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PLC and Automation (B.E. E & TC) additional
topic for Unit 6
Hart Protocol
A6.1 WHAT IS HART?
HART Field Communications Pr...
PLC and Automation Additional Topic for Unit 66-2
Figure 6.1: HART uses frequency shift keying to encode digital informati...
HART Protocol and CAN bus 6-3
The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superim...
PLC and Automation Additional Topic for Unit 66-4
HART Networks
HART devices can operate in one of two network configurati...
HART Protocol and CAN bus 6-5
A6.2 THE HART MESSAGE STRUCTURE
The structure of a HART message is shown in figure 6.5. The ...
PLC and Automation Additional Topic for Unit 66-6
master and the field devices. This openness and the interchangeability o...
HART Protocol and CAN bus 6-7
11. Device Integration.
The DD will be developed by the device manufacture or a service part...
PLC and Automation Additional Topic for Unit 66-8
HART communication devices.
3. Device specific: Device-specific commands...
HART Protocol and CAN bus 6-9
Universal Commands
PLC and Automation Additional Topic for Unit 66-10
A6.2.3 Continuous HART Communication Increases Safety Integrity Level (...
HART Protocol and CAN bus 6-11
Continuous Fault Monitoring: HART-capable control systems and interfaces can continuously m...
PLC and Automation Additional Topic for Unit 66-12
condition will be quickly recognized. However, if it were left at 50%, ...
HART Protocol and CAN bus 6-13
Two smart transmitters are installed on each lauter tub one on the bottom of the tank and t...
PLC and Automation Additional Topic for Unit 66-14
identifies and establishes the availability of intended communication p...
HART Protocol and CAN bus 6-15
Physical layer Coding Data transmission between the masters and the field devices is physic...
PLC and Automation Additional Topic for Unit 66-16
Communication services
The HART protocol provides standard and broadcas...
HART Protocol and CAN bus 6-17
instrument suppliers.
Unlike other digital communication technologies, the HART protocol pr...
PLC and Automation Additional Topic for Unit 66-18
Numerous device parameters are available from HART-compatible instrumen...
HART Protocol and CAN bus 6-19
4. Reading the values of additional measurements provided by the device
5. Device Health an...
PLC and Automation Additional Topic for Unit 66-20
Twisted Pair (UTP), or Ribbon cable. Each node uses a Male 9-pin D conn...
HART Protocol and CAN bus 6-21
The pinout for the 9-pin D connector is shown in the table 5.10 below. Additional connector...
PLC and Automation Additional Topic for Unit 66-22
Figure 5.25: CAN Bus Interface IC Logic Transition Levels
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Hart and can bus for unit 6 plc (b.e. e & tc) 2015

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This is the additional file for HART and CAN bus topics for Unit 6 in PLC and Automation (b.e. e & tc) 2015 syllabus from Chinttan Publications.

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Hart and can bus for unit 6 plc (b.e. e & tc) 2015

  1. 1. PLC and Automation (B.E. E & TC) additional topic for Unit 6 Hart Protocol A6.1 WHAT IS HART? HART Field Communications Protocol is widely recognized as the industry standard for digitally enhanced 4-20 mA smart instrument communication. Use of the technology is growing rapidly and today virtually all major global instrumentation suppliers offer products with HART communication. The HART protocol provides a uniquely backward compatible solution for smart instrument communication as both 4- 20 mA analog and digital communication signals are transmitted simultaneously on the same wiring. HART provides many benefits promised by fieldbus, while retaining the compatibility and familiarity of existing 4-20 mA systems. This paper provides a brief overview of the HART protocol and the benefits achievable with this important technology. Per instrument cost savings benefits of $300-500 in initial installation/commissioning and $100-200 per year in ongoing maintenance/operations are commonly reported.
