outputSingleScan() causing errors in R2018b?

Hi, I am using Data Acquisition Toolbox with National Instrument's 'USB-6221oem' board. My aim is to configure two different sessions, one for handling the Analogue I/O and other for Digital Output in the user interface that I am developing. In R2018b the Data Acquisition Toolbox function 'outputSingleScan(s,data)' is causing errors, this error was not present in R2018a. When I run the code below

% Session for Digital Output
D = daq.createSession('ni');
Dout = D.addDigitalChannel('Dev1','Port0/Line0:4','OutputOnly');

% Session for Analogue Input/Output
A = daq.createSession('ni');
A.Rate = 100000;
Ain = A.addAnalogInputChannel('Dev1','ai0','Voltage');
Ain.TerminalConfig = 'SingleEnded';
Aout = A.addAnalogOutputChannel('Dev1','ao0','Voltage');

% Data for Analogue Output
DataOut = ones(50,1);

% Execution
outputSingleScan(D,[0 0 0 1 0]);
queueOutputData(A,repmat(DataOut,20,1));
DataIn = startForeground(A);
outputSingleScan(A,0);
outputSingleScan(D,[0 0 1 0 0]);

% If successful execution
disp('Done!');

Matlab gives me the error 'The data argument must contain one column for each output channel in the session.' at line 20 which is the line outputSingleScan(D,[0 0 1 0 0]); (3rd last line in above code, this line of code is to send a single output for the session configured for Digital Output). However, when I comment out line 19: outputSingleScan(A,0); (4th last line in above code, this line of code is to send a single output for the session configured for Analogue I/O) the script runs successfully.

It doesