How to Connect HART Communication to Instrumentation: A Practical Guide
HART (Highway Addressable Remote Transducer) communication protocol is a widely adopted technology in process automation for smart instrumentation. It allows digital communication over the same 4-20 mA analog wiring used for transmitting measurement signals. Understanding how to connect HART communication to your instrumentation can significantly improve diagnostics, calibration, and configuration capabilities. This guide outlines the steps, requirements, tools, and best practices for setting up HART communication in your industrial environment.
What is HART Communication?
HART Protocol Overview
HART is a hybrid communication protocol developed by Rosemount in the 1980s. It overlays digital communication signals on top of standard 4-20 mA analog signals, allowing two-way field communication with smart instruments.
Why Use HART?
- Non-intrusive: Uses existing 4-20 mA wiring
- Cost-effective: No need for extra cabling or major infrastructure changes
- Bidirectional communication: Supports diagnostics, configuration, and calibration
- Multivariable support: Transmits multiple data points over a single loop
Components Required for HART Communication
To successfully connect and use HART, you’ll need the following:
1. HART-Compatible Field Instrument
Most modern transmitters, such as pressure, flow, or level transmitters, include HART capabilities. Confirm the instrument datasheet or nameplate.
2. HART Communicator or Software
- Handheld Communicator (e.g., Emerson 475)
- PC-Based Software (e.g., Pactware, FieldCare, AMS Device Manager)
3. HART Modem or Interface
- USB HART Modem or Bluetooth HART Modem
- Converts PC signals to HART protocol
4. Loop Power Supply (if bench setup)
HART communication requires the loop to be powered, either by the control system or a dedicated bench power supply.
5. 250-ohm Resistor
HART requires a minimum impedance of 230 ohms in the loop for digital communication. Most systems use a 250-ohm resistor.
How to Connect HART Communication
Step-by-Step Connection Process
A. In Field Instrumentation (Live Plant Environment)
- Identify the instrument’s signal wiring terminals (usually + and – of the 4-20 mA loop).
- Connect the HART communicator in parallel to the instrument signal loop:
- Use the clip leads or HART test jacks on the transmitter terminal block.
- Ensure the system impedance (≥ 230 ohms) is met (typically already present in DCS input cards).
- Start the HART communicator or software and begin the handshake process to detect the device.
B. In Bench Testing (Offline)
- Connect a power supply (typically 24 VDC) to the transmitter.
- Place a 250-ohm resistor in series with the loop.
- Connect the HART communicator across the resistor or in parallel to the instrument terminals.
- Start your communicator or software and initiate communication.
Best Practices for Reliable HART Communication
- Use shielded twisted-pair cable to reduce electrical noise.
- Maintain a loop impedance of at least 230 ohms.
- Ensure the device is properly grounded.
- Update device descriptor (DD) files in your HART communicator to support newer devices.
- Avoid connecting to active control loops during plant operation without proper planning
Troubleshooting HART Connectivity Issues
| Issue | Possible Cause | Solution |
|---|---|---|
| No communication | Insufficient loop resistance | Add or verify 250-ohm resistor |
| Device not recognized | Missing or outdated DD file | Update software/device descriptor |
| Communication dropout | Electrical noise or grounding issue | Use shielded cables and proper grounding |
| Cannot write configuration | Loop powered by isolated source | Ensure loop shares common ground |
HART Communication Modes
Point-to-Point Mode
- One master (communicator or DCS) communicates with one field device.
- Analog 4-20 mA used for control; digital signal used for diagnostics.
Multi-Drop Mode
- Multiple devices share the same pair of wires.
- Analog current fixed at 4 mA; digital signal carries process data.
Advantages of Using HART in Instrumentation
- Remote configuration and calibration
- Real-time diagnostics
- Reduced downtime through predictive maintenance
- Fewer field trips for technicians
- Supports legacy 4-20 mA infrastructure
Real-World Application Example
At a petrochemical plant, HART-enabled pressure transmitters were installed across several production units. By integrating HART communicators with the plant’s asset management system (AMS), maintenance teams could remotely monitor calibration drift, perform loop tests, and detect potential failures. This reduced site visits by 40% and improved plant availability.
Conclusion
Connecting HART communication to your instrumentation setup enhances both diagnostics and operational efficiency. By understanding the necessary hardware, wiring methods, and communication protocols, engineers and technicians can seamlessly integrate HART into both field and bench environments. Proper implementation ensures reliable data exchange, reduced maintenance costs, and smarter decision-making across industrial facilities.
Whether you’re troubleshooting an installed system or setting up new smart instruments, mastering HART communication is a valuable skill in today’s automation landscape.