  2. 2. PLC and Automation Additional Topic for Unit 66-2 Figure 6.1: HART uses frequency shift keying to encode digital information on top of the 4-20 mA analog signal Analog + Digital Communication For many years, the field communication standard used by process automation equipment has been a milliampere (mA) analog current signal. In most applications the milliampere signal varies within a range of 4-20 mA in proportion to the process variable being represented. Virtually all installed plant instrumentation systems use this international standard to communicate process variable information. HART Field Communications Protocol extends the 4-20 mA standard to enhance communication with intelligent measurement and control instrumentation. A major step in the evolution of process control, the HART protocol is fostering significant innovation in process instrumentation capabilities. The enhanced communication characteristics of this important technology are reflected in the protocol name, HART which stands for Highway Addressable Remote Transducer. The HART protocol enables two-way digital communication with smart instruments without disturbing the 4-20 mA analog signal. Both the 4-20 mA analog and HART digital communication signals can be transmitted simultaneously over the same wiring. Primary variable and control signal information is carried by the 4-20 mA (if desired), while additional measurements, process parameters, device configuration, calibration, and diagnostics information is accessible through the HART protocol over the same wires at the same time. Unlike other open digital communication technologies for process instrumentation, HART is compatible with existing systems. The HART Technology Figure 6.2: HART digital communication signal superimposed on the 4-20 mA analog signal
  3. 3. HART Protocol and CAN bus 6-3 The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20 mA. Since the digital FSK signal is phase continuous, it does not interfere with the 4-20 mA signal. A logical 1 is represented by a frequency of 1200 Hz and a logical 0 is represented by a frequency of 2200 Hz as shown in figure 6.1 and 6.2. The HART FSK signaling enables two-way digital communication and makes it possible for additional information beyond just the normal process variable to be communicated to or from a smart field instrument. The HART protocol communicates at 1200 bits per second without interrupting the 4-20 mA signal and allows a host application (master) to get two or more digital updates per second from a field device. The most important performance features of the HART protocol include 1. Proven in practice, simple design, easy to maintain and operate, 2. Compatible with conventional analog instrumentation, 3. Simultaneous analog and digital communication, 4. Option of point-to-point or multidrop operation, 5. Flexible data access via up to two master devices, 6. Supports multivariable field devices, 7. Sufficient response time of approximately 500 ms, 8. Open de-facto standard freely available to any manufacturer or user. HCF user organization The HART protocol is an open communication protocol which interfaces the master device with the field device and can be implemented by any manufacturer and freely employed by the user. The required technical support is provided by the HART Communication Foundation (HCF). This manufacturer-independent, not-for-profit organization encourages widespread use of the HART technology. HCF assumes the responsibility of coordinating and supporting the open protocol standard and manages within this framework the device descriptions of all registered devices. A6.1.1 Communication Modes Master-Slave Mode HART is a master-slave communication protocol, which means that during normal operation, each slave (field device) communication is initiated by a master communication device. Two masters can connect to each HART loop. The primary master is generally a distributed control system (DCS), programmable logic controller (PLC), or a personal computer (PC). The secondary master can be a handheld terminal or another PC. Slave devices include transmitters, actuators and controllers that respond to commands from the primary or secondary master. BURST MODE Some HART devices support the optional burst communication mode. Burst mode enables faster communication (3–4 data updates per second). In burst mode, the master instructs the slave device to continuously broadcast a standard HART reply message (e.g., the value of the process variable). The master receives the message at the higher rate until it instructs the slave to stop bursting.
  4. 4. PLC and Automation Additional Topic for Unit 66-4 HART Networks HART devices can operate in one of two network configurations—point to point or multidrop. POINT-TO-POINT In point-to-point mode, the traditional 4–20 mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digital using the HART protocol figure 6.3. The 4–20 mA analog signals is not affected by the HART signal and can be used for control in the normal way. The HART communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance, and diagnostic purposes. MULTIDROP The multidrop mode of operation requires only a single pair of wires and, if applicable, safety barriers and an auxiliary power supply for up to 15 field devices figure 6.3. All process values are transmitted digitally. In multidrop mode, all field device polling addresses are > 0, and the current through each device is fixed to a minimum value (typically 4 mA). Use multidrop connection for supervisory control installations that are widely spaced, such as pipelines, custody transfer stations, and tank farms. Figure 6.3: Point-to-point mode of operation Figure 6.4: Multidrop mode of operation
