1. A brief study Mrs. Tannistha Kapoor Engr. O&M/C&I
HART COMMUNICATIONS
A brief study
Mrs. Tannistha Kapoor
Engr. O&M/C&I

2. AGENDA 1. WHAT IS HART 2. HOW HART WORKS 3. HART COMMUNICATIONS
4. HART DATA
5. CALIBRATION
6. BENEFITS OF USING HART
7. WIRELESS HART

3. WHAT IS HART
Hart is an acronym for Highway addressable remote transducers.
HART is a bidirectional master-slave field communications protocol developed in the late 1980's to facilitate communication between intelligent field instruments and host systems by Rosemount Inc. Later it was developed into an open protocol
It makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA.
It communicates without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device
HART is a master/slave protocol which means that a field (slave) device only speaks when spoken to by a master.
HART provides for up to two masters (primary and secondary). This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system.

4. WHAT IS HART contd
A TYPICAL HART SETUP

5. WHAT IS HART contd POINT TO POINT CONFIGURATION MULTIDROP

6. HOW HART WORKS CONVENTIONAL PROCESS LOOP PROCESS LOOP WITH HART ADDED
HART is sometimes best understood by looking at how it evolved from a conventional process loop.  Figure 1 is a simplified diagram of the familiar analog current loop.  The process transmitter signals by varying the amount of current flowing through itself.  The controller detects this current variation by measuring the voltage across the current sense resistor.  The loop current varies from 4 to 20 mA at frequencies usually under 10 Hz.
With HART added both ends of the loop now include a modem and a "receive amplifier. To send a HART message, the process transmitter turns ON its AC-coupled current source.  This superimposes a high-frequency carrier current of about 1 mA p-p onto the normal transmitter output current.  The current sense resistor at the controller converts this variation into a voltage that appears across the two loop conductors.   The voltage is sensed by the controller's receive amplifier and fed to the controller's demodulator (in block labeled "modem").
To send a HART message in the other direction (to the process transmitter), the HART Controller closes its transmit switch.  This effectively connects the "Xmit Volt Source" across the current loop conductors, superimposing a voltage of about 500 mV p-p across the loop conductors.  This is seen at the process transmitter terminals and is sent to its receive amplifier and demodulator.
This implies that a Master transmits as voltage source, while a Slave transmits as a current source

7. How Hart Works contd HART PROCESS TRANSMITTER:-.
The EEPROM stores fundamental hart parameters
UART is used to convert between serial & parallel data
Network interface is a current regulator which implement the two current sources.

8. How hart works contd.
The HART signal is typically isolated with a high-pass filter having a cut-off frequency in the range of 400 Hz to 800 Hz.  The analog signal is similarly isolated with a low-pass filter.  The separation in frequency between HART and analog signaling means that they can coexist on the same current loop.

9. HART COMMUNICATIONS
The transmitting device begins by turning ON its carrier and loading the first byte to be transmitted into its UART.
The UART converts each transmitted byte into a 11 bit serial character.
The serial character stream is applied to the Modulator of the sending modem.
The Modulator operates such that a logic 1 applied to the input produces a Hz periodic signal at the Modulator output.  A logic 0 produces 2200 Hz.
After transmission of the first byte , the transmitter loads the next byte. And so on.
After the last byte is serialized and transmitted the transmitter turns off the source.

10. HART COMMUNICATIONS
SIGNAL PATH:-

11. HART COMMUNICATIONS PROTOCOL Only one HART device can talk at a time.
A Master typically sends a command and then expects a reply. A Slave waits for a command and then sends a reply. A Slave accesses the network as quickly as possible in response to a Master.   The command and associated reply are called a transaction. 
There are typically periods of silence (nobody talking) between transactions.
Network access by Masters requires arbitration which is based on monitoring of network traffic and implementation of timers.
If two Masters are present and both are synchronized, then they will use the network alternately .

