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digital output NI 9474

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Hello,

 

I am trying to learn labVIEW DAQ, and right now I am trying to figure out how to use NI 9474 with labview. NI 9474 is a device with digital outputs. I have attached a (poorly drawn) schematic of how I have my real circuit wired. For some reason, the voltage across the resistor is 9v instead of whatever the digital output should be when i set daq assistant to be 1 sample on demand. When I change daq assistant to continuous samples, it reads 4.6v. So I am wondering what am I doing wrong and what should my digital output be?

 

I have attached my vi file, so can someone please help me figure out how to use NI 9474 with labview?

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@kenypatel wrote:

... 

 For some reason, the voltage across the resistor is 9v instead of whatever the digital output should be when i set daq assistant to be 1 sample on demand. When I change daq assistant to continuous samples, it reads 4.6v. So I am wondering what am I doing wrong and what should my digital output be?

 

I have attached my vi file, so can someone please help me figure out how to use NI 9474 with labview?


If not 9V, what did you expect the Digital Output to be? For a supply voltage of 9V, I expect the digital output when on will produce 9V. For your change to continuous samples, what input data are you sending to the express VI? 4.6 V sounds a lot like you've got a toggling state between 9V and 0V that looks like 4.5V when checked with a meter. The VI you posted is for the working Sample on Demand case. Can you post the non-working Express VI with the 4.6V behavior?

 

Also, if you really want to learn DAQmx, you should try and do this without the Express VI.

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You are supplying 9V, so you should have 9V going across the resistor.  I'm thinking what is happening with the Continuous Samples is that it is toggling between High and Low.  If you measured that with a DMM, you will get around 4.5V.  Measure with an oscilloscope to make sure what is happening here.


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I thought the digital output of the ni 9474 outputs a voltage of 5v. The 9v is just the power supply for the NI 9474, at least that is what I thought. 

 

I expected that when a digital output is on, it outputs a certain voltage. When the digital output is off, it does not output a voltage. The power supply has nothing to do with the digital output. The power supply just powers the device. That is how I thought digital outputs work?

 

It seems like from the feedback I am getting is that if I use a 9v power supply, then the digital output, outputs 9v. If I use a 3v power supply, then the digital output, outputs 3v? Is that correct?

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Also, when I changed it from on demand to continuous, which is when it would output as 4.6v, I could not figure out how to wire the vi. So, I don't have a vi file to share with that. But, All I did was change the assistant vi from on demand to continuous. And I could not figure out how to wire that new assistant vi.

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Then the lesson here is to read the manual.  You could also just look on the side of the module to see that the digital outputs are nothing more than relays to connect to the supply.


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The manual says that the digital outputs source voltage. I thought that sourcing means it sends voltage, whereas sinking is just meant to turn on and off relays.

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You need to re-read the manual If you think it is supposed to output 5 volts when you have 9 volts as the supply voltage.
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I just went through it again, it says 5, 12, 24 V sourcing output. And it says a continuout output of 1Amp. Maybe I just do not understand what the datasheet is telling me?

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Ah, I saw output characteristics at the bottom and it says output voltage = vsup - (io*ro)

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