  5. 5. HART Protocol and CAN bus 6-5 A6.2 THE HART MESSAGE STRUCTURE The structure of a HART message is shown in figure 6.5. The preamble, of between 5 and 20 bytes of hex FF (all 1's), helps the receiver to synchronise to the character stream. The start character may have one of several values, indicating the type of message: master to slave, slave to master, or burst message from slave; also the address format: short frame or long frame. The address field includes both the master address (a single bit: 1 for a primary master, 0 for a secondary master) and the slave address. In the short frame format, the slave address is 4 bits containing the ―polling address‖ (0 to 15). In the long frame format, it is 38 bits containing a ―unique identifier‖ for that particular device. (One bit is also used to indicate if a slave is in burst mode.) The command byte contains the HART command for this message. Universal commands are in the range 0 to 30; common practice commands are in the range 32 to 126; device-specific commands are in the range 128 to 253. Figure 6.5: Message Structure The byte count byte contains the number of bytes to follow in the status and data bytes. The receiver uses this to know when the message is complete. (There is no special ―end of message‖ character.) The status field (also known as the ―response code‖) is two bytes, only present in the response message from a slave. It contains information about communication errors in the outgoing message, the status of the received command, and the status of the device itself. The data field may or may not be present, depending on the particular command. A maximum length of 25 bytes is recommended, to keep the overall message duration reasonable. (But some devices have device-specific commands using longer data fields.) Finally, the checksum byte contains an ―exclusive-or‖ or ―longitudinal parity‖ of all previous bytes (from the start character onwards). Together with the parity bit attached to each byte, this is used to detect communication errors. A6.2.1 DDL device description The HART commands are based on the services of the lower layers and enable an open Communication between the
  6. 6. PLC and Automation Additional Topic for Unit 66-6 master and the field devices. This openness and the interchangeability of the devices independents of the manufacturer are available only as long as the field devices operate exclusively with the universal and common-practice commands and the user does not need more than the simple HART standard notation for the status and fault messages. When the user wants the message to contain further device-related information or that special properties of a field device are also used, the common-practice and universal commands are not sufficient. Using and interpreting the data requires that the user know their meaning. However, this knowledge is not available in further extending systems which can integrate new components with additional options. To eliminate the adaptation of the master devices software whenever an additional status message is included or a new component is installed, the device description language (DDL) was developed. The DDL is not limited to the use for HART applications. It was developed and specified for fieldbusses, independent of the HART protocol, by the Human Interface workshop of the International Fieldbus Group (IFG). The developers of the device description language DDL aimed at achieving versatile usability. The DDL finds also use in field networks. The required flexibility is ensured in so far as the DDL does not itself determine the number and functions of the device interfaces and their representation in the control stations. The DDL rather is a language, similar to a programming language which enables the device manufacturers to describe all communication options in an exact and complete manner. The DDL allows the manufacturer to describe 1. Attributes and additional information on communication data elements, 2. All operating states of the device, 3. All device commands and parameters, 4. The menu structure, thus providing a clear representation of all operating and functional features of the device. Having the device description of a field device and being able to interpret it, a master device is equipped with all necessary information to make use of the complete performance features of the field device. So device and manufacturer-specific commands can also be executed and the user is provided with a universally applicable and uniform user interface, enabling him to clearly represent and perform all device functions. Thanks to this additional information, clear, exact and, hence, safer operation and monitoring of a process is made possible DDL- Use and Benefits Usage of devices which are described with DDL 1. Manufacture and process industry in many branches, 2. For maintenance, repair and fault removal in the plant or shopfloor, 3. For large installations and single device usage, 4. For simple to complex devices, 5. Further use is possible e.g. for configuration of network and PCs, 6. Classes of Devices, 7. Sensors e.g. for pressure, temperature, flow, level, gas and liquid analysis, 8. Actors e.g. positioners, switchgears, close loop controller, motor management systems, 9. Remote I/Os, 10. Gateways with standard communication protocols,
  7. 7. HART Protocol and CAN bus 6-7 11. Device Integration. The DD will be developed by the device manufacture or a service partner. The DD is distributed by the HCF on a disk, via the internet for download or in the device catalog of a DD application. Benefit for ... ... customer ... device manufacture unified User interface User guidance Multi language support Device integration Consistent online device help Suitable for simple and complex devices Efficient development of the DD Easy to learn (device developer can implement the DD by him self) By using already existing DD‘s, dictionaries and standard DD‘s By support for generation of the online help It‘s only necessary to use the DD application and a text editor. The DD and the device can be tested with the DD application Low cost for user training. Safe operation Protection of the Investment Available for many devices. Protection of Investment. One tool for all devices and all usages. Validation of all inputs. High level of quality. Stable EDD standard Operating system independent Easy to expand Low cost for maintenance. Reduction of overall costs. Reduction of cost A6.2.2 HART Commands The HART command set provides uniform and consistent communication for all field devices. The command set includes three classes: 1. Universal, 2. Common practice, 3. Device specific. Host applications may implement any of the necessary commands for a particular application. Table 6.1: HART commands 1. Universal: All devices using the HART protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units). 2. Common practice: Common practice commands provide functions implemented by many, but not necessarily all,
