Emerson AMS Trex Device Communicator User Manual page 81

User Guide
3.11
User Guide
1.800.561.8187
Internal resistors
The Trex unit has internal resistors that can be used when the Trex unit powers a HART
transmitter, or when the HART transmitter is on a 4-20 mA current loop without a resistor.
The resistors are enabled through the device connection wizard in the Field
Communicator application.
Note
For positioners, the internal resistors in the Trex unit are disabled.
Available resistors
Resistor Use
167 Ohm Used when the Trex unit powers a HART transmitter. This is the only resistor
that is available when the Trex unit powers a transmitter.
The 167 Ohm resistor is available due to Intrinsic Safety limitations when
the Trex unit powers a device. If the current is high (for example 22.5 mA),
the voltage drops to a level that may not be enough to power the device.
The use of the lower valued resistor can reduce the amount of the voltage
drop. The HART signal is lower, but the shout/deaf mode helps increase the
HART signal amplitude for devices that do not meet the HART
specifications.
250 Ohm Used when you select to increase loop resistance in the device connection
wizard when connecting to a device on an externally-powered 4-20 mA
current loop.
500 Ohm Used when the Trex unit detects the current is too high for the 250 Ohm
resistor on an externally-powered 4-20 mA current loop. If the current
continues to be too high after adding this resistor, the Trex unit displays an
error message indicating the detected current exceeds the capacity of the
resistor. The 500 Ohm resistor is used for externally-powered current loops.
Connection to a transmitter powered by the Trex unit
To enable the 167 Ohm resistor, the Trex unit must be connected to the device using the
HART + pwr terminals.
Figure 3-24: Connection for using the Trex internal resistor when the Trex unit
powers the transmitter
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September 2021
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