Converting NI 9211 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 9211 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 fixed-point CJC data to binary CJC data and then convert the binary CJC data 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 values to voltage and convert the binary CJC data to temperature. You must convert these values in the host VI.

Note Note  The equations below apply only to the NI 9211. If you are using the NI 9211E board-only module, refer to the KnowledgeBase for the correct equations.

Using a VI to Convert Data to Temperature

Refer to the Convert to Temperature (NI 9211) polymorphic VI in the labview\examples\CompactRIO\Module Specific\NI 9211\NI 9211 Support Files.llb for an example of converting calibrated or raw data to temperature. You can use the Convert to Temperature (NI 9211) VI as a subVI in the host VI.

Using an Equation to Convert Binary Thermocouple Values to Voltage

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

Voltage = Binary Value × 80 mV ÷ 8,388,607

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

Using an Equation to Convert Fixed-Point CJC Data to Binary CJC Data

You can use the following equation in the host VI to convert fixed-point CJC data to binary CJC data:

Binary CJC Data = (Fixed-Point CJC Data) ÷ (0.160 ÷ (224 – 1))

where Fixed-Point CJC Data is the value returned by the FPGA I/O Node.

Using Equations to Convert Binary CJC Data to Temperature

You can use the following equations in the host VI to convert binary CJC data to temperature:

Calculate the resistance of the thermistor:

RT = (10000 × Binary CJC Data) ÷ (223Binary CJC Data)

Calculate the CJC temperature:

T = [ 1 ÷ [A + B(ln(RT)) + C(ln(RT))3]] – (273.15 + OffsetConstant)

where T = temperature in °C
A = 1.2873851 × 10-3
B = 2.3575235 × 10-4
C = 9.4978060 × 10-8
RT = thermistor resistance reading
OffsetConstant* = 0.7

*The OffsetConstant is the typical temperature gradient between the CJC sensor and the thermocouple cold junction.

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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