Take Discrete DAQ sample while doing Continuous Sampling

Hello,

I am doing a background sampling of a DAQ USB6003 in Matlab. What I need to do is periodically pull the latest data point from the background sampling. However, when I try to do this using the "read" function during the background sessions, I get the error:

Error using roboAnterograde_speedStop (line 159)

NI Error -50103:

The specified resource is reserved. The operation could not be completed as specified.

Task Name: _unnamedTask<3FF>

Status Code: -50103

I also tried creating a second DAQ object with only the channel added that I need to continously check during the background sesions and it also doesn't work.

So, does anyone have a good way to pull the latest data points during the background sampling without stopping the sampling?

Hopefully this makes sense.

Thank you

Answer 1

[akshatsood]

Hi @Holden,

I see that you are performing a background sampling of a DAQ USB6003 in MATLAB and you need to periodically pull the latest data point from the background sampling.

The error you are encountering, "NI Error -50103", indicates that the resource is reserved, meaning that the DAQ device is already in use by another task. This is a common issue when attempting to access the same DAQ device from multiple tasks simultaneously.

Here are a few strategies you can try to resolve this issue and periodically access the latest data points:

Use a Listener to Access Data - MATLAB allows you to set up a listener that can be triggered whenever new data is available. This approach avoids directly accessing the DAQ device while it is in use.

s = daq.createSession('ni'); % Create a DAQ session

addAnalogInputChannel(s, 'Dev1', 'ai0', 'Voltage');

s.Rate = 1000; % 1000 Hz
s.DurationInSeconds = 10;

% Add a listener for DataAvailable event
lh = addlistener(s, 'DataAvailable', @(src, event) processData(event));

function processData(event)
    latestData = event.Data(end, :); % Get the latest data point
    disp(latestData); 
end

s.startBackground();

Use a Circular Buffer - Implement a circular buffer to store the latest data points and access them as needed. This approach can be combined with the listener method.

bufferSize = 1000;
circularBuffer = zeros(bufferSize, 1);
bufferIndex = 1;

function processData(event)
    global circularBuffer bufferIndex bufferSize;
    data = event.Data;
    numDataPoints = size(data, 1);
    
    for i = 1:numDataPoints
        circularBuffer(bufferIndex) = data(i);
        bufferIndex = mod(bufferIndex, bufferSize) + 1;
    end
    
    % Access the latest data point
    latestData = circularBuffer(mod(bufferIndex - 2, bufferSize) + 1);
    disp(latestData);
end

Please refer to the following MATLAB Answer for better understanding

https://www.mathworks.com/support/search.html/answers/6258-nidaq-continuous-and-background-acquisition.html?fq%5B%5D=asset_type_name:answer&fq%5B%5D=category:support/national-6856&page=1

I hope this helps.

Answer 2

[Holden]

Thanks for the info.

It seems like I should be able to get away with a simpler solution such as just calling the "read" function during the background acquisition. They do that in the example code on the documentation. In my code below, I first start the continuous acquisition, and then the underlined code lines have to do with the discrete calls. What is very odd is that by the end of the while loop, the variable 'cnt' which increases for every iteration of the while loop is at ~300, whereas the variable 'spot' only is at around 80. They should be the same number by the end. Is it because the discrete "read" command is slow and cannot keep up with the while loop perhaps?

% Start Continuous Data Sampling

start(d2,"Continuous"); % start analog channels

disp('Start');

sampling = true; % boolean that sampling is running

cnt = 1; % initialize for logging Zaber position

timerStart= tic; % start of timer for when position is logged

% Initialize for Movement

begin = 1;

moving = true;

% On the fly data

dataHold = [];

timestampsHold = [];

spot = 1;

% Forward Peck

while (moving == true)

posTime(cnt) = toc(timerStart);

pos(cnt) = axis.getPosition(Units.LENGTH_MILLIMETRES);

if (begin == 1) % if on first iteration, perform movement

axis.moveRelative(travelDistance, Units.LENGTH_MILLIMETRES, false, travelSpeed, Units.VELOCITY_MILLIMETRES_PER_SECOND);

end

check = axis.isBusy(); % check if device is moving

if (check == false)

moving = false; % end sampling

end

cnt = cnt + 1; % increase count

begin = begin + 1;

[dataTemp, timestampsTemp] = read(d2,1,OutputFormat="Matrix");

dataHold(spot) = dataTemp(2);

timestampsHold(spot) = timestampsTemp;

spot = spot + 1;

end

speedHold = dataHold(1,:)/5 * 220000; % convert to rpm

% Retrive Continuous Data

scanData = read(d2,"all"); % read data

Answer 3

[Holden]

For instance in the documentation for continuous background sampling, it states

start(d,"Continuous") starts the background operation running continuously. If there is data already available from the preload function, output generation begins immediately along with acquisition on any input channels. Otherwise, acquisition begins when you execute write. The operation continues until you call stop. As output scan data is generated or input scan data is acquired, you might need to call write or read while the DataAcquisition is still running.

This seems to indicate that you should be able to read data while it is still scanning. Is it just not working for me because I am trying to call read too quickly perhaps?

[akshatsood]

Hi @Holden,

It sounds like the issue might indeed be related to the speed at which the "read" function is called. If the discrete "read" command is slower than the loop, it could cause the discrepancy between "cnt" and "spot". You might want to try adding a brief pause in the loop or optimizing the read process to ensure it can keep up with the loop iterations.

[Holden]

Hello,

It turns out adding a 10 [ms] pause after the read command did solve the issue, thank you!