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Question about DAQmxCreateDOChan / DAQmxWriteDigitalLines (NI-USB 6501)


I'm creating an application in Java in which i call (trough JNI) the functions DAQmx C functions. I've been searching the forums and other documentation, but couldn't find an answer to my question.

I need to write an algorithm that (very simlply) puts an array of outputs to hi. However, i can't figure out how the DAQmxCreateDOChan and the DAQmxWriteDigitalLines functions work exactly. Let me explain myself through an example:

- phase1: i create a channel with the DAQmxCreateDOChan, specifying a list of lines (sparated by commas as stated in the C reference help). Lets say i put lines 0, 1, 2, and 4to HI. I guess the function call in this case will look llike this, assuming all operations are on dev0:

DAQmxCreateDOChan(taskHandle,"dev0/line0, dev0/line1, dev0/line2, dev0/line4",DAQmx_Val_ChanForAllLines));
I call the DAQmxWriteDigitalLines, and lines 0, 1, 2 and 4 are set to HI. No problems there, i guess.

- phase2: i create another digital channel with DAQmxCreateDOChan, specyfying another listof lines. Lets say lines 1, 2 and 4. Functon call will look like this then:

DAQmxCreateDOChan(taskHandle,"dev0/line1, dev0/line2, dev0/line4",DAQmx_Val_ChanForAllLines));
Again, I call the DAQmxWriteDigitalLines function.

But then my questions is: how is the DAQmxWriteDigitalLines function going to handle this ? 

- Will line1 be set to LOW?
- And will lines 1, 2 and 4 stay HI thoughout the whole process, without interuption ?

The reason why I ask is the following: in the above example, I can not (and this is very important) have an interuption at any given moment in the 'HI-state' of lines 1, 2 and 4. I absolutely have to make sure that, in between switching, there is no interuption in the HI state of lines 1, 2 and 4. If there is a small interuption, my program is worthless. 

Another questin: in my example: do i need to clear the task through the DAQmxClearTask functon? I assume i have to ?

Thanks in advance for your ideas and hints. These are my first steps in the NI-DAQmx libraries, so i'm still learning a lot. I'm sorry if some of my questions are too obvious and are  traceable in the documentation.

Kind regards,


Message Edited by SanderG on 03-25-2009 03:42 PM
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Hi Sander


When you are going to create a second task using some of the same lines, while another one is already running, you are most likely to receive an error, because the output lines are already reserved. So I would advise to simply create one task containing all the lines you will need to write to.


Also, when a state has been set on a certain line, it will stay in that state untill a new state has been set or you reset the line states. See the following article about this:


The clear task also would not be necessary to use troughout the program. It is best to just start your task once, and stop/clear it only at the end of your program, when all I/O operations are finished.


A nice and quick tutorial about the most needed DAQmx functions can be found here:


Best Regards

Applications Engineer
NI Belgium
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Hi Michiel,


Thanks for your answer, it certainly gave me a lot of ideas. It's good to know that there will not be an interruption when a line was already set to 1.


After reading the articles, i know what to do !


Many thanks,



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