Ams trex™ Device Communicator

Power a transmitter
The Trex unit can power one transmitter. When the Trex unit powers a transmitter, an
internal 167 Ohm resistor is used, so an external resistor is not necessary. If you already
connected an external resistor in series with the red or black HART terminal on the Trex
unit, there will be less voltage available for the device, especially at 20 mA. This may cause
the device to power off. The Trex unit begins providing power when you tap 
OK
.
 WARNING
Do not insert a resistor in parallel with either the HART + pwr or HART terminals on the
Trex unit or in parallel with the device's communication terminals. This will interfere with
the current measurements.
Power a positioner
The Trex unit can power one positioner and set the current to 4 mA, 20 mA, or another
value between 3.5 and 22.5 mA. This allows you to set the position of the valve. The Trex
unit begins providing power and current when you tap OK.
User Guide 
Loop Diagnostics application
September 2021
User Guide 
131

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When the Trex unit provides power to a positioner, the 
Trex Unit Current
option is also
enabled. This allows you to control the current to the device by using the quick buttons,
the pencil icon, or the up/down arrow keys.
Simulate a transmitter with 4 mA
The Loop Diagnostics application lets you simulate a transmitter to verify an analog input
loop before installing a device. This option is useful to perform a loop check on an analog
input that will be wired to a 4-wire transmitter, or on an unpowered loop that has 2-wire
devices. The Trex unit provides power and 4 mA to the current loop.
Disable power to a device
To disable power, tap 
Trex Unit Power
to set it to 
OFF
. The Trex unit also stops providing
power if you tap 
Exit
or remove the lead set from the device or the Trex unit.
If the Trex unit is powering a positioner, it is recommended that you disable power by
tapping 
Trex Unit Power
. This allows you to move the positioner to a safe position without
causing any unexpected moves from the valve when the Trex unit stops providing power.
You can select 4 mA, 20 mA, or the present value. After the desired current level is
reached, the Trex unit stops providing power.
Note
The Loop Diagnostics application has a 
Duration of Current Change
option that lets you
set how quickly the device will reach the set current level. If the 
Duration of Current
Change
option is not set to zero, the current changes gradually to the selected current.
This is intended to protect large equipment from potential damage from abrupt
movement.