Only 1 VI can run at the same time?

Question

Hi there, this is for my school project which requires to acquire analog input from 2 sensor circuits(1 with LDR,1 with LM35 for temperature sensing) and generate an output so that a light bulb would light up if the voltage in the LDR circuit is too low and a fan would be turned on if temperature is over a certain limit.
I managed to come up with the VIs and it worked independently but when i try to integrate it only the LDR circuit worked, and i could not troubleshoot it.
Attached is the 2 VIs that i come up with and another VI which i tried to integrate but failed to work properly.
Thanks
Solved!
Go to Solution.
Attachments:
VaryingFan.vi ‏40 KB
VaryingLight.vi ‏35 KB
2VIs.vi ‏58 KB

Answer 1

You need to create a DAQmx task to be able to read two voltages from two AIs on the same device.  Same for writing two AOs on the same device.  Here is an example of how to create a DAQmx task.  Create one task for AI and another for AO.  You must set the AI Read and AO Write to Multiple Channels - Single Sample - 1D DBL.  The first element in the 1D array will correspond to the first element in the List of AI Channels array shown in my example:
Here is another tip.  On the loop borders, change the DAQmx task/channel in/out and the error in/out terminals to shift registers.  Using shift registers here is good practice recommended by NI.  The reason being is this.  If there happens to be an error in one loop iteration, the error gets lost with regular terminals because the input terminal will have no error condition.  On the next iteration, the error from the previous iteration is lost.  For the task in/out terminal, if the task changes, you will lose the change.  Shift registers will propogate all errors and changes to the next loop iteration.  Another reason is if the loop runs 0 times (doesn't run at all).  The outputs will be default values.  So if there was an error coming into the loop, it will be lost if the loop does not iterate at least one time.  So use shift registers here.
One more thing, change your numeric type to DBL inside your case structures.  Notice the coercion dot (small red dot) on the input to the DAQmx Write funtion.  It is expecting a DBL data type.
OK, just one more.  Your output from AI Read is a DBL data type.  You can't have a DBL input to a case selector.  Notice again the red dot on the case selector question mark.  Labview is converting the DBl to an integer (probably an I32).  You may have rounding off errors.  You should round off and change to an integer before the question mark so you know exactly what the outcome will be.  What if the AI Read outputs 0.5 votls?  Will the 0 case be executed, or will it be the 1,2 case?  What if the voltage is 2.5?  Which case will be exectuted?  You should fix this.
I think that is enough for now. 
- tbob
Inventor of the WORM Global