12. During this process a given Master knows that it is free to use the network when it sees the end of the Slave response to the other Master.  If a Master doesn't take its turn, the other Master can have another turn, provided it waits a length of time called RT2.  The time interval RT2 is illustrated in figure 2.2.  The Master whose turn it is to use the network has this much time in which to start.  Otherwise the Master that last used the network may start.  This is how things role merrily along when there are no problems and when both Masters have almost continuous business to transact.   Although not explicitly shown in figure 2.2 and subsequent figures, both Masters start their timers at the end of any network activity.  Any fresh activity cancels the timers.  Also, it is implicit in these explanations that a Master will not begin talking if someone else is talking

13. a Slave doesn't respond to Master 1
a Slave doesn't respond to Master 1.  Master 2 must now wait a length of time called RT1 before it tries to use the network.  Master 1, while waiting for the Slave response, sees the Master 2 command instead.  It then waits until Master 2 is done and then it can retry.  Here, Master 2 has lost synchronization because it did not see a Slave Response to Master 1.  It regains synchronization at the end of RT1. Slave maximum response time, which is designated TT0, is slightly shorter than RT1.  This ensures that a Master and Slave will not start transmitting simultaneously.

14. Since Master 2 didn't see a good Slave Response, it begins waiting a length of time RT1 from the end of the Slave Response.  Master 1, which saw a good Slave Response and is still synchronized, starts RT2.  At the end of RT2, Master 1 sees that Master 2 isn't using the network and decides to use it again.  Master 2 sees this new transmission by Master 1 and becomes resynchronized.  Had Master 1 not wanted to re-use the network again, then Master 2 would have become resynchronized at the end of RT1 and could have begun its transaction then. Suppose that both Masters are new to the network or are both unsynchronized and try to use the network at the same time (after waiting for RT1).  The respective commands will be garbled and there will be no response.  Both Masters will start RT1 again at about the same time.  And both will collide again at the end of RT1.  To prevent this from going on endlessly, the Primary and Secondary Masters have different values for RT1.  The Primary Master uses a value designated RT1(0).   The Secondary Master uses a value designated RT1(1).

15. HART COMMUNICATIONS Timer Description Symbol Value (character times)
Master Wait Before Re-Using Network
RT2 8
Primary Master Wait from Unsynched
RT1(0) 33
Secondary Master Wait from Unsynched
RT1(1) 41
Slave Max time to Respond
TT0 28
Slave Time Between Bursts BT

16. HART COMMUNICATIONS
A Slave (normally) has a unique address to distinguish it from other Slaves. 
Addresses are either 4 bits or 38 bits.
The long address consists of the lower (least significant) 38 bits of a 40-bit unique identifier.
Each command or reply is a message, varying in length from 10 or 12 bytes to typically 20 or 30 bytes
This address is incorporated into the command message sent by a Master and is echoed back in the reply by the Slave. and are called short and long or "short frame" and "long frame" addresses, respectively.  A Slave can also be addressed through its tag (an identifier assigned by the user). 
Early HART protocol used only a 4 bit address.  This meant there could be 16 field instruments per network.  In any Field Instrument the 4-bit address could be set to any value from 0 to 15 using HART commands.  If a Master changed the address of a Field Instrument, it would have to use the new address from then on when talking to that particular Field Instrument.
    Later, HART was modified to use a combination of the 4-bit address and a new 38 bit address.  In these modern devices, the 4-bit address is identical to the 4-bit address used exclusively in earlier devices, and is also known as a polling address or short address.  The 38 bit address is also known as the long address, and is permanently set by the Field Instrument manufacturer.  A 38-bit address allows virtually an unlimited number of Field Instruments per network.   Older devices that use only a 4-bit address are also known as "rev 4" Field Instruments.   Modern devices, that use the combined addresses, are also known as "rev 5" instruments.  These designations correspond to the revision levels of the HART Protocol documents.  Revision 4 devices are now considered obsolete.  Their sale or use or design is discouraged and most available software is probably not compatible with revision 4.
    So, why the two forms of address in modern Field Instruments?   The reason is that we need a way of quickly determining the long address.  We can't just try every possible combination (2 to the 38th power).  This would take years.  So, instead, we put the old 4-bit address to work.  We use it to get the Field Instrument to divulge its long address.  The protocol rules state that HART Command 0 may be sent using the short address.  All other commands require the long address.  Command 0, not surprisingly, commands a Field Instrument to tell us its long address.  In effect the short address is used only once, to tell us how to talk to the Field Instrument using its long address.

