Converting NI 9214 Data (FPGA Interface)

NI CompactRIO Device Drivers Help

Edition Date: December 2017

Part Number: 373197L-01

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Set the Calibration Mode to Calibrated in the C Series Module Properties dialog box for the NI 9214 if you want the FPGA I/O Node to return calibrated, fixed-point data from the module in units of volts. If you set the Calibration Mode to Calibrated, you must convert the CJC data from voltage to temperature. Set the Calibration Mode to Raw if you want the FPGA I/O Node to return calibrated, binary data from the module. If you set the Calibration Mode to Raw, you must convert the binary thermocouple and CJC values to voltage and then convert the CJC data from voltage to temperature. You must convert these values in the host VI.

Using a VI to Convert Data to Temperature

Refer to the NI 9214 Convert to Temperature polymorphic VI in the labview\examples\CompactRIO\Module Specific\NI 9214\NI 9214 Getting Started\NI 9214 Getting Started.lvproj for an example of converting calibrated or raw data to temperature. You can use the NI 9214 Convert to Temperature VI as a subVI in the host VI.

Using an Equation to Convert Binary Values to Voltage

You can use the following equation in the host VI to convert the binary thermocouple and CJC values to voltage:

Voltage = Binary Value × 78.125 mV ÷ 8,388,607

where Binary Value is the value returned by the FPGA I/O Node.

Converting CJC Data from Voltage to Temperature

The NI 9214 Convert Thermistor Reading VI is a subVI in the NI 9214 Convert to Temperature polymorphic VI that converts CJC data from voltage to temperature.

The VI uses the following equation to calculate the resistance of the thermistor:

RT = [(CJC Data ÷ 0.078125) ÷ (1 – (CJC Data ÷ 0.078125))] × 20,000

Using the resistance of the thermistor, the VI references a look-up table to interpolate the CJC temperature.

Refer to the National Institute of Standards and Technology (NIST) Monograph 175 thermocouple reference tables for more information about converting and adjusting thermocouple values.

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