  8. 8. PLC and Automation Additional Topic for Unit 66-8 HART communication devices. 3. Device specific: Device-specific commands represent functions that are unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on device-specific commands is available from device manufacturers. Common Practice Commands used in HART
  9. 9. HART Protocol and CAN bus 6-9 Universal Commands
  10. 10. PLC and Automation Additional Topic for Unit 66-10 A6.2.3 Continuous HART Communication Increases Safety Integrity Level (SIL) By Bud Adler Moore Industries International, Inc. Despite the reliability delivered by today‘s process transmitters and valve controllers, devices do fail. The more risk associated with a failure, the more important it is to ensure the operational integrity of the device. For example, a runaway exothermic reaction is (of course) far more serious than an overcooked batch of cookies (although to some, burnt sweets is a major tragedy as well!). Figure 6.6 HART Diagnostic Alerts All HART smart devices have diagnostic indicators that can alert users to a change in instrument status from a remote location. This data, in the form of status bits, is embedded in the HART digital messages superimposed on the 4-20 mA signal. The diagnostic status bits available in a HART communicating device are: Bit 7—Device Malfunction Bit 6—Configuration Changed Bit 5—Cold Start Bit 4—More Status Available Bit 3—Primary Variable Analog Output Fixed Bit 2—Primary Variable Analog Output Saturated Bit 1—Non-Primary Variable Out of Limits Bit 0—Primary Variable Out of Limits Many users have discovered the value of accessing these diagnostics with their Hand-Held Configurator (HHC). Good News: Users can detect a problem when the HHC is connected to the loop. Bad News: Unless the control system or a loop monitor is communicating with the device on a continuous basis, the ability to detect problems ceases as soon as the HHC is disconnected.
  11. 11. HART Protocol and CAN bus 6-11 Continuous Fault Monitoring: HART-capable control systems and interfaces can continuously monitor the diagnostic status bits in important field devices and provide early warning if problems are detected. HART- capable Loop Monitors, such as the Moore Industries SPA, provide a cost-effective alternative if the control system is not HART- capable. The SPA is typically mounted behind the panel and connected across the loop just like an HHC. When the HART status bits change, the SPA provides both LED indication and relay output(s). This relay action can warn of the situation and/or institute a shutdown, or transfer the instrument to a safe mode of operation pending resolution of the situation. In addition to monitoring the diagnostic status bits, the SPA can also initiate an alarm or provide a 4-20 mA signal based on any three of the Dynamic Process Variables available in HART devices. Device manufacturers define up to four process-related variables to be communicated in these Dynamic Variables. Multi-Variable Devices 1. Pressure: Pressure, Temperature and Differential Pressure. 2. PH Transmitters: Electrode output, compensation temperature and sensor impedance. 3. Coriolis Meters: % solids, density and temperature. 4. Valve Positioner: Actual Stem Position, Actuator Pressure, and Target Stem Position. 5. Temperature Transmitter: Cold junction compensation value HART communication allows monitoring of these Status Bits and Dynamic Variables on a continuous basis providing valuable insight into both hard failures and subtle offsets. Example #1: Excess friction in a control valve often leads to surging conditions that can result in dangerous process upsets. Loss of actuator pressure from a clogged air filter or a torn diaphragm may also lead to a dangerous or costly control offset. The HART Loop Monitor can be configured to alarm on either or both of these conditions. It can also annunciate any of a variety of other performance-related situations. Example #2: Potentially catastrophic results can occur when an emergency shutdown valve does not close when triggered by a dangerous process upset. These critical valves often go for months, or even years, without being stroked to assure proper operation. Clogged air filters, corroded shafts or failed control wiring can all lead to a malfunction. Where the operation of this valve is safety critical, a prudent strategy is to upgrade it with a smart HART positioner complemented with a HART Loop Monitor. With this combination, the presence of adequate air supply can be verified and the valve can be partially stroked on a regular basis to insure its ability to move off of the seat. The loop monitor provides stem position feedback alarms to insure that the valve is only partially stroked thus avoiding a process upset. Example #3: Most temperature transmitters incorporate sensor diagnostics. In general, the main task of sensor diagnostics is to drive the 4-20 mA output either upscale or downscale upon sensor failure. In a safety critical application, this high or low action would often trigger an expensive (and perhaps unnecessary) process shutdown. A HART Loop Monitor can be configured to use the status bits to provide a relay output indicating sensor failure. To avert a process shutdown, this strategy provides differentiation between a non-serious sensor problem and potentially dangerous process condition. For more safety critical applications, a dual non-voting scheme or a two-out-of-three scheme provides even more reliability. Example #4: A subtle failure that may go overlooked for days is a transmitter lock-up characterized by the signal being frozen at a given value. This can occur when a Hand-Held Communicator is used to perform a loop test and is disconnected before returning the transmitter to automatic operation. If the signal happens to be at either 0% or 100%, the