17. Hart communications contd
HART MESSAGE STRUCTURE :-
Part of Message
Length in Bytes
Purpose
Preamble
5 to 20
Synchronization & Carrier Detect
Start Delimiter 1
Synchronization & Shows Which Master
Address
1 or 5
Choose Slave, Indicate Which Master, and Indicate Burst Mode
Command
Tell Slave What to Do
Number Data Bytes
Indicates Number Bytes Between Here and Checksum
Status
0 (if Master) 2 (if Slave)
Slave Indicates Its Health and Whether it did As Master Intended
Data
0 to 253
Argument Associated with Command (Process Variable, For Example)
Checksum
Error Control

18. Hart Communications contd.
The preamble is allowed to vary in length, depending on the Slave's requirements.
The status field (2 bytes) occurs only in replies by HART Slave devices.  If a Slave does not execute a command, the status shows this and usually indicates why.  Several possible reasons are:
        1.    The Slave received the message in error.  (This can also result in no reply.)
        2.    The Slave doesn't implement this command.
        3.    The Slave is busy.
        4.    The Slave was told to do something outside of its capability                 (range number too large or small, for example).
        The Slave is write-protected and was told to change a protected parameter.
Commands are one of 3 types:  Universal, Common Practice, and Device Specific (Proprietary). 
A Master will use the longest possible preamble when talking to a Slave for the first time.  Once the Master reads the Slave's preamble length requirement (a stored HART parameter), it will subsequently use this new length when talking to that Slave.  Different Slaves can have different preamble length requirements, so that a Master might need to maintain a table of these values.
    A longer preamble means slower communication.  Slave devices are now routinely designed so that they need only a 5 byte preamble; and the requirement for a variable preamble length may  now be largely historical.
Universal and Common Practice commands implement functions that were either part of an original set or are needed often enough to be specified as part of the Protocol.   Among the Universal commands are commands to read and write the device's serial number, tag, descriptor, date; read and write a scratch memory area; read the device's revision levels; and so on.  These parameters are semi-permanent and are examples of data that is stored in EEPROM.

19. Hart communications contd.
SLAVE REPLY ALGORITHM

20. HART DATA OVERVIEW
DIGITAL DATA: valuable data items standard in every HART device
DEVICE IDENTIFICATION: device tag, supplier, device type and revision, device serial number
CALIBRATION DATA: upper and lower range values, upper and lower sensor limits, PV damping, last calibration date
PROCESS VARIABLES: primary variable plus secondary measurements and multivariable parameters
STATUS/DIAGNOSTIC ALERTS: device malfunction, configuration change, power fail restart, loop current fixed or saturated, primary or secondary variable out of limits, communication error, plus more
When this data is integrated with control, asset management or safety systems, you are able to improve plant operations, lower cost and increase plant availability

21. HART DATA OVERVIEW CONTD
PROCESS VARIABLE VALUES
Primary Process Variable (analog) ma current signal continuously transmitted to host
Primary Process Variable (digital) - Digital value in engineering units, IEEE floating point, up to 24 bit resolution
Percent Range - Primary Process Variable expressed as percent of calibrated range
Loop Current - Loop current value in milliamps
Secondary Process Variable 1 - Digital value in engineering units available from multivariable devices
Secondary Process Variable 2 - Digital value in engineering units available from multivariable devices
Secondary Process Variable 3 - Digital value in engineering units available from multivariable devices
When this data is integrated with control, asset management or safety systems, you are able to improve plant operations, lower cost and increase plant availability

22. HART DATA OVERVIEW CONTD
COMMANDS FROM HOST TO DEVICE
Set Primary Variable Units
Set Upper Range
Set Lower Range
Set Damping Value
Set Message
Set Tag
Set Date
Set Descriptor
Perform Loop Test - Force loop current to specific value
Initiate Self Test - Start device self test
Get More Status Available Information
Codes vary by manufacturer/device

23. HART DATA OVERVIEW CONTD
STATUS AND DIAGNOSTIC ALERTS:-
Device Malfunction - Indicates device self-diagnostic has detected a problem in device operation
Configuration Changed - Indicates device configuration has been changed
Cold Start - Indicates device has gone through power cycle
More Status Available- Indicates additional devices status data available
Primary Variable Analog Output Fixed - Indicates device in fixed current mode
Primary Variable Analog Output Saturated - Indicates 4-20mA signal is saturated
Secondary Variable Out of Limits - Indicates secondary variable value outside the sensor limits
Primary Variable Out of Limits - Indicates primary variable value outside the sensor limits

24. HART DATA OVERVIEW CONTD
DEVICE IDENTIFICATION:-
Instrument Tag - User defined, up to 8 characters
Descriptor - User defined, up to 16 characters
Manufacturer Name (Code) - Code established by HCF and set by manufacturer
Device Type and Revision - Set by manufacturer
Device Serial Number - Set by manufacturer
Sensor Serial Number - Set by manufacturer

