Characteristics of HART
HART-Highway Addressable Remote Transducer
HART is a digital industrial Automation Protocol or Communication Protocol.
A HART device is a microprocessor-based process transmitter which supports a two-way communication with the Host.
HART digital signal is modulated onto the 4-20 mA analog signal at a higher frequency and is observed by the process control equipment
For any communication protocol explanation we have to consider OSI (Open System Interconnection) model of ISO, this gives us a general architecture of network specification,
OSI model has 7 layers
1.PHYSICAL LAYER- SAYS ABOUT ELECTRICAL CONNECTIONS
2.DATA LAYER SAYS ABOUT DATA TRANSFER FOR ADJACENT NETWORK
3.NETWORK LAYER SAYS ABOUT ENCAPSULATION OF DATA INTO PACKETS FOR TRANSMISSIONS AND RECEPTION
4.TRANSPORT LAYER SAYS ABOUT DATA HANDLING OVER ENTIRE NETWORK
5.SESSION LAYER SAYS DATA ORGANIZING START AND END OF SPECIFIC TRANSMISSION
6.PRESENTATION LAYER DEFINES CHARACTER SETS AND CONTROL AND GRAPHIC CONTROLS
7.APPLICATION/SOFTWARE LAYER SAYS ABOUT ACTUAL SOFTWARE PROGRAMS
HART uses layer 1 layer 2 layer 4 and layer 7 and if wireless uses an additional layer 3
Layer 1 Physical layer
Layer 1 Physical layer
Data transmission between masters and field devices is physically realized by superimposing an encoded digital signal on the 4–20 mA current loop.
The physical layer defines an asynchronous half-duplex interface that operates on the analog current signal line. To encode the bits, the FSK method, based on the Bell 202 communication standard, is used. Digital value 0 is assigned frequency 2200 Hz, and digital value 1 is assigned frequency 1200 Hz.
HART masters are connected in parallel to the field devices.
HART wiring in the field usually consists of twisted pair cables.
The network technology used is
a) Either FSK for HART versions 1, 2,3,4,5
The HART Communication Protocol is based on the Bell 202 telephone communication standard and operates using the frequency shift keying (FSK) principle. The digital signal is made up of two frequencies: 1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications
b) PSK for HART 6 and 7 versions
PSK Physical Layer—a higher speed physical layer option—is available in the HART 6 and HART 7 versions of the HART Protocol. PSK supports significantly faster communications with standard command/response throughput of up to 10-12 transactions per second simultaneous with the 4-20mA signal.
c) TDMA for wireless HART
A Wireless HART network utilizes Time Division Multiple Access (TDMA) technology to ensure that only one instrument is talking on a channel at any given time. This prevents message collisions within the Wireless HART network. A network is provided with an overall schedule which is divided into 10 ms timeslots. At any time, only one pair of instruments are communicating on the same frequency channel, however, it is possible that more than one pair of instruments can communicate at the same time using different channels. In most cases, only one pair of instruments is communicating in a given timeslot so the Wireless HART network will not monopolize the frequency spectrum that is shared with other wireless networks.
In HART individual bus devices can communicate via the topology specified below.
The physical layer can be of 5 types. This means that the network configuration used are
In point-to-point mode, the 4-20mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART Protocol. The 4-20mA analog signal is not affected by the HART signal and can be used for control. The HART Communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance and diagnostic purposes
The multi-drop 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 (HART 5) or 62 field devices (HART 7) All process values are transmitted digitally. In multi-drop mode, all field device polling addresses must be unique in a range of 1-63 (depending on the HART Protocol Revision) and the current through each device is fixed to a minimum value (typically 4mA).
Use multi-drop connection for supervisory control installations that are widely spaced such as pipelines, custody transfer stations, and tank farms
iii)Split range network
Split range control is a single control loop divided into two or more independent final control elements such as valves acting in different directions or in different steps. There are many ways to implement a split range control: software, valve calibration or by connecting two or more positioners to a single control signal (usually 4-20mA). A typical split range loop with two valves will be configured as follows:
Intelligent Valve Positioner #1
Action: ATO (Air to Open)
Input current range: 4-12mA
Intelligent Valve Positioner #2
Action: ATC (Air to Close)
Input Current range: 12-20mA
When more than one positioner is installed in a single current loop, the HART loop address of each device must be set to 1, 2, or 3 (or other non-zero values) to allow a HART master to recognize each intelligent valve positioner when connected to all three devices on a single current loop.
As the need for additional process measurements increases, users seek a simple, reliable, secure and cost-effective method to deliver new measurement values to control systems without the need to run more wires.
Each Wireless HART network includes three main elements:
Wireless field devices connected to process or plant equipment. These devices can be a device with Wireless HART built in or an existing installed HART-enabled device with a Wireless HART adapter attached to it.