  12. 12. PLC and Automation Additional Topic for Unit 66-12 condition will be quickly recognized. However, if it were left at 50%, the oversight may go unnoticed and possibly cause a dangerous situation. The HART Loop Monitor would call attention to this condition immediately with a relay output. How Safe Is Safe? When performing a Risk Analysis on a process operation, each loop is analyzed for its potential contribution to an unsafe condition should a failure occur. This assessment will define an acceptable Safety Integrity Level (SIL) for each loop in that process. It is up to the design team to select the proper products and procedures to demonstrate the achievement of the required SIL. Guidelines are offered in the ISA standard SP84.01 and in the IEC standard 61508 for methods to improve loop reliability. Every device in a loop has potential failure conditions. Sometimes increased maintenance will insure a higher degree of reliability. For many devices, online diagnoses of failures or potentially dangerous conditions are required to insure the level of reliability demanded by the SIL of the process. Using HART communication allows the diagnosis of potentially dangerous failures and conditions to be significantly increased. This increases loop reliability. By increasing the detection of potentially dangerous failures, the Safety Failure Fraction (SFF) is increased. This results in a reduced Probability of Failure upon Demand (PFD). A6.2.3.1 Cost-Saving Applications Appliance Manufacturing with Multidrop A consumer appliance manufacturer used the networking capability of the HART protocol to measure level, flow, and pressure. HART multidrop provided substantial wiring and installation savings as well as digital accuracy with the elimination of the analog to digital (A/D) and digital to analog (D/A) conversions of the instrument and PLC I/O. The following figure shows pressure transmitters connected to a PLC via smart transmitter interface multiplexers. Figure 6.7: Multidrop Network Example Remote Rezeroing in a Brewery The benefits of remote monitoring and rezeroing of smart transmitters using the HART protocol are dramatically illustrated in this example of two smart transmitters that control the fluid level in lauter tubs in a brewhouse application. Similar benefits would be realized in any application involving a closed vessel.
  13. 13. HART Protocol and CAN bus 6-13 Two smart transmitters are installed on each lauter tub one on the bottom of the tank and the other about nine inches from the bottom. The bottom transmitter is ranged ±40 in H2O; the upper transmitter is ranged 0-30 in H2O. As the lauter tub is filled, the bottom transmitter senses level based on pressure. When the level reaches the upper transmitter, that point is marked as the new zero-level point, and the upper transmitter becomes the primary sensing instrument for the lauter-tub level. The nine-inch zero-level offset from the bottom of the tank is necessary to accommodate loose grain that settles in the bottom of the tank. Transmitters that are coordinated and working together control fluid level in each lauter tub to within a few barrels. However, the upper transmitter requires periodic maintenance or replacement and rezeroing. An undetected false upper- transmitter level reading can cause a tank level error of up to 40 gallons. The usual procedure for transmitter rezeroing takes about 95 minutes and has been required as frequently as twice a day. Rezeroing a transmitter using configuration software and PLC interface modules eliminates the need to locate and identify the problem at the site as well as the need for verification by control-room personnel and greatly reduces the chance for inadvertent errors. Estimated total time to rezero each transmitter is reduced to 15 minutes. Through the configuration software's instrument-status and diagnostic capabilities, a false level indication can be automatically detected while a lauter tub fill is in progress. The affected transmitter can then be automatically rezeroed by programming logic in the programmable controller to issue the appropriate command to the instrument. A6.3 OSI MODEL The OSI (Open Systems Interconnection) Reference Model (developed by the International Standards Organization, or ISO; the two should not be confused), shown in figure 6.8, has been developed to define this structure, and to help in the understanding of the process and nuances of communication. It contains seven layers, and is independent of technology: 1,2 Layer 1 Physical: Defines the mechanical and electrical characteristics necessary to establish, maintain, and terminate the link between physical devices. It includes voltage levels, modulation techniques, etc. Layer 2, Data link: Concerned with physical addressing; network topology; establishing, maintaining and releasing links between nodes; assembling data into packets; as well as error detection and correction at the bit level. Layer 3, Network: Provides connectivity and path selection between two end systems with logical addressing. It is the layer at which routing across subnets occurs. Layer 4, Transport: Responsible for reliable network communication between end nodes, midlevel control of message delivery, including handshaking, message-level error detection, and retransmission. Layer 5, Session: Establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities, the higher level addressing of messages, as well as system control that controls communication sequencing and timing. Layer 6, Presentation: Ensures that information sent by the application layer of one system will be readable by the application layer of another by translating the message into the proper format. It is concerned with the data structures used by programs. Layer 7, Application: Provides services to application processes, the user programs that make a transmission request (such as e-mail, file transfer, and terminal emulation) that are outside the OSI model. The application layer
  14. 14. PLC and Automation Additional Topic for Unit 66-14 identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. Figure 6.8: Diagram of OSI model OSI Model for HART Figure 6.9: HART protocol implementing the OSI model The HART protocol utilizes the OSI reference model. As is the case for most of the communication systems on the field level, the HART protocol implements only the layers 1, 2 and 7 of the OSI model. The layers 3 to 6 remain empty since their services are either not required or provided by the application layer 7 (see figure 6.9).
