• Simplify the problem by removing one or more components from the HART loop.
Attempt tasks from the "Checks after installation" section.
• Isolate the device and perform tasks from "Checks before installation."
4.11.1 Measure voltage on an externally-powered loop
You can use the HART terminals on the Trex unit to measure the voltage on an externally-
powered device/loop.
Note
Use the HART + pwr terminals if you want the Trex unit to power the device and measure
voltage. See Power a 2-wire transmitter.
CAUTION
Do not connect lead sets to the HART and HART + pwr terminals at the same time.
Procedure
1. Connect the lead set to the HART terminals on the Trex unit and to the externally-
powered device.
2. View the Measured Voltage section on the Loop Diagnostics screen.
4.11.2 Measure the control system output
Measure loop current to ensure a device or distributed control system is sending the
correct value. You can measure loop current from 0 to 25 mA on a 4-20 mA current loop.
Note
Measurements are for reference purposes only. Do not make critical process-control
decisions based on these measurements.
WARNING
Do not connect the mA terminals on the Trex unit to a power supply that is not current
limited to 250 mA. This can blow a fuse inside the Trex unit. The repair/replacement will
need to be completed at an authorized service center.
WARNING
Do not connect the mA terminals (ammeter) in parallel with a powered 4-20 mA current
loop. Ammeters have low resistance. This can disrupt the loop and cause devices to report
incorrect values or positioners to move unexpectedly.
Procedure
1. Connect the lead set to the mA terminals on the Trex unit and to a connection point
on the loop.
The Trex unit can be connected at various point along the loop to measure current.
Loop Diagnostics application User Guide
September 2021
148 User Guide
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