Gateways enabling communication between the field devices and host applications connected to a high-speed backbone or other existing plant communications network.
A Network Manager responsible for configuring the network, scheduling communications between devices, managing message routes, and monitoring network health. The Network Manager can be integrated into the gateway, host application, or process automation controller.
Each device in the mesh network can serve as a router for messages from other devices. In other words, a device doesn’t have to communicate directly to a gateway, but just forward its message to the next closest device. This extends the range of the network and provides redundant communication routes to increase reliability
v)Control in field (PID)
Microprocessor-based smart instrumentation enables control algorithms to be calculated in the field devices, close to the process. Some HART transmitters and actuators support control functionality in the device, which eliminates the need for a separate controller and reduces hardware, installation, and start-up costs. Accurate closed-loop control becomes possible in areas where it was not economically feasible before. While the control algorithm uses the analog signal, HART communication provides the means to monitor the loop and change control set point and parameters. Placing control in the field enhances control functionality. Measurement accuracy is maintained because there is no need to transmit data to a separate controller. Control processing takes place at the high update rate of the sensor and provides enhanced dynamic performance.
Layer 2 (data layer)
Layer 2 (Data layer)
The data link layer provides a reliable, transaction-oriented communication path to and from field devices for digital data transfer.
The data link layer supports the application layer above it and requires services from the physical layer below it.
Divided into two sub layers: the logical link control responsible for addressing, framing, and error detection; and the medium access control that controls the transmission of messages across the physical link.
The elements of the HART frame are summarized as follows:
The delimiter is the first field in a HART message. It is used for message framing by indicating the position of the byte count. Three frame types are supported by the HART data link layer STX(0x2) indicates master to a field device, STX is generally Start of the transaction ACK(0x6) Slaves response to the STX, and finally the BCK(0x1) burst acknowledge frame periodically transmitted by a burst-mode device.
The address field can be short or long. The protocol supports both five (5) byte unique addresses and one (1) byte polling addresses. The expansion bytes are optional. This field is 0–3 bytes long and its length is indicated in the delimiter.
The command byte encodes the master commands of the three categories: universal, common practice, and device-specific commands. The byte count character indicates the message length, which is necessary since the number of data bytes per message can vary from 0 to 25.
The data field is optional and consists of an integral number of bytes of application layer data.
The response message includes two status bytes at the beginning of the data portion of the message.
This check byte field is 1 byte long. The check byte value is determined by a bitwise exclusive OR of all bytes of a message including the leading delimiter.
HART protocol is Master/Slave based communications protocol. Slave communication is initiated only when the Master requests. Two Masters can connect to each HART loop.
Primary Master can be the Distributed Control System, Programmable Logic Controller (PLC) or any Personal Computer. Secondary Master is generally a Handheld Terminal or another PC.
Slave Devices consist of Transmitters, Actuators, and controllers which respond to commands from Master.
Types of data link layer communications are Request/Response, Burst Mode, Events and Event Notifications.
Request response mode
HART Communications protocol uses Request/Response messages to access and change parameter values, invoke device methods, configure devices and in wireless HART manage the network devices
Allows the master to instruct the slave device to continuously broadcast a standard HART reply message.
Master receives the message in burst mode until it tells the slave to stop bursting.
Wireless HART devices support Burst mode whereas in Wired it is optional.
Events and event notification
Event notification publishes changes in the Device Status.
It is possible to specify limited set of bits that will trigger event notifications.
A de-bounce interval is configured.
Once the event is released, it is transmitted repeatedly until it is acknowledged.
Event notifications are built upon burst mode operation. The two distinct methods to display events are: Device Status and Common Practice Command 48.
Layer 3 (only for wireless communication) Network Layer
Network Layer (only for wireless Communication)
DLL moves packets between devices, hop by hop, the network layer moves packets end-to-end within the wireless network.
The 2.4 GHz ISM frequency band is divided into 16 non-overlapping frequency channels. Wireless HART instruments use a pseudo-random channel hopping sequence to reduce the chance of interference with other networks. Network layer security provides end-to-end data integrity and privacy across the wireless network.
Wireless HART is beyond the scope of this article
Layer 4(Transport layer)
Layer 4(Transport layer)
Block Data Transfer
It allows the device to transfer blocks of information.
It is classified as a Transport layer service.
Establishes connection between host and slave and transfers stream of data.
It maximizes the utilization of HART Communication.
Connection for this kind of communication is established by the command 111 to a specific port.
Command 112 is used to transfer data to and from the field device
HART-IP, an Internet protocol (IP) enabled version of HART, was developed. HART-IP gives enterprise level systems and applications access for block data transfer, DATA HANDLING OVER ENTIRE NETWORK and integration of runtime measurement and device diagnostics information from HART devices through existing plant IP networks using Ethernet, Wi-Fi, fibre optic, packet-radio, satellite or 3G/4G cellular.