SLAAEH8 October   2024 AFE781H1 , AFE782H1 , AFE881H1 , AFE882H1 , DAC8740H , DAC8741H , DAC8742H

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 The 4-20mA Loop
    2. 1.2 The HART Protocol
      1. 1.2.1 Adding HART to the 4-20mA Loop
      2. 1.2.2 HART FSK
  5. 2AFE881H1 HART Modem
    1. 2.1 AFE881H1 HART Transmitter
    2. 2.2 Detailed Schematic
      1. 2.2.1 Input Protection
      2. 2.2.2 Startup Circuit
      3. 2.2.3 Voltage-to-Current Stage
      4. 2.2.4 Voltage-to-Current Calculation
      5. 2.2.5 HART Signal Transmission
      6. 2.2.6 HART Input Protection
      7. 2.2.7 Current Consumption
      8. 2.2.8 HART Transmitter Board
      9. 2.2.9 HART Protocol Stack
  6. 3HART Testing and Registration
    1. 3.1  HART History and the FieldComm Group
    2. 3.2  HART Testing Overview
      1. 3.2.1 HART Protocol Specifications
      2. 3.2.2 HART Protocol Test Specifications
      3. 3.2.3 Remote Transmitter Device Testing
    3. 3.3  HART Test Equipment
    4. 3.4  HART Physical Layer Testing
      1. 3.4.1 FSK Sinusoid Test
      2. 3.4.2 Carrier Start and Stop Time Tests
      3. 3.4.3 Carrier Start and Stop Transient Tests
      4. 3.4.4 Output Noise During Silence
      5. 3.4.5 Analog Rate of Change Test
      6. 3.4.6 Receive Impedance Test
      7. 3.4.7 Noise Sensitivity Test
      8. 3.4.8 Carrier Detect Test
    5. 3.5  Data Link Layer Tests
      1. 3.5.1 Data Link Layer Test Specifications
      2. 3.5.2 Data Link Layer Test Logs
    6. 3.6  Universal Command Tests
    7. 3.7  Common-Practice Command Tests
    8. 3.8  Device Specific Command Tests
    9. 3.9  HART Protocol Test Submission
    10. 3.10 HART Registration
  7. 4Other TI HART Modem Designs
  8. 5Summary
  9. 6Acknowledgments
  10. 7References

4 Other TI HART Modem Designs

There are a few TI designs that have been created to demonstrate HART communications. First, the DAC8740H device was the first of TI’s HART modem devices used in factory automation and control. The DAC8740H is one device of a family of HART modems detailed in Table 4-1.

Table 4-1 DAC874xH HART Modem Devices
DEVICE Communication
DAC8740H UART
DAC8741H SPI
DAC8742H UART/SPI

The DAC8740H has two associated designs that have HART implementations. First is the Highly-Accurate, Loop-Powered, 4mA to 20mA Field Transmitter With HART Modem Reference Design. This design was registered with FieldComm as a HART device. Figure 4-1 shows the board that was used for the TI design.

Board used for Highly-Accurate, Loop-Powered, 4mA to 20mA Field Transmitter With HART Modem Reference Design Figure 4-1 Board used for Highly-Accurate, Loop-Powered, 4mA to 20mA Field Transmitter With HART Modem Reference Design

The second design is HART-Enabled PLC Analog Input Module Reference Design. This design was not registered with FieldComm, but the design guide does have measurements and test procedures to show that the design passes the various HART tests.

Board used for HART-Enabled PLC Analog Input Module Reference Design Figure 4-2 Board used for HART-Enabled PLC Analog Input Module Reference Design

As shown previously in the transmitter design, the AFE881H1 is a HART modem with an integrated DAC. The AFE881H1 is part of a family of HART modem devices detailed in Table 4-2.

Table 4-2 AFE881H1 Family of HART Modem Devices
DEVICE DAC RESOLUTION DAC OUTPUT RANGE
AFE881H1 16-Bit 0.3V to 2.5V
AFE781H1 14-Bit 0.3V to 2.5V
AFE882H1 16-Bit 0V to 2.5V
AFE782H1 14-Bit 0V to 2.5V

Evaluation modules (EVMs) are available for the AFE881H1 and the AFE882H1. For the AFE881H1EVM, the board outputs the DAC voltage for evaluation. For the AFE882H1EVM which is a similar device but with a DAC output of 0V to 2.5V), the EVM uses an XTR305 to create a voltage or current output.

A front-end board EVM incorporating a V-to-I conversion stage can be released. These front-end boards operate as transmitter on a 4-20mA loop when a supply voltage is applied. Figure 4-3 shows an AFE881H1 modular front-end board.

AFE881H1 modular front-end board Figure 4-3 AFE881H1 modular front-end board

The connector on the right side of the board connects to the loop power, and a load resistor can be added to either side of the loop connection. The left side of the EVM is connected to a digital back-end board. For GUI control from a PC, the digital back end board has a USB connection. An FTDI device sends SPI or UART signals back and forth from the device to the PC.

Digital back-end boards have also been created with a launchpad connector. TI launchpad kits can be used to write firmware and control the device.

As a test of the front-end/back-end board ecosystem, these boards were used as a vehicle to run HART tests for the AFE882H1. The modular EVM for the AFE882H1 is shown in Figure 4-4.

AFE882H1 Front-End Board With Digital Back-end Board and MSP430FR5969 Launchpad as a HART Transmitter Figure 4-4 AFE882H1 Front-End Board With Digital Back-end Board and MSP430FR5969 Launchpad as a HART Transmitter

As before, the front-end board is connected to a power supply and acts as the controller for a 4-20mA loop. In this modular setup, an EXP-MSP430FR5969 launchpad board is programmed for the HART stack. The HART stack from the AFE881H1 transmitter design was reused for the AFE882H1. This launchpad is connected to a digital back-end (LP-DBE) interface board and connected to a front-end board which contains the AFE882H1 device. Similar to the AFE881H1 transmitter board, the loop power is supplied by a connector that connects to the right side of the board. HART tests were successfully run on this board, and FieldComm has also completed testing of this board. Using this setup, a AFE882H1 design was registered as a HART transmitter.