Error 200294 in Daqmxstart task

Question

Hi,
   I am trying to write and read data at the same time form a cdaq chassis 9172. I have 5 NI A/D cards and 2 NI D/A cards. I manage to code it in a way I can generate and acquire from the same chassis. Though I read data fine but when I am trying to write I get error 200294 at the daqmxstart task.vi. I always thought that I shouldnt start a task everytime I write data but I realize, the task wouldnt start unless i write data first. That is why my start task.vi is inside the loop which is now causing the errror. I am only assumming. can anyone help me code this in a better way to avoid that error?
First pic is how I initialize the channels and set sample clock
second pic is my producer loop, writting and reading.
The remaining part of the code works fine besides the writing data to daqmx write.
Solved!
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Attachments:
A2D and D2A pic.png ‏47 KB
producer loop.png ‏42 KB

Answer 1

My friend, you have a few problems here.
First of all. The reason you're getting the error at DAQmx Start is because you start the task with your first loop iteration...then you start it again on your next loop iteration, without stopping it in between.  It's already running, so you can't start it again.  Hence your problem, and why you need to take DAQmx Start out of the loop.  Now, for a Write operation, you need to preload the buffer, so that when you call DAQmx Write, there's something to write.  So!  Outside your loop, where you set up your AO task?  You'll need to generate some data to write, preload the buffer, then call DAQmx Write. After that, call DAQmx Start.  THEN go into your loop.
Another thing.  It looks as though you'd like to output new data via DAQmx Write with each loop iteration.  Assuming that's true, you need to configure your task to do that, using a DAQmx Write property node.  Set "Regeneration" to "Do Not Allow Regeneration".  This will allow you to output new data with each loop iteration.
That Dequeue function you have is set to time out after 10msec.  Suppose it does indeed time out.  You'll be trying to write an empty array of waveforms to your AO channels.  That would likely be a problem, so you need to handle that possibility.  Do it however you like, the easiest way is to put your previous valid data into a shift register, check your Dequeue function for timeout, and either use the newly dequeued data (if Dequeue did NOT time out) or use the previous valid data (if Dequeue DID time out).  When you get new, valid data from your Dequeue function, write it to the shift register.  That will ensure you're always writing valid data.
Your Wait function is redundant.  Your loop timing is already dictated by the Dequeue function and your DAQmx Write function.  Take the Wait function out.  It's only going to goof up your timing.
Also, you might not want your Read and Write functions in the same loop, since you're not doing anything to synchronize their timing.  You'll probably run into buffer overflow/underflow problems with the way you have things set up.  Do a forum search for synchronized AI-AO to find out how to synchronize your tasks.
Have a look.  Modify as needed, but this is how you should do a non-regenerative DAQmx Write.  Did you by chance research this particular error on the forums previous to posting?  You probably would have found at least one post very similar to this one.