Emerson AMS Trex User Manual page 139

Figure 4-4:
The Trex unit measures both voltage and current from the HART + pwr terminals. An internal 167 Ohm
resistor is used when the Trex unit powers the device.
CAUTION!
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 and may prevent communication.
Figure 4-5:
A.
User Guide
Power a HART transmitter
Measure voltage on an externally-powered HART transmitter
Voltage source
Loop Diagnostics application
131