  15. 15. HART Protocol and CAN bus 6-15 Physical layer Coding Data transmission between the masters and the field devices is physically realized by superimposing an encoded digital signal on the 4 to 20 mA current loop. Since the coding has no mean values, an analog signal transmission taking place at the same time is not affected. This enables the HART protocol to include the existing simplex channel transmitting the current signal (analog control device field device) and an additional half-duplex channel for communication in both directions. The bit transmission layer defines an asynchronous half-duplex interface which operates on the analog current signal line. To encode the bits, the FSK method (Frequency Shift Keying) based on the Bell 202 communication standard is used. The two digital values .0. and .1. are assigned to the following frequencies (see figure 6.10): Logical .0.: 2200 Hz Logical .1.: 1200 Hz Figure 6.10: HART signal superimposed on the analog current signal Each individual byte of the layer-2 telegram is transmitted as eleven-bit UART character at a data rate of 1200 bits/s. The HART specification defines that master devices send voltage signals, while the field devices (slaves) convey their messages using load-independent currents. The current signals are converted to voltage signals at the internal resistance of the receiver (at its load). To ensure a reliable signal reception, the HART protocol specifies the total load of the current loop including the cable resistance to be between minimum 230 ohms and maximum 1100 ohms. Usually, the upper limit is not defined by this specification, but results from the limited power output of the power supply unit. Services of layer 2 Access control The HART protocol operates according to the master-slave method. Any communication activity is initiated by the master, which is either a control station or an operating device. HART accepts two masters, the primary master usually the control system and the secondary master a PC laptop or handheld terminal used in the field. HART field devices the slaves never send without being requested to do so. They respond only when they have received a command message from the master. Once a transaction, i.e. a data exchange between the control station and the field device, is complete, the master will pause for a fixed time period before sending another command, allowing the other master to break in. The two masters observe a fixed time frame when taking turns communicating with the slave devices.
  16. 16. PLC and Automation Additional Topic for Unit 66-16 Communication services The HART protocol provides standard and broadcast commands: Application layer HART commands, the communication routines of HART master devices and operating programs are based on HART commands which are defined in the application layer of the HART protocol. Pre-defined commands enable the master device to give instructions to a field device or send messages/data. So set points, actual values and parameters can be transmitted and various services for start-up and diagnostics performed. The field devices immediately respond by sending an acknowledgement telegram which can contain requested status reports and/or the data of the field device. The example in figure 6.11 shows what the transmitted bytes mean in a transaction initiated using the command 33. This HART command enables the master to read four transmitter variables of the field device and the corresponding units of measurement with only one command. To enable a universal communication, the HART commands are classified according to their function into commands for master devices and for field devices Figure 6.11: Example of HART transaction A6.4 BENEFITS OF HART COMMUNICATIONS The HART protocol is a powerful communication technology used to exploit the full potential of digital field devices. Preserving the traditional 4–20 mA signal, the HART protocol extends system capabilities for two-way digital communication with smart field instruments. The HART protocol offers the best solution for smart field device communications and has the widest base of support of any field device protocol worldwide. More instruments are available with the HART protocol than any other digital communications technology. Almost any process application can be addressed by one of the products offered by HART
