read function in DAQ is hidden by a built in function

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Lars Eriksson 2024-1-30

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评论： Walter Roberson 2024-2-1

I want to run read in DAQ toobox but get an error message:

Incorrect number of types of input or output for function 'read'.

When I type "help read" I get:

read - read one or more video frames

when I run "which read"

read is a built in method

So the daq read function is shadowed by something else how can I get rid of the shadowing so that I can use read from DAQ toolbox?

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Lars Eriksson 2024-1-31

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Everything works fine with the card and the session based communication, I can put outputs in a queue and get simultaneous signal generation and sampling to work. But when I just wanted to acquire some analog channels the read-command barfed at me. I have a lot of toolboxes installed and have the feeling that the wrong read command is selected by Matlab.

% This is the code I had,
T_t=120;     % Total collection time in [s].
f_S=100;     % Sampling frequency
%% Connect with the NI card
disp('Connecting to the instrument')
s=daq.createSession('NI');
disp('Session created')
ch(1)=addAnalogInputChannel(s,theDevice,'ai0','Voltage');
s.Channels.TerminalConfig='SingleEnded';
ch(2)=addAnalogInputChannel(s,theDevice,'ai1','Voltage');
s.Channels(2).TerminalConfig='SingleEnded';
ch(3)=addAnalogInputChannel(s,theDevice,'ai2','Voltage');
s.Channels(3).TerminalConfig='SingleEnded';
ch(1).Range=[-5,5];
ch(2).Range=[-5,5];
ch(3).Range=10*[-1,1];
s.Rate=f_S;
[out,mytime]=read(s,f_S*T_t);

This is where things go wrong...

Lars Eriksson 2024-2-1

在 MATLAB Online 中打开

Thanks for the feedback and input.

The s=daq.createsession() works fine for me (when I wrote my last comment I wasn't sitting at the measurement computer with the daq card to see if I wrote 'NI'/'ni'/"ni", it was done in a separate script, the rest was copied verbatim from the script I used). Whatever string I used, I got the session working and could control the device, and I could do most things with outputs/inputs. I could get the work done and acquired a sample of the right size by setting up an output and queueing a dummy output with the right length using queueoutput(s,dummy) and then running it with out=daq.startforeground(s). However, this is like going over the river for water when it is only the result of the vanilla read-function that I needed for my usage.

The read statement proposed doesn't have any way to control the length of acquisition. I need to have the right sampling time and length to match the data. I want to use this from the help.

scanData = read(d,span)

Unfortunately I'm out travelling so I can't test if this particular call to the read function gives the same error message or if it get passed to the right read function, I will do it as soon as I'm back.

There are plenty of variants of the "read" function. Writing "help read" gives "read - Read one or more video frames" on my machines. Clicking on "other uses of read" gives a batch of functions that are named "read". Matlab didn't detect the one I wanted to use, I'll experiment further when I am back after my travels.

Walter Roberson 2024-2-1

在 MATLAB Online 中打开

s = daq("ni");

potentially creates a different kind of object.

I agree that read(s) does not permit you to specify the number of scans. The point is to test whether read(s) will give the error about incorrect number of parameters.

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