Enable or disable the internal resistors

If the Trex unit is powers the transmitter, there is no prompt/option to enable or disable the internal 167 Ohm resistor. The Trex unit automatically applies the resistor when you enable power to a transmitter from the device connection wizard.

If the Trex unit is connected to an externally-powered HART transmitter, the device connection wizard prompts you if you want to increase loop resistance. The Trex unit then selects the 250 Ohm or 500 Ohm resistor; you cannot select a specific value. The selected resistor value is not displayed on the screen.

  1. If you want the Trex unit to power a 2-wire HART transmitter, do the following:
    1. Connect the lead set to the HART + pwr terminals on the Trex unit and to the communication terminals on the device.

      Connection for providing power to a 2-wire HART transmitter

    2. In the Field Communicator application, tap HART.
    3. Tap Yes when asked if you intend to provide power.
    4. Tap Transmitter. The Trex unit applies the internal resistor.
    5. Follow any additional prompts to connect to the device.
  2. If the Trex unit is connected to an externally-powered 2-wire HART transmitter, do the following:
    1. Connect the lead set to the HART terminals on the Trex unit and to the communication terminals on the device in-series. (If you are connecting to a 4-wire transmitter, connect the Trex unit in parallel.)

      Connection for an externally-powered 2-wire transmitter

      1. Voltage source
    2. Tap HART.
    3. Tap No when asked if you intend to provide power.
    4. Tap Transmitter.
    5. Tap No when asked if you want to change polling. (This assumes the device is at address zero.)
    6. Tap Yes when asked if you want to increase loop resistance.
    7. Tap Next.
      A message is displayed to indicate the internal resistor is being enabled.
  3. To disable the internal resistor, disconnect the device from the Trex unit, or close the Field Communicator application.