  17. 17. HART Protocol and CAN bus 6-17 instrument suppliers. Unlike other digital communication technologies, the HART protocol provides a unique communication solution that is backward compatible with the installed base of instrumentation in use today. This backward compatibility ensures that investments in existing cabling and current control strategies will remain secure well into the future. Benefits outlined in this section include 1. Improved plant operations. 2. Operational flexibility. 3. Instrumentation investment protection. 4. Digital communication. 1. Improved Plant Operations The HART protocol improves plant performance and increases efficiencies in a. Commissioning and installation, b. Plant operations, c. Maintenance. HART -based field devices can be installed and commissioned in a fraction of the time required for a traditional analog-only system. Operators who use HART digital communications can easily identify a field device by its tag and verify that operational parameters are correct. Configurations of similar devices can be copied to streamline the commissioning process. A loop integrity check is readily accomplished by commanding the field transmitter to set the analog output to a preset value. The HART protocol supports the networking of several devices on a single twisted wire pair. This configuration can provide significant savings in wiring, especially for applications such as tank monitoring. Multivariable devices reduce the number of instruments, wiring, spare parts, and terminations required. Some HART field instruments embed PID control, which eliminates the need for a separate controller, and results in significant wiring and equipment cost saving. Figure 6.12: Examples of device parameters sent to control room HART -communicating devices provide accurate information that helps improve the efficiency of plant operations. During normal operation, device operational values can be easily monitored or modified remotely. If uploaded to a software application, these data can be used to automate record keeping for regulatory compliance (e.g., environmental, validation, ISO 9000 and safety standards).
  18. 18. PLC and Automation Additional Topic for Unit 66-18 Numerous device parameters are available from HART-compatible instruments that can be communicated to the control room and used for control, maintenance, and record keeping (figure 6.12). Some HART devices perform complex calculations, such as PID control algorithms or compensated flow rate. Multivariable HART-capable instruments take measurements and perform calculations at the source, which eliminates time bias and results in more accurate calculations than are possible when performed in a centralized host. Some HART field devices store historical information in the form of trend logs and summary data. These logs and statistical calculations (e.g., high and low values and averages) can be uploaded into a software application for further processing or record keeping. 2. Operational Flexibility The HART protocol allows two masters (primary and secondary) to communicate with slave devices and provide additional operational flexibility. A permanently connected host system can be used simultaneously, while a handheld terminal or PC controller is communicating with a field device (figure 6.13). Figure 6.13: Multimaster System The HART protocol ensures interoperablility among devices through universal commands that enable hosts to easily access and communicate the most common parameters used in field devices. The HART DDL extends interoperability to include information that may be specific to a particular device. DDL enables a single handheld configurator or PC host application to configure and maintain HART-communicating devices from any manufacturer. The use of common tools for products of different vendors minimizes the amount of equipment and training needed to maintain a plant. HART extends the capability of field devices beyond the single-variable limitations of 4–20 mA in hosts with HART capability. 4. Digital Communication A digital instrument that uses a microprocessor provides many benefits. These benefits are found in all smart devices regardless of the type of communication used. A digital device provides advantages such as improved accuracy and stability. The HART protocol enhances the capabilities of digital instruments by providing communication access and networking (Table 6.2). Advantages 1. Device Configuration or re-configuration 2. Device Diagnostics 3. Device Troubleshooting
  19. 19. HART Protocol and CAN bus 6-19 4. Reading the values of additional measurements provided by the device 5. Device Health and Status Table 6.2: Digital instruments versus HART instruments Limitations HART communicators need to be maintained. As most of their logic rests in the communicator itself, you need to regularly update (an annual update fee applies) to keep your communicator compatible with newer gear. HART communicators are more expensive than some others. Many sites are already familliar with their existing protocol and may be unwilling to move to HART. REVIEW QUESTIONS 1. Explain OSI/ISO reference model in communication system and layers used in HART. 2. What is ―HART‖? Explain the message structure for HART. 3. Explain foundation field bus system against traditional 4-20 mA System. 4. Write in short ―ControlNet and Devicenet‖. 5. What is ―Device Description Language‖? Explain the benefits of the same. 6. List and explain at least five common commands used in HART. 7. Write a short note on ―Profibus‖. 8. Write a short not eon ―AS-I‖. 9. Explain overview of HART. 10. Explain HART commands.  A6.5 CAN BUS Description The Controller Area Network (CAN) specification defines the Data Link Layer, ISO 11898 defines the Physical Layer. The CAN bus is a Balanced (differential) 2-wire interface running over either a Shielded Twisted Pair (STP), Un-shielded
