Understanding the fixed-point values from an NI-9203

[Thoric]

I have a cRIO-9073 with an NI 9203 module. Within LabVIEW FPGA, values from the card are returned as fixed-point. The LabVIEW-chosen properties of the fixed-point are (+/-, 21, -4) which apparently provides a range of (-3.125E-2, 3.125E-2, 2.98E-8), and the card measures current in the range 4-20mA with a resolution of only 16-bits. My understanding of fixed-point values is only slight, so can anyone explain to me why a 21-bits value is required for 16-bits acquisition data?

[ThomasC · Accepted solution]

Hi Thoric,

Sorry it's taken a while to get a response to you.   The extra bits needed with fixed point representation are for the overflow functionality.  There is some more information in the link below.

http://zone.ni.com/reference/en-XX/help/371361E-01/lvconcepts/numeric_data/#FXP_overflow

I hope this answers your question but if you require any more help please let me know.

Regards,

Thomas Clark

[Barbu]

Hello,

What does the FXP value of a NI-9203 chanel in 4-20mA stand for ? The actual value of the current (between 4 and 20 mA), or a scale factor (between 0 and 1)

I'm coding away from my lab, so I cannot setup my cRIO just to check that. I did use it few month ago to measure the output of a LEM current transducer and I kinda recall that the FXP value of the NI-9203 was not the actual current but some kind of scale factor.

If anybody knows... otherwise I'll find out tomorrow and keep you posted 🙂 

Etienne. 

[Barbu]

Hi,

Turns out I remembered wrong 🙂

The FXP value represent the value of the current and is between 0.004 and 0.02

Cheers