Specifications

SQ: 1-44
RS3: Service Quick Reference Guide
Troubleshooting
RTD and Thermocouple I/O Troubleshooting
Condition
Action
RTD and thermocouple
field problems.
For RTD and thermocouple field problems, the following
items should be checked carefully:
Loose or c orroded connections or FEM not
securely mounted in MUX. This can cause all
input points to read as negative numbers. For
FEM mounting instructions see SV: 5--3.
Reverse polarity
Wires shorted to case at temperature sensor
Wrong type of thermocouple wire
Noise c aused by close proximity to AC wires
High common node voltage. Measure from all
device l eads to chassis with a digital voltmeter
using both AC and DC meter scales.
Universal Voltage FEM
shows positive overload on
open circuit.
This appears when Universal Voltage FEM is connected
through a remote communication link and configured as a
thermocouple input. The system cannot differentiate
between a thermocouple with a high temperature reading
and one with a broken or open loop.
Check that the MUX CPU PROM is Doric
(Beckman) part number 278--7000--03B.
Communication failure to
MUX CPU and the
Coordinator Processor.
Ensure that TransZorbs CR17 through CR20 are bipolar
(PGKE47CA). The correct units have no polarity marking.
Low thermocouple reading
(or reading at low value) for
thermocouples attached to
Universal FEM.
This appears when MIB is configured for TC or mv input.
Replace the FEM.
Incorrect
RTD readings with two or
three-wire RTDs.
1. Check for correct wiring at FEM. See SV:5.
2. Check for correct wiring at RTD Marshaling Panel
and at FEM. See SV: 5.
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