6.2 HART Protocol & Smart Transmitters

HART Protocol & Smart Transmitters

1. Microprocessor-Based Smart Transmitters

Traditional analog transmitters utilized purely analog electronics (operational amplifiers, potentiometers, and force-balance mechanisms) to condition sensor signals. Modern smart transmitters are microprocessor-based digital instruments that bridge the gap between traditional 4-20 mA analog signaling and digital fieldbus networks.

Internal Hardware Architecture

1. Primary Sensor Element: Piezoresistive silicon diaphragm, differential capacitance cell, or resonant quartz crystal.
2. Analog-to-Digital Converter (ADC): High-resolution (typically 16-bit to 24-bit Sigma-Delta) ADC converts raw analog sensor signals (millivolts or capacitance) into digital values.
3. Microprocessor & Memory: Executes non-linear digital characterization, factory sensor temperature compensation math, damping filters, unit conversions, and diagnostics stored in non-volatile EEPROM.
4. Digital-to-Analog Converter (DAC): Converts compensated digital process values into a precision 4-20 mA DC loop current regulation signal.
5. HART FSK Modem: Superimposes high-frequency digital communication signals onto the analog output lines and demodulates incoming digital commands.

[Sensor] -> [ADC] -> [Microprocessor] -> [DAC] -> [Current Regulator] -> (4-20mA Loop)
                           ^                               |
                           +-------- [HART Modem] <--------+

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2. Multi-Variable Digital Capabilities (PV, SV, TV, QV)

A major advantage of smart transmitters is their ability to monitor multiple process parameters over a single physical wire pair using HART digital queries. HART defines four standard dynamic variables:

1. Primary Variable (PV): The primary process measurement mapped directly to the 4-20 mA analog current loop output.
2. Secondary Variable (SV): Secondary process parameter measured by the instrument.
3. Tertiary Variable (TV): Auxiliary process or sensor parameter.
4. Quaternary Variable (QV): Diagnostic, internal electronics, or ambient temperature parameter.

Practical Application Example: Multivariable Mass Flowmeter

Consider a smart differential pressure multivariable transmitter installed across an orifice plate in a steam line:

• PV (Primary Variable): Calculated Mass Flow Rate (e.g., 0-50,000 lb/hr), mapped to the 4-20 mA analog output.
• SV (Secondary Variable): Raw Differential Pressure (e.g., 0-250 inH2O), accessible via HART digital query.
• TV (Tertiary Variable): Line Static Pressure (e.g., 0-600 psig), accessible via HART digital query.
• QV (Quaternary Variable): Process Temperature measured via integrated RTD (e.g., 500°F), used internally for real-time steam density compensation.

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3. HART Bell 202 FSK Modulation Physics

The HART (Highway Addressable Remote Transducer) protocol uses Frequency Shift Keying (FSK) based on the Bell 202 telecom standard to superimpose digital communications directly on top of the 4-20 mA DC analog current loop.

FSK Frequency & Waveform Parameters

• Logic '1' (Mark): Represented by a 1200 Hz sine wave pulse.
• Logic '0' (Space): Represented by a 2200 Hz sine wave pulse.
• Signal Amplitude: Approximately $0.5\text{ mA}_{p-p}$ (about $\pm 0.25\text{ mA}$ about the DC baseline) for conventional HART FSK.
• Baud Rate: 1200 bits per second (bps).

  HART Bell 202 FSK Modulation:
  Logic '1' = 1200 Hz  |  Logic '0' = 2200 Hz
  +0.25 mA --+   +--      +--+  +--+
   0.00 mA   |---|   |----|  |--|  |-- (DC Baseline: 4 to 20 mA)
  -0.25 mA ------+          +----+  +--

Why HART Does Not Distort the Analog Reading

The FSK sine wave is perfectly symmetrical above and below the DC baseline current. When integrated over time by slow-responding analog control loops, low-pass PLC input filters, or mechanical meter movements, the net DC offset introduced by the AC sine wave is exactly zero milliamps ($0.0\text{ mA}$):

$\Delta I_{\text{DC\_avg}} = \frac{1}{T} \int_{0}^{T} A \sin(2\pi f t) \, dt = 0.000\text{ mA}$

This mathematical property allows continuous analog control to proceed without interruption while digital configuration and diagnostics occur simultaneously.

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4. HART Loop Impedance and Compliance

HART communication requires enough AC impedance for a receivable FSK voltage while the DC loop still has enough compliance for its maximum current. A 250-ohm load is common, and many conventional installations use a range near 230-1100 ohms, but the connected device, barrier, host, and wiring specifications govern the permitted network.

Derivation of 250 Ohm Requirement

HART handheld communicators and host modems are voltage-sensing devices. They decode digital signals by measuring the AC voltage signal ($V_{\text{FSK}}$) developed across the loop resistance by the transmitter's $0.5\text{ mA}_{p-p}$ FSK current modulation:

$V_{\text{FSK\_p-p}} = I_{\text{FSK\_p-p}} \times R_{\text{loop}}$ $V_{\text{FSK\_p-p}} = 0.0005\text{ A}_{p-p} \times 250\ \Omega = 0.125\text{ V}_{p-p} = 125\text{ mV}_{p-p}$

This example develops $125\text{ mV}_{p-p}$ across 250 ohms. Compare measured signal and noise with the device/host specifications and approved HART troubleshooting guidance rather than treating this example as a universal receiver threshold.

