time lag in plotting and acquiring data with Instrumentation toolbox

I'm using the instrumentation toolbox to acquire data from a NI USB 6366. I want to periodically plot the output to screen so I can verify it is good. To do this, I have to stop the acquisition.. By the time I can plot the data and restart the session, there is data that is missed creating a discontinuity. Futher, I try to log the data to disk, it also misses a huge chunk of data.

Ive attached a screen shot of a 3Hz sinewave showing how it is hacked up by this process.

In NI labview, the data is logged continuously while it is displayed. I don't see anything in the demos on how to write a practical program with this toolbox, some help is really appreciated.

I need to keep the sampling rate at about 10k and record for a few minutes.. I dont necessarily need to log data to disk but want to atleast plot the data now and then.

program.

daq.getDevices;
s = daq.createSession('ni');
s.Rate = 10000;   %sample rate
s.DurationInSeconds = .5   %time length of acquisition
duration = 300
display('acquiring data')
[ch, idx] = addAnalogInputChannel(s,'Dev1',[0, 1],'Voltage');   %addAnalogInputChannel(s,deviceID,channelID,measurementType)%turning on all 8 channels for now
data = []
divsor = 3;
figure,

for n = 1:1:duration
newdata = s.startForeground();
newdata(:,1) = 100*newdata(:, 1);  %CURRENT PROBE 0.01 volts per amp*
newdata(:,2) = 1*newdata(:, 2);   %
data =  [data; newdata];
plot (data)
time = 1/s.Rate:1/s.Rate: s.DurationInSeconds;
plot(data)
xlim([0 divsor*size(newdata ,1)])
legend('Current', 'Volts')

if mod(n,divsor) == 0 
     display (['Seconds' , num2str(s.DurationInSeconds*n)])  %progress
     dlmwrite (filename,data, '-append');
     data = data(divsor*size(newdata,1):  size(data,1), :);
end
end

Answer 1

[Walter Roberson]

Instead of executing in the foreground, execute in the background after having created a DataAvailable listener;

lh = addlistener(session,'DataAvailable',callback);

http://www.mathworks.com/help/daq/ref/dataavailable.html

You might want to alter the notification threshold; see http://www.mathworks.com/help/daq/ref/notifywhendataavailableexceeds.html

[jim private]

hi walter.

thanks for the help.. I think your right, this is the right way to do it. However, I still cant make it work right. I want to have it both plot the data and also save the data to disk. What you are slowing will keep plotting it to disk. How does the data ever get saved to disk? I tried making a separate function called getAIdata.. really just hung up on this and need an example

Also would like to write the data to disk periodically in a while loop so it wont fill up memory. Any ideas on examples or how to do this?

daq.getDevices;

s = daq.createSession('ni');

s.Rate = 10000;   %sample rate

s.DurationInSeconds = 1   %time length of acquisition

duration = 300

display('acquiring data')

   [ch, idx] = addAnalogInputChannel(s,'Dev1',[0, 1],'Voltage');  
   %addAnalogInputChannel(s,deviceID,channelID,measurementType)
   %turning on all 8 channels for now

   ai_data=[];

lh = addlistener(s,'DataAvailable', @plotData);

lh2 = s.addlistener('DataAvailable', @getaiData);  %listener handle

for n = 1:1:duration

end

----- ----

function [ai_data]=getaiData(src,event)

global ai_data

       ai_data=[ai_data;event.Data];

end

Answer 2

[Vinod]

Take a look at this example

http://www.mathworks.com/help/daq/examples/software-analog-triggered-data-capture.html

You can modify it to plot and write data to the disk. You won't be using the software defined triggers, but this should help you get started writing your code.