HART COMMUNICATION

 

 

HART Communication

By mobin Ahmed ETO, BP marine Academy, CBD Belapur

1. Introduction to HART Communication

HART (Highway Addressable Remote Transducer) is a widely-used communication protocol in industrial automation that enables digital data to be transmitted along with the traditional 4-20 mA analog signal. This unique setup allows HART-enabled devices to communicate with control and monitoring systems, transmitting additional information such as device diagnostics, status, and configuration parameters in parallel with the primary process variable (like temperature or pressure).

This combination of analog and digital transmission means that HART can enhance existing infrastructure by adding digital communication without replacing the analog signal system. As a result, it's compatible with legacy systems and offers easy integration with newer technologies.

 

2. Hybrid Communication in HART

Analog Signal:

• The conventional 4-20 mA analog signal is the backbone of HART communication, carrying the primary process variable (like temperature or flow rate) as a continuous current signal. This current loop is reliable, interference-resistant, and serves as the primary measurement method in industrial systems.
• Analog communication in HART is one-way, meaning it can only send information from the field device to the control system, primarily representing the main process value.

 

 

Digital Signal:

• HART adds a digital communication channel by superimposing a Frequency Shift Keying (FSK) signal onto the 4-20 mA loop. This digital signal transmits data without interrupting or distorting the analog signal.
• The digital channel operates at 1200 bps, with a low-level modulation that enables two-way communication between the host system (like a PLC or Distributed Control System) and the smart field device.
• With the digital channel, the control system can receive additional data, including device diagnostics, multiple process variables, and configuration data. This enhances system capabilities by making maintenance and monitoring more proactive and accurate.

 

 

 

3. Communication Modes in HART

HART devices support two communication modes:

• Request-Response Mode: The standard operating mode where the host sends a command, and the device responds with data. This mode is useful for periodic data polling and one-time requests like device setup or configuration.

• Burst Mode: Some HART devices offer a burst mode in which the device sends continuous digital updates of a specific variable without waiting for a request from the host. For example, a flow transmitter could be set to transmit flow rate continuously, which is valuable for real-time monitoring. The host can stop burst mode by sending a specific command to the device.

 

4. Master/Slave Protocol in HART

• HART is structured around a master-slave communication protocol, where each field device acts as a slave that only responds to commands from a master. There can be two masters per HART loop:
• Primary Master: Generally, a DCS, PLC, or PC, it initiates commands and handles continuous communication with the field devices.
• Secondary Master: Often a handheld communicator or another PC used for on-site device configuration and troubleshooting without interrupting the primary control loop.
• Slave devices in a HART system typically include transmitters, actuators, and other field devices. Each respond to commands from either the primary or secondary master.

 

 

5. Frequency Shift Keying (FSK) in HART

• HART uses Frequency Shift Keying (FSK) to transmit digital data at 1200 bps, based on the Bell 202 standard.
• The FSK signal encodes digital bits using two distinct frequencies:
• 1200 Hz represents a digital "1".
• 2200 Hz represents a digital "0".
• This FSK signal is phase-continuous and low-level, meaning it doesn’t interfere with the primary 4-20 mA analog signal, allowing both channels to coexist on the same loop without causing disruption.

 

 

6. HART Configurations: Point-to-Point and Multi-Drop Connections

HART supports two types of configurations:

• Point-to-Point Connection: In this setup, a single field device is connected to the host, and the current loop functions normally (transmitting the 4-20 mA analog signal). A secondary host can also connect to send commands, but only one field device is on the loop.

 

 

• Multi-Drop Connection: Multiple HART devices can be connected in parallel in a multi-drop network, which is useful for applications requiring multiple devices on the same line. In this mode, each device is assigned a unique address, and the current in the loop is fixed at 4 mA, making the analog signal unavailable. Communication occurs purely through digital data.

 

 

 

7. Advantages of HART Protocol

• Enhanced Data Accessibility: HART devices offer diagnostic information, allowing users to access advanced data that can be used to optimize operations and prevent failures.
• Improved Maintenance and Inventory Costs: Real-time device status helps prevent unexpected downtime and reduces inventory by identifying exact replacement needs.
• Increased Safety and Diagnostics: Advanced diagnostics improve safety by detecting potential issues early.
• Interoperability: HART’s compatibility with multiple device types and systems makes it highly versatile in various industrial environments.

 

 

8. Disadvantages of HART Protocol

• Slower Transmission Speeds: With digital transmission at 1200 bps, HART is slower than other protocols like Profibus, limiting its use in fast-changing processes.
• Limited Device Count in Multi-Drop Mode: HART can only support up to 15 devices in multi-drop mode, and it disables the analog current signal in this setup.
• Single Process Variable at a Time: While HART can communicate multiple variables, the protocol is best suited to applications where only one variable is monitored per loop.

 

 

9. Applications of HART Protocol

• Process and Instrumentation Systems: HART is widely used in industrial automation, especially for integrating smart sensors and transmitters in sectors like oil, gas, and chemical industries.
• Smart Device Diagnostics: HART’s diagnostic capabilities make it ideal for monitoring device health in real-time.
• Multivariable Instruments: HART can handle instruments that measure multiple parameters, such as mass flow meters that track volumetric flow, mass flow, density, and temperature.

 

10. OSI Layer Implementation in HART

HART implements several layers of the OSI model, which governs data communication:

• Physical Layer: Utilizes FSK signaling, based on Bell 202, allowing data to overlay on the 4-20 mA current loop without interference.
• Data Link Layer: Defines the master-slave protocol structure, allowing multiple masters and slaves on the same network.
• Network and Transport Layers: Handle routing and transport for reliable data communication.
• Application Layer: Defines specific commands, including Universal Commands (mandatory), Common Practice Commands, Device-Specific Commands, and Device Family Commands, ensuring flexibility and standardization across devices.

 

11.Conclusion

 

HART communication adds a layer of digital communication to the traditional 4-20 mA analog loop, providing enhanced capabilities for diagnostics, configuration, and data monitoring. Its compatibility with existing analog systems makes it a powerful yet cost-effective solution for industrial automation, blending the reliability of analog with the versatility of digital data transfer.