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Using a 4-20mA CO2 sensor with a Xenta 102-AX

Issue

Configuring the Xenta 102-AX to accept a 4-20mA CO₂ sensor input on Universal Input 3. A negative value is required on the CO₂ offset value, but it is not possible to enter a negative value.

Product Line

TAC Vista, EcoStruxure Building Operation

Environment

  • Xenta 102-AX
  • 4-20mA output CO₂ sensor

Cause

The Xenta 102-AX universal inputs are designed to work with a 0-5V input. The best option is to always buy a CO₂ sensor that is compatible. However, if 4-20mA CO₂ sensors have already been purchased and installed, they can be made to work.

Resolution

In this example, a 4-20mA CO₂ sensor with a 0-2000ppm range is assumed.

  1. In the Xenta 102-AX Plug-In > Hardware Configuration Tab, set Input 3 Selection to CO₂ Sensor
  2. Wire a 250Ω resistor across U3 and M. This converts the 4-20mA signal to 1-5V.
  3. In Vista Workstation, navigate to the 102-AX > UnivIn3_Sense.
  4. Open SCPToffsetCO2. This value needs to be -500, but negative values cannot be entered. To get around this:
    1. Change Value to 0
    2. Change Scaling > Offset to 500. This alters the display of the offset value. Click OK.
    3. Open SCPToffsetCO2 a second time. The value is still 0, but it is being displayed as 500, due to the scaling offset.
    4. Change the value to 0 (because of the offset, 0 represents -500).
    5. Click OK
  5. Open SCPTgain.
    1. Set multiplier to 2444
    2. Set divisor to 1000
    3. Click OK. This value comes from 2000 (range of sensor) + 500 (offset) / 1023

Note: An input of 0V (a sensor that has not been wired yet) will result in an nvoCO2sensor value of 65535 (invalid).

Also see Using a 0-10 volt CO2 sensor with a Xenta 102-AX to configure the Xenta 102-AX to accept a 0-10 volt CO₂ sensor input on Universal Input 3.

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