Ams trex™ Device Communicator

Figure 3-25: Connection for using the Trex internal resistor for an externally-powered
2-wire transmitter
A. Voltage source
For 4-wire transmitters, the Trex unit must be connected in parallel. In the diagram below,
the Trex internal resistor is enabled in the HART terminals.
Figure 3-26: Connection for using the internal resistor for a 4-wire transmitter
A. Voltage source
Field Communicator application
User Guide
September 2021
82
User Guide

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3.11.1
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.
Procedure
1. If you want the Trex unit to power a 2-wire HART transmitter, do the following:
a) Connect the lead set to the HART + pwr terminals on the Trex unit and to the
communication terminals on the device.
Figure 3-27: Connection for providing power to a 2-wire HART
transmitter
b) In the Field Communicator application, tap 
HART
.
c) Tap 
Yes
when asked if you intend to provide power.
d) Tap 
Transmitter
. The Trex unit applies the internal resistor.
e) 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:
a) 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.)
User Guide 
Field Communicator application
September 2021
User Guide 
83

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