25. HART DATA OVERVIEW CONTD
CALIBRATION INFORMATION FOR 4-20MA TRANSMISSION OF PRIMARY PROCESS VARIABLE
Date - Date of last calibration, set by user
Upper Range Value - Primary Variable Value in engineering units for 20mA point, set by user
Lower Range Value - Primary Variable Value in engineering units for 4mA point, set by user
Upper Sensor Limit - Set by manufacturer
Lower Sensor Limit - Set by manufacturer
Sensor Minimum Span - Set by manufacturer
PV Damping - Primary Process Variable Damping Factor, set by user
Message - Scratch pad message area (32 characters), set by user
Loop Current Transfer Function - Relationship between Primary Variable digital value and 4-20mA current signal
Loop Current Alarm Action - Loop current action on device failure (upscale/downscale)
Write Protect Status - Device write-protect indicator

26. HART ADVANTAGES
Key benefits of this unique open standard communication technology are:
-4-20mA compatibility with simultaneous digital information available
- Easy to use and understand
-Low risk
- highly accurate and robust
-Cost-effective implementation for both users and suppliers
-Available in a wide variety of device types
- Supported by most industry device and systems suppliers
-Fully interoperable and reliable
Users be aware—intelligent data is no longer available only through the handheld communicator or a proprietary program, but can be monitored continuously 24 hours a day, 7 days a week.
Through continuous monitoring of the HART Communication digital signal, potential system failures or quality assurance anomalies can be easily detected at the device level. HART-detectable failure scenarios include in-range sensor failures, 4-20mA distortion (analog output does not equal PV), incorrect setup and a device / input mismatch.

27. CALIBRATION
TRANSDUCER BLOCK:- Generates the actual digital signal representation of the process parameter.
ZERO & SPANNING BLOCK:- The upper and lower range values are used to produce the transducer value from above to correspond to a 4mA signal for the lower range and a 20mA signal for the upper range in the % form. In addition an appropriate transfer function (e.g., linear, square root, quadratic, cubic spline, etc.) may be applied .
DAQ BLOCK:- Produces the 4-20ma signal, insuring that 0% equals exactly 4 ma and 100% equals 20ma.

28. WIRELESS HART

29. Wireless HART contd
Wireless HART Networks consists of WirelessHART field devices, at least one WirelessHART gateway, and a WirelessHART network manager.
These components are connected into a wireless mesh network supporting bi-directional communication from HART host to field device and back.

30. Wireless HART contd
Network Manager The Network Manager is an application that manages the mesh network and Network Devices. The Network Manager performs the following functions:
- Forms the mesh network Allows new devices to connect to the network Sets the communication schedule of the devices Establishes the redundant data paths for all communications Monitors the network
Gateway The Gateway Device connects the mesh network with a plant automation network, allowing data to flow between the two. The Gateway Device provides access to the WirelessHART devices by a system or other host application.
Field Devices The Field Device may be a process connected instrument, a router or Hand Held device. The WirelessHART network connects these devices together. -Router Device:A device to improve network coverage (to extend a network) capable of forwarding messages from other Network Devices Process Connected Instrument:Typically a measuring or positioning device used for process monitoring and control. It is also capable of forwarding messages from other Network Devices WirelessHART Adapter:A device that allows a HART instrument without wireless capability to be connected to a WirelessHART network.

31. Wireless HART contd TECHNOLOGY BASICS:-
- Time Synchronized Communication:- WirelessHART devices communicate using Time Division Multiple Access. All device-to-device communication is done in a pre-scheduled time window which enables very reliable (collision-free), power-efficient, and scalable communication - Self-Organizing and Self-Healing:- It means every device has the intelligence to discover neighbors, measure RF signal strength, acquire synchronization and frequency hopping information, and then establish paths and links with neighboring devices. This enables very simple and robust network installation, reliable long-term performance, and simple network expansion. - Frequency Hopping Spread Spectrum:- It uses the unlicensed part of the radio spectrum in the 2.4GHz ISM band. - Secure Communications There are three pillars of secure communication: encryption, authentication and integrity. Encryption keeps the information carried by the message from being read by other parties; authentication ensures that the sender is actually the sender; and integrity ensures that the message was delivered unaltered. -Redundant Mesh Routing WirelessHART implements a “full-mesh” topology in which every device has multiple redundant communication paths.

36. HART SYSTEM