Distech and Trend 4-20mA Sensor Setup on APEX UIO

This guide covers configuring 4-20mA analog inputs on Distech APEX UIO controllers, specifically addressing common issues with Trend T/PI sensors.

Understanding 4-20mA Configuration

4-20mA current loop sensors require specific configuration:

Parameter	Typical Setting
Input Type	4-20mA (Active)
Scaling	Linear
Low Range	4mA = minimum engineering value
High Range	20mA = maximum engineering value
Units	Engineering units (°F, PSI, etc.)

APEX UIO Input Configuration

Hardware Setup

Important: 4-20mA inputs on APEX UIO require proper jumper/DIP switch configuration:

1. Locate input configuration switches on the UIO module
2. Set to current mode (mA) not voltage mode
3. Verify loop power - Some sensors are 2-wire (loop-powered), others are 4-wire (externally powered)

Software Configuration in EC-Net

1. Navigate to the analog input point
2. Set Input Type to "4-20mA" or "Current"
3. Configure Scaling:
   • Min Raw Value: 4 (representing 4mA)
   • Max Raw Value: 20 (representing 20mA)
   • Min Engineering Value: (sensor minimum, e.g., 0°F)
   • Max Engineering Value: (sensor maximum, e.g., 100°F)
4. Set appropriate Engineering Units

Common Issues and Solutions

Issue 1: Reading Shows Zero or Negative

Cause: Input configured for thermistor instead of current

Solution:

• Verify input type is set to 4-20mA, not thermistor
• Check hardware jumper/switch position

Issue 2: Reading is Erratic or Out of Range

Cause: Missing or incorrect loop power

Solution for 2-wire sensors:

• Verify 24VDC loop power is provided
• Check polarity (+ and - correct)
• Measure loop current with meter in series

Solution for 4-wire sensors:

• Verify external power supply to sensor
• Signal wires connect to input terminals only

Issue 3: Value Stuck at Minimum or Maximum

Cause: Open or shorted wiring

Diagnosis:

• Open loop reads ~0mA (value at minimum)
• Shorted loop reads >20mA (value at maximum or error)

Solution:

• Check wiring continuity
• Verify sensor is operating (measure current at sensor)
• Look for damaged conductors

Issue 4: Incorrect Scaling/Offset

Cause: Sensor range doesn't match configuration

Example - Trend T/PI Sensor: The Trend T/PI (temperature/pressure indicator) may have different ranges depending on model:

Model	Range	4mA Value	20mA Value
T/PI Temp	0-100°F	0°F	100°F
T/PI Temp	-20-120°F	-20°F	120°F
T/PI Pressure	0-150 PSI	0 PSI	150 PSI

Solution:

• Find exact sensor model number
• Verify sensor range from datasheet
• Match scaling in controller configuration

Issue 5: Using Linear Mode Incorrectly

Correct linear scaling setup:

Raw Input (mA)    Scaled Output (°F)
4.0               32.0  (freezing)
12.0              72.0  (midpoint)
20.0              112.0 (max)

Configuration:

• Raw Min: 4
• Raw Max: 20
• Scaled Min: 32
• Scaled Max: 112

Verification Procedure

Step 1: Measure Actual Current

1. Disconnect signal wire at controller
2. Place ammeter in series
3. Read actual current (should be 4-20mA range)

Step 2: Compare to Displayed Value

1. Calculate expected value based on current and scaling
2. Compare to controller display
3. If mismatch, reconfigure scaling

Step 3: Signal Simulation

1. Use 4-20mA signal generator (or calibrator)
2. Apply known current values: 4mA, 12mA, 20mA
3. Verify controller reads expected engineering values

Wiring Best Practices

For reliable 4-20mA signals:

• Use shielded twisted pair cable
• Ground shield at one end only (controller end)
• Keep signal wiring away from power cables
• Maximum recommended distance: 1000ft (300m) for typical gauges

Troubleshooting Summary

Symptom	Likely Cause	Check
Reads zero	Wrong input type	Hardware switch, software config
Erratic	Wiring/noise	Shield grounding, cable routing
Stuck min	Open circuit	Wiring continuity
Stuck max	Short circuit	Wiring for shorts
Wrong value	Scaling mismatch	Sensor range vs config