• Low-impedance example (below about $230\ \Omega$ in many conventional networks): For instance, if a technician connects a handheld communicator directly across the output terminals of a regulated 24 VDC power supply (which has an internal AC impedance near $0.1\ \Omega$): $V_{\text{FSK}} = 0.0005\text{ A} \times 0.1\ \Omega = 0.05\text{ mV}_{p-p}$ The AC signal voltage collapses, preventing communication ("No Device Found" error).
• High-resistance example (above about $1100\ \Omega$ in many conventional networks): Compliance can become limiting; calculate actual terminal voltage at maximum required current.

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5. Handheld Communicator Connection Rules

A handheld communicator is connected in parallel at an approved access point on a loop with adequate AC impedance and DC compliance. A 250-ohm load is common; verify the loop and communicator documentation:

1. Valid Connection Point A: Directly across the transmitter's COMM terminals (or signal terminals), provided the loop contains a $250\ \Omega$ load resistor.
2. Valid Connection Point B: Directly across the $250\ \Omega$ precision sense resistor inside the control panel.
3. Power-Supply Terminals: Connecting across a low-impedance regulated supply usually provides too little HART signal voltage and may not be an approved hazardous-area hookup point; use the designated access point and procedure.

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6. Point-to-Point vs. Multidrop Mode

HART protocols operate in two distinct modes:

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7. Burst Mode & Device Descriptions (EDDL / FDI)

• Burst Mode: An optional mode in which a device publishes selected HART response data without an individual request for every update. The achievable update rate depends on protocol revision, command, device, and network traffic.
• Electronic Device Description Language (EDDL) & FDI: Vendor-independent files that define parameter menus, diagnostic methods, graphical interface windows, and calibration wizards for host communicators and Asset Management Systems (AMS).

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8. Remote Re-ranging vs. Physical Calibration (Trim Adjustments)

A major source of confusion on certification exams is the difference between re-ranging and trimming a smart transmitter.

[Physical Process Input] --(1) Sensor Trim--> [Digital PV Value] --(2) Rerange--> [DAC Trim] --(3)--> (4-20mA Output)

1. Remote Re-ranging (Software Configuration)

Re-ranging changes the lower range value ($LRV = 4\text{ mA}$) and upper range value ($URV = 20\text{ mA}$) stored in memory without applying physical pressure or temperature standards.

• Example: A 0-200 psi transmitter is software re-ranged to 0-100 psi. The physical sensor calibration remains unchanged; the microprocessor simply maps 0 psi to 4.0 mA and 100 psi to 20.0 mA.
• Reference Standard Required? NO.

2. Sensor Trim (Physical Calibration)

Sensor trim adjusts the internal digital reading produced by the Analog-to-Digital Converter (ADC) to match known physical reference standards applied to the sensor.

• Zero Trim / Lower Sensor Trim: Applying 0.000 psi from a deadweight tester and commanding the device to calibrate its digital zero baseline.
• Upper Sensor Trim: Applying 100.000 psi span reference pressure and commanding the device to calibrate its digital span baseline.
• Reference Standard Required? YES (Traceable physical calibration standard).

3. DAC Trim (Digital-to-Analog Converter Trim / Current Trim)

DAC trim adjusts the digital-to-analog output stage to correct for analog current loop amplifier drift.

• Procedure: The communicator commands the transmitter to output exactly 4.000 mA. The technician measures actual current with a calibrated DMM connected in series. If the DMM reads 4.024 mA, the technician enters "4.024" into the communicator, allowing the microprocessor to correct its DAC offset.
• Reference Standard Required? YES (Precision series mA meter).

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9. Worked Field Diagnostic Scenario

Problem: A Smart Differential Pressure Flow Transmitter is connected to a DCS. The DCS operator reports that flow is reading 36.0% ($9.76\text{ mA}$). However, when a technician connects a HART communicator to the transmitter terminals, the HART digital display shows PV = 50.0% ($12.00\text{ mA}$ equivalent).

Step-by-Step Diagnostic Analysis:

1. Establish a reference: Confirm with an approved process/calibration reference that 50.0% is the expected PV; a HART display alone does not prove sensor accuracy.
2. Compare paths: The HART digital PV is 50.0%, while a calibrated series-current measurement is $9.76\text{ mA}$. The DCS represents that measured current as 36.0%, so the discrepancy lies before the DCS scaling display.
3. Isolate the analog path: Compare the device's commanded/expected analog output with the measured loop current and check compliance, configuration, and forced-output status. If 12.00 mA is commanded but 9.76 mA is delivered, DAC trim or electronics is suspect.
4. Correct under procedure: Perform the approved DAC verification/trim with a calibrated meter. Replace or repair only if configuration, wiring, load, and trim checks support that disposition.