  20. 20. PLC and Automation Additional Topic for Unit 66-20 Twisted Pair (UTP), or Ribbon cable. Each node uses a Male 9-pin D connector. The Bit Encoding used is: Non Return to Zero (NRZ) encoding (with bit-stuffing) for data communication on a differential two wire bus. The use of NRZ encoding ensures compact messages with a minimum number of transitions and high resilience to external disturbance. Figure 5.23: CAN Bus Electrical Interface Circuit A number of different data rates are defined, with 1Mbps (Bits per second) being the top end, and 10kbps the minimum rate. All modules must support 20 kbps. Cable length depends on the data rate used. Normally all the devices in a system transfer uniform and fixed bit-rates. The maximum line length is 1Km, 40 meters at 1Mbps. Termination resistors are used at each end of the cable. The worst-case transmission time of an 8-byte frame with an 11- bit identifier is 134 bit times (that‘s 134 microseconds at the maximum baud rate of 1Mbits/sec). Figure 5.24: Can Message Frame The CAN Bus interface uses an asynchronous transmission scheme controlled by start and stop bits at the beginning and end of each character. This interface is used, employing serial binary interchange. Information is passed from transmitters to receivers in a data frame. The data frame is composed of an Arbitration field, Control field, Data field, CRC field, ACK field. The frame begins with a ‗Start of frame‘ [SOF], and ends with an ‗End of frame‘ [EOF] space. The data field may be from 0 to 8 bytes. The frame check sequence is derived from a Cyclic Redundancy Code (CRC); the coefficients are generated modulo-2: X15 + X14 + X10 + X8 + X7 + X4 + X3 + 1. CAN implements five error detection mechanisms; 3 at the message level and 2 at the bit level [Also incorporates error flags]. At the message level: Cyclic Redundancy Checks (CRC), Frame Checks and Acknowledgment Error Checks. At the bit level: Bit Monitoring, Bit Stuffing. The CANbus pinout is shown in the table below. The Application for CAN bus in the automotive area include; A low speed CANbus may be employed to operate window and seat controls. A high speed CANbus may be employed for engine management or brake control. Many other applications are possible [Engine Sensors, Anti-Skid Systems]. For additional information refer to: CAN Bus Specification; Version 2.0, or ISO 11898/11519. CANbus is used as a vehicle bus. CANbus is also used as an Industrial Field bus. CAN may also sometimes be seen as Car Area Network. A6.6.1 CAN Bus Pin Out
  21. 21. HART Protocol and CAN bus 6-21 The pinout for the 9-pin D connector is shown in the table 5.10 below. Additional connector styles are listed on the CAN Bus Connector Pin out or CAN Bus Round Connector Pin out. Many of the additional connector pin outs are used with CANopen and include: 10-pin header [5 2 multipole], RJ10 [Modular Connector Jack], RJ45 [Modular Connector Jack], 5-pin mini [circular], 5-pin micro [circular], Open Style, 7/8/9-pin round connectors. Table 5.10 9 Pin (male) D-Sub CANbus PinOut Pin #Signal NamesSignal Description 1 Reserved Upgrade Path 2 CAN_L Dominant Low 3 CAN_GND Ground 4 Reserved Upgrade Path 5 CAN_SHLD Shield, Optional 6 GND Ground, Optional 7 CAN_H Dominant High 8 Reserved Upgrade Path 9 CAN_V+ Power, Optional Some systems may use pin 8 as an error line, to indicate an error on the net. CAN Bus Standard Organizations CiA - CAN In Automation - International Users and Manufactures Group. ISO - International Organization for Standardization. A6.6.2 CAN Bus Standard/Specifications Information ISO/DIS 11898-1: Road vehicles -- Controller area network (CAN) -- Part 1: Data link layer and physical signaling ISO/DIS 11898-2: Road vehicles -- Controller area network (CAN) -- Part 2: High-speed medium access unit ISO/CD 11898-3: Road vehicles -- Controller area network (CAN) -- Part 3: Low-speed fault tolerant medium dependent interface ISO/CD 11898-4: Road vehicles -- Controller area network (CAN) -- Part 4: Time triggered communication CAN Bus Specification Version 2.0 A6.6.3 CAN Bus Interface ICs CAN Bus uses a Drive Voltage: High; 2.75v to 4.5 volts, Low; 0.5 to 2.25 volts, Differential 1.5v to 3.0 volts
  22. 22. PLC and Automation Additional Topic for Unit 66-22 Figure 5.25: CAN Bus Interface IC Logic Transition Levels                                                      

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