operations are performed between scheduled transfers.

Live List Maintenance

The list of all devices that are properly responding to the Pass Token (PT) is called the “Live List”. New devices may be added to the fieldbus at any time. The LAS periodically sends Probe Node (PN) messages to the addresses not in the Live List. If a device is present at the address and receives the PN, it immediately returns a Probe Response (PR) message. If the device answers with a PR, the LAS adds the device to the Live List and confirms its addition by sending the device a Node Activation message.

The LAS is required to probe at least one address after it has completed a cycle of sending PTs to

all devices in the Live List. The device will remain in the Live List as long as it responds properly to the PTs sent from the LAS. The LAS will remove a device from the Live List if the device does not either use the token or immediately return it to the LAS after three successive tries. Whenever a device is added or removed from the Live List, the LAS broadcasts changes to the Live List to all devices. This allows each device to maintain a current copy of the Live List.

Fieldbus Access Sublayer (FAS)

The FAS uses the scheduled and unscheduled features of the Data Link Layer to provide a service for the Fieldbus Message Specification (FMS). The types of FAS services are described by Virtual Communication Relationships (VCR).

The VCR is like the speed dial feature on your memory telephone. There are many digits to dial for an international call such as international access code, country code, city code, exchange code and finally the specific telephone number.

This information only need to be entered once and then a “speed dial number” is assigned.After setup, only the speed dial number needs to be entered for the dialing to occur. Likewise, after configuration, only the VCR number is needed to communicate with another fieldbus device.

Fieldbus Message Specification (FMS)

Fieldbus Message Specification (FMS) services allow 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 tobuild messages for the User Application.

Data that is communicated over the fieldbus is described by an “object description.” Object

descriptions are collected together in a structure called an “object dictionary” (OD).

The object description is identified by its “index” in the OD. Index 0, called the object dictionary

header, provides a description of the dictionary itself, and defines the first index for the object

descriptions of the User Application. The User Application object descriptions can start at any

index above 255. Index 255 and below define standard data types such as Boolean, integer, float, bitstring, and data structures that are used to build all other object descriptions.

Devices are configured using Resource Blocks and Transducer Blocks. The control strategy is built using Function Blocks.

Resource Block

The Resource Block describes characteristics of the fieldbus device such as the device name, manufacturer, and serial number. There is only one Resource Block in a device.

Function Block

Function Blocks (FB) provide the control system behavior. The input and output parameters of

Function Blocks can be linked over the fieldbus.

Transducer Blocks

Like the Resource Block, the Transducer Blocks are used to configure devices.

Transducer Blocks decouple Function Blocks from the local input/output functions required to read sensors and command output hardware. They contain information such as calibration date and sensor type.

Supporting Objects

The following additional objects are defined in the

User Application:

Link Objects define the links between Function Block inputs and outputs internal to the device and across the fieldbus network.

Trend Objects allow local trending of function block parameters for access by hosts or other devices.

Alert Objects allow reporting of alarms and events on the fieldbus.

Multi-Variable Container (MVC) Object serves to “encapsulate” multiple Function Block parameters in order to optimize communications for Publishing- Subscriber and Report Distribution transactions. It has a user-configured list to define the required parameters, whose data values are referenced in a variable list.

View Objects are predefined groupings of block

parameter sets that can be displayed by the human/machine interface. The function block specification defines four views for each type of block. Figure 15 shows an example of how common Function Block variables map into the views. Only a partial listing of the block parameters is shown in the example.

FIELDBUS DEVICE FUNCTION

 The device functions are made visible to the fieldbus communication system through the User Application Virtual Field Device (VFD).

The header of the User Application object dictionary points to a Directory which is always the

first entry in the function block application. The Directory provides the starting indexes of all of

the other entries used in the Function Block application.

The VFD object descriptions and their associated data are accessed remotely over the fieldbus

network using Virtual Communication Relationships (VCRs) as shown below . 

  

SYSTEM MANAGEMENT 

Function Blocks must execute at precisely defined intervals and in the proper sequence for correct control system operation. System management synchronizes execution of the Function Blocks to a common time clock shared by all devices. System management also handles other important system features such as publication of the time of day to all devices, including automatic switchover to a redundant time publisher and searching for parameter names or “tags” on the fieldbus. Fieldbus devices do not use jumpers or switches to configure addresses. Instead, device addresses are set by configuration tools using System Management services. All of the configuration information needed by System Management such as the Function Block schedule is described by object descriptions in the Network and System Management Virtual Field Device (VFD). This VFD provides access to the System Management Information Base (SMIB), and also to the Network Management Information Base (NMIB).

SYSTEM CONFIGURATION 

Fieldbus system configuration consists of two phases:

1) System Design and 2) Device Configuration.

 System Design

The system design for fieldbus-based systems is very similar to today’s Distributed Control System (DCS) design with the following differences. The first difference is in the physical wiring due to the change from 4-20 mA analog point-to-point wiring to a digital bus wiring where many devices can be connected to one wire. Each device on the fieldbus must have a unique physical device tag and a corresponding network address.

The second difference is the ability to distribute the control and input/output (I/O) subsystem functions from the control system to the fieldbus devices. This may reduce the number of rack mounted controllers and remote mounted I/O equipment needed for the

system design.

 Device Configuration

After the system design is completed and the instruments have been selected, the device configuration is performed by connecting Function Block inputs and outputs together in each device as required by the control strategy.After all of the function block connections and other

configuration items such as device names, looptags, and loop execution rate have been entered,

the configuration device generates information foreach fieldbus device.A stand-alone loop can beconfigured if there is a field device that is a Link Master. This will allow continued operation of the loop without the configuration device or a central console. The system becomes operational after the fieldbus devices have received their configurations.

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 CONCLUSION

There have emerged literally hundreds of fieldbuses developed by different companies and organisations all over the world. The term fieldbus covers many different industrial network protocols. Most fieldbus protocols have been developed and supported by specific PLC manufacturers.

Foundation Fieldbus is a sophisticated, object-oriented protocol that uses multiple messaging formats and allows a controller to recognize a rich set of configuration and parameter information ("device description") from devices that have been plugged into the bus. Foundation Fieldbus even allows a device to transmit parameters relating to the estimated reliability of a particular piece of data. Foundation Fieldbus uses a scheduler to guarantee the delivery of messages, so issues of determinism and repeatability are solidly addressed (determinism means knowing absolute worst-case response times with 100% certainty). Each segment of the network contains one scheduler.

In the United States, DeviceNet, Profibus DP, and Foundation Fieldbus H1 have taken their places as the dominant open systems for connecting industrial devices. while Foundation Fieldbus is taking the lead in process control.