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Plotting analog input continuously on a given axis

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Niraj Desai
Niraj Desai on 6 Jun 2020
Commented: Niraj Desai on 10 Jun 2020
I'm trying to understand the behavior of the Data Acquisition Toolbox when I try to acquire data from a National Instruments board and plot it continuously. My code is printed below. It works -- but only if I declare daqObj as a global variable. If I don't, then I don't get an error, but nothing happens. I'm using Matlab 2020a and Data Acquisition Toolbox 4.1 on a Windows 10 Pro desktop.
function [] = myTest(ax)
% ax is an axes in a GUI made with App Designer
global daqObj % <--- why do I need this?
daqObj = daq("ni");
addinput(daqObj,"Dev1","ai0","Voltage");
daqObj.Rate = 1000;
daqObj.ScansAvailableFcn = @plotMyData;
daqObj.ScansAvailableFcnCount = 5000;
start(daqObj,"Continuous")
function [] = plotMyData(obj,~)
[data,timestamps] = read(obj,obj.ScansAvailableFcnCount,"OutputFormat","Matrix");
plot(ax,timestamps,data)
end
end

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Accepted Answer

Geoff Hayes
Geoff Hayes on 10 Jun 2020
Niraj - if you don't declare daqObj as a global variable, then it will be a local variable. When your function "completes" (after calling start(...), then all local variables will be deleted/destoyed. If daqObj is local, then it will be destroyed and will stop trying to acquire data so this is probably why "nothing happens". But if you declare it as a global variable then it will "exist" outside of the function (so that other functions could access/share this variable) and so the data acquisition will work.
You mention that you are using App Designer, so why not make the daqObj a property/member of the GUI so that you don't have to declare this as global? That way, you can control from within the GUI when to start and stop the acquisition.

  1 Comment

Niraj Desai
Niraj Desai on 10 Jun 2020
Geoff,
Thank you for this thoughtful answer. What you wrote makes a lot of sense. Thanks also for the suggestion about App Designer. Good idea.
Niraj

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