Why do industrial sites need the HART protocol?
Although traditional 4-20mA analog signals can transmit over long distances (up to 1000 m), they can only convey a single process variable and cannot remotely read instrument configuration or perform calibration. When instruments are widely distributed, going on site for calibration is very troublesome. The HART protocol superimposes digital communication on the 4-20mA analog signal, solving this pain point.
Through HART, engineers can remotely read instrument information, modify parameters, and perform zero and specified-point calibration in the control room, greatly improving maintenance efficiency.
How does the HART protocol achieve remote communication?
Basic wiring and resistor requirements
HART communication requires a resistor of 250Ω or more (usually about 230–600Ω) to be connected in series on the PLC side, and then a HART terminal (such as a hand-held communicator or commissioning software) is connected. This enables digital communication without interrupting the analog signal.
- Resistance range: 230–600Ω, 250Ω recommended
- Communication distance: Same as the 4-20mA signal, up to 1000 m
- Terminal devices: HART hand-held communicator, HART module of DCS/PLC
Comparison with traditional serial communication
| Communication method | Transmission distance | Typical application | Commonly used? |
|---|---|---|---|
| HART | ≤1000m | Remote calibration of smart instruments | Very commonly used |
| RS-485 serial port | ≤1200m | DCS systems | Less commonly used |
| Pulse output | Shorter | Flow totalization | Not commonly used |
Traditional serial signals are limited by distance and are generally only used in DCS mode. HART is compatible with existing analog wiring and is a more economical choice.
Which E+H instrument devices require the HART protocol?
E+H (Endress+Hauser) devices basically all support the HART protocol, including but not limited to the following types:
- Pressure transmitters: such as the Cerabar series, used for remote zero calibration and range adjustment.
- Flowmeters: such as Promag electromagnetic flowmeters and Promass Coriolis mass flowmeters, supporting HART to read parameters such as flow rate and total.
- Level meters: such as Levelflex guided wave radar and Micropilot radar, performing echo curve diagnostics via HART.
- Temperature transmitters: such as the iTEMP series, supporting HART configuration and sensor fine-tuning.
- Smart valve positioners: some models support HART communication to achieve valve stroke calibration and diagnostics.
Therefore, when selecting models, if remote commissioning or integration into a DCS is desired, it is recommended to prioritize the HART protocol rather than pulse output.
Selection recommendations and precautions
When to choose HART?
- Instruments are widely distributed and on-site calibration costs are high.
- Diagnostic information beyond process variables needs to be read remotely.
- The existing system is 4-20mA and you want to upgrade to smart communication.
Precautions
- Ensure the loop resistance meets HART communication requirements (≥230Ω).
- The power supply and cable shielding must comply with the HART specification.
- If using a DCS, confirm that its HART module supports the required functions.
In short, the vast majority of E+H smart instruments support the HART protocol, and you only need to select it according to project requirements during model selection. For occasions requiring remote maintenance, HART is currently the most mature and most economical choice.