HART COMMUNICATIONS

A brief study

Mrs. Tannistha KapoorEngr. O&M/C&I

AGENDA

1. WHAT IS HART1. WHAT IS HART

2. HOW HART WORKS 2. HOW HART WORKS

3. HART COMMUNICATIONS3. HART COMMUNICATIONS

7. WIRELESS HART7. WIRELESS HART

5. CALIBRATION5. CALIBRATION

4. HART DATA4. HART DATA

6. BENEFITS OF USING HART6. BENEFITS OF USING HART

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

WHAT IS HART contd

A TYPICAL HART SETUP

WHAT IS HART contd

MULTIDROP

CONFIGURATION

POINT TO POINT

CONFIGURATION

HOW HART WORKSCONVENTIONAL PROCESS LOOP

PROCESS LOOP WITH HART ADDED

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.

How hart works contd.

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 1200 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.

HART COMMUNICATIONS

SIGNAL PATH:-

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 .

Timer Description SymbolValue (character

times)

Master Wait Before Re-Using Network

RT2 8

Primary Master Wait from Unsynched

RT1(0) 33

Secondary Master Wait from

UnsynchedRT1(1) 41

Slave Max time to Respond

TT0 28

Slave Time Between Bursts

BT 8

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

HART COMMUNICATIONS

Part of MessageLength in

BytesPurpose

Preamble 5 to 20 Synchronization & Carrier Detect

Start Delimiter 1Synchronization & Shows Which

Master

Address 1 or 5Choose Slave, Indicate Which

Master, and Indicate Burst Mode

Command 1 Tell Slave What to Do

Number Data Bytes 1Indicates Number Bytes Between

Here and Checksum

Status0 (if Master)

2 (if Slave)

Slave Indicates Its Health and Whether it did As Master Intended

Data 0 to 253Argument Associated with Command

(Process Variable, For Example)

Checksum 1 Error Control

Hart communications contdHART MESSAGE STRUCTURE :-

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).

    5.    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). 

SLAVE REPLY ALGORITHM

Hart communications contd.

HART DATA OVERVIEW

DIGITAL DATA: 35-40 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

HART DATA OVERVIEW CONTD

PROCESS VARIABLE VALUESPrimary Process Variable (analog) - 4-20 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

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

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

HART DATA OVERVIEW CONTD

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

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

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

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.

WIRELESS HART

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.

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

• GatewayThe 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 DevicesThe 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.

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 CommunicationsThere 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 RoutingWirelessHART implements a “full-mesh” topology in which every device has multiple redundant communication paths.

HART SYSTEM