Sending Desired Signal to DAQ Device with Data Acquisition Toolbox

Hey everyone, I am working on controlling current sent to a pair of Helmholtz coils through Matlab with Data Acquisition Toolbox. Looking at related documentation, I was able to generate code that would queue desired signal to NI DAQ device and sustain it but what I want to do is the flexibility of changing the current I send to the device whenever desired. When the data queued with queueOutputData() is finished, the program seems to queue back whatever is in the listener. I actually want it to continue with whatever I queued until I change it and if I change it I want to continue with that. Do I have to update the listener and if so how can I do that? Also, I understand that when I queue a certain signal, I can't cancel it and replace with another signal before it's processed. Is there any way I can overcome it? Below I share the part of the code that generates desired signal, initiates a listener and queues some output data.

s=daq.createSession('ni');
out1=addAnalogOutputChannel(s,'Dev1',0,'Voltage');
s.Rate = 20000; %Device initialization

f=5; 
w=2*pi*f;
out1_amp=0.8;
out2_amp=out1_amp;
out1_signal=out1_amp*sin(linspace(0,2*pi*f,s.Rate+1)');
out1_signal(end)=[]; %Signal generation

lh=addlistener(s,'DataRequired', @(src, event) src.queueOutputData([out1_signal]));

s.IsContinuous=true;
queueOutputData(s,repmat([out1_signal],5,1));

s.startBackground();

[Walter Roberson]

https://www.mathworks.com/help/instrument/flushoutput.html

Removing queued data requires the Instrument Control toolbox instead of the daq toolbox.

So the trick for daq is to only queue as much data as the maximum you are willing to allow to continue to go through after you determine that you need to change.

[Hakan Caldag]

Thank you for your comment. If I flush the output and queue another signal afterwards, will the listener act and queue the old signal before I queue the new one?

[Walter Roberson]

You cannot flush the output because you are not using the instrument control toolbox. The routine I posted about is the only output routine that allows flushing, but it is for a different toolbox than you are using.

I suggest that you use something like nested functions with shared variables in order to provide a buffer whose contents are to be sent upon every invocation of the callback. If you do not have any updates, do not change the shared variable so it will just end up queuing the same data again. If you do have an update, change the shared variable and the new value will be picked up the next time the callback fires to queue data.

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

To avoid there being too great of a lag between the time you decide to make the change and the time the new data is sent out, do not use a large buffer. But do not use too small of a buffer either, since you will get a callback every time the buffer empties and callbacks are not "free"

[Dora Mahecic]

Hi, I am trying to do something similar. I managed to write a callback function which changes the data sent to the queue whenever the waveform properties are changed. However, it seems that the queue somehow remembers the previous data as well, since instead of switching from the first waveform to the second, then third (1 1 1 1 2 2 2 3 3 3 3 ), it starts iterating between the three (1 1 1 1 2 1 2 1 2 3 1 2 3 1 2 3 1 2 3). Why does the queue contain the old data too? Any suggestion on how to solve this would be appreciated, thanks

[Walter Roberson]

Please post your code, Dora.

[Hakan Caldag]

I was able to solve this problem somehow but I cannot guarantee that it works at all times and I am not sure which one of the things below made it work. And since I do not have access to the code right now, I may be missing a few things.

-Firstly, instead of using "addlistener" function, I use "listener" function to define my listener.

- I stop() and release() whenever I want to update my signal.

- After this, I remember redefining the listener based on the new signal but I am not sure whether I delete() the listener and then redefine with listener() function or overwrite it without deleting.

I hope these help.