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What Units Are Returned from C-Series Analog Modules When Using Fixed-Point Data?



Hardware: CompactRIO>>Analog Input Modules

Problem:
I am using Fixed-Point (FXP) Data in my FPGA project for my C-series analog modules. What units will be returned with this data?

Solution:
This KnowledgeBase relates the units returned when using FXP acquisition with different types of C-series modules.  If you are using LabVIEW 8.5 or later and NI RIO 2.4 or later, you will see calibrated FXP data used with I/O nodes in your FPGA VI. If you are using older versions of LabVIEW or NI RIO, you will see raw integers that need to be converted to either voltage, current, resistance or voltage/voltage using the Binary to Nominal VI.  

Thermocouple Input Module
The NI 9211 is an example of a cRIO thermocouple module. It will read calibrated FXP data from the I/O node in units of Volts. Those voltage readings can then be turned into temperature measurements using the reading from the CJC and knowledge of the thermocouple type. These calculations are shown in the module specific LabVIEW example project NI 9211 Getting Started.

RTD Input Module
The NI 9217 is an example of a cRIO RTD module. It will read calibrated FXP data from the I/O node in units of Ohms. Those resistance readings can then be turned into temperature measurements using a standard 100 Ohm RTD polynomial conversion function. These calculations are shown in the module specific LabVIEW example project NI 9217 Getting Started.



Strain Input Module
The NI 9237 is an example of a cRIO Bridge module. It will read calibrated FXP data from the I/O node in units of Volts/Volts (Channel Voltage/Excitation Voltage). Those voltage/voltage readings can then be turned into strain measurements using knowledge of the bridge configuration (full, half or quarter) and gauge factor. The White Paper: Measuring Strain with Strain Gauges shows the details of such calculations.

Current Input/Output Module
The NI 9203 is an example of a cRIO current input module. It will read calibrated FXP data from the I/O node in units of Amps.

Voltage Input/Output Module
The NI 9205 is an example of a cRIO voltage input module. It will read calibrated FXP data from the I/O node in units of Volts. Note that the 9234 is also an analog input module which incorporates software-selectable AC/DC coupling and IEPE signal conditioning for accelrometers and microphones. The 9234 will still read calibrated FXP data in units of Volts. The manufacturer of the sensor connected to the 9234 should document which voltage values correspond to values for acceleration.


Related Links:
Product Documentation: Preventing Software Compatibility Issues for CompactRIO Systems
White Paper: Measuring Strain with Strain Gauges
KnowledgeBase 4DPGKPKO: How Can I Transfer My Fixed-Point Data Using a FIFO in LabVIEW?
White Paper: Choosing an Accelerometer Measurement System

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Report Date: 03/24/2008
Last Updated: 09/20/2017
Document ID: 4JNCQO4H

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