Back

HART

Loading views...
← PreviousNext →

HART (Highway Addressable Remote Transducer)

Overview

HART is a hybrid analog/digital protocol overlaying FSK digital signals on the 4–20 mA analog current loop. It allows digital configuration, diagnostics, and multi-variable measurement on the same wiring as legacy 4–20 mA instruments.

1. Theory & Fundamentals

  • Physical: 4–20 mA analog loop + FSK digital (Bell 202 modem: 1200 Hz=1, 2200 Hz=0)
  • Simultaneous: Analog carries primary variable; digital adds additional data
  • Master-slave: Host (DCS/handheld) initiates; field device responds
  • Multi-drop: Up to 15 devices at fixed 4 mA current; no analog signal
  • Addressing: Short address 0–15; long address (38-bit unique ID)
  • Commands: Universal (0–30), Common Practice (32–127), Device-specific (128–253)
  • WirelessHART: HART over wireless mesh (IEEE 802.15.4)

2. Frame / Packet Structure

HART Frame:
  Preamble (≥5 bytes of 0xFF) | Start Byte | Address(1 or 5B) | Command(1B) | Byte Count | Data | Checksum

Start byte:
  0x02 = STX (master→slave, short addr)
  0x82 = STX (master→slave, long addr)
  0x06 = ACK (slave→master, short)
  0x86 = ACK (slave→master, long)

Command 0: Read Unique Identifier
  Response: 254 | Mfr code | Device type | Req preambles | HART revision | SW revision | HW revision | Flags | Device ID(3B)

Command 3: Read Dynamic Variables
  Response: 254 | PV units | PV | SV units | SV | TV units | TV | QV units | QV

3. Protocol Mechanics

  • Collision avoidance: Master waits for line quiet before transmitting
  • Preamble: synchronizes receiver clock; minimum 5 bytes
  • Checksum: XOR of all bytes from address through data
  • Response time: typically 250–300 ms per transaction
  • Burst mode: field device sends periodic responses without master polling

4. Hardware Implementation

  • HART modem: AD5700, HCF_TOOL-46 (reference design)
  • Coupling: 470 µH inductor in series with 4–20 mA loop; HART signal couples across resistor
  • Receive: bandpass filter (1200/2200 Hz), FSK demodulator
  • Transmit: FSK modulator, current limited to ±0.5 mA superimposed
  • MCU UART at 1200 baud; UART RX → demodulator output; UART TX → FSK modulator

5. Register-Level / Configuration

// HART command 0 via UART + AD5700 modem
void HART_SendCmd0(void) {
    uint8_t frame[]={
        0xFF,0xFF,0xFF,0xFF,0xFF, // Preamble ×5
        0x02,                     // STX, short addr
        0x00,                     // Address 0 (polling)
        0x00,                     // Command 0
        0x00,                     // Byte count 0
        0x02                      // Checksum = 0x00^0x00^0x00^0x02
    };
    // Assert RTS to key AD5700 TX
    GPIO_Write(HART_RTS, 1);
    UART_Send(frame, sizeof(frame));
    while(!UART_TxComplete()); delay_us(300);
    GPIO_Write(HART_RTS, 0); // Release
    // Receive response
}

6. Driver / Software Development

  • Implement init, TX, RX functions; use interrupts or DMA
  • Handle errors: timeout, CRC mismatch, arbitration loss
  • Use circular/ring buffers for high-throughput RX
  • Add retry logic and watchdog for reliability
  • Separate hardware layer from protocol logic

7. Debugging & Testing

  • HART Communicator (Emerson 475): handheld HART tester
  • SDC-625 HART modem + PC with HART master software
  • Logic analyzer: decode Bell 202 FSK if supported; or decode via UART after demodulator
  • Common issues: incorrect preamble count; line loading (too much capacitance); address conflict

8. Real-World Applications

  1. Smart pressure transmitters (Emerson Rosemount)
  2. Flow meters (Coriolis, magnetic, vortex)
  3. Level transmitters and radar gauges
  4. Control valves with positioners
  5. Motor protection relays

9. Advanced Topics & Edge Cases

  • WirelessHART: HART over mesh network; same command set
  • HART 7: Latest revision; adds burst publishing, enhanced status
  • FDT/DTM: Universal device driver framework for HART devices
  • HART-IP: HART over TCP/IP for modern DCS integration
  • EDDL: Electronic Device Description Language for HART device descriptions

10. Standards & Variants

Feature HART PROFIBUS PA FF
Physical 4–20mA+FSK IEC 61158-2 IEC 61158-2
Speed 1.2 kbps 31.25 kbps 31.25 kbps
Legacy Yes (4–20mA) No No
Intrinsic safety Easy Yes Yes

💡 Practical Examples

Example 1

Command 0: read unique identifier → get device type, revision, ID

Example 2

Command 3: read 4 dynamic variables → PV=flow, SV=temperature, TV=density, QV=viscosity

Example 3

Command 35: write primary variable range limits (0–100% span calibration)

🧪 Practice Questions

Beginner

  1. What analog signal does HART ride on?
  2. What frequencies does HART FSK use?
  3. What is the maximum number of multi-drop devices?
  4. What is the HART response time?
  5. What is WirelessHART?

    6. Intermediate

  6. Implement HART command 0 framing and checksum.
  7. Decode command 3 response to extract 4 process variables.
  8. How do you calculate the HART checksum?
  9. Design the HART modem coupling circuit.
  10. Implement HART burst mode in slave firmware.

    11. Advanced

  11. Build a HART master reading all commands 0–7 from a field device.
  12. Implement a HART slave (simulated transmitter) responding to commands.
  13. Design WirelessHART gateway converting to Modbus TCP.
  14. Implement HART over IP (HART-IP) server.
  15. Add HART to an existing 4–20 mA sensor design.

    16. Hands-on Projects

  16. HART Reader: AD5700 + STM32, read PV from pressure transmitter.
  17. HART Slave: simulate a HART temperature transmitter responding to commands.
  18. HART to MQTT: gateway reading HART devices, publish via MQTT.

Checklist

  • [ ] Understand HART frame structure and checksum
  • [ ] Calculate Bell 202 FSK timing (1200/2200 Hz)
  • [ ] Design HART modem coupling circuit
  • [ ] Implement HART master sending command 0
  • [ ] Parse unique identifier response
  • [ ] Read process variables (command 3)
  • [ ] Handle multi-drop addressing
  • [ ] Implement burst mode receive
  • [ ] Debug with HART Communicator
  • [ ] Add HART to custom sensor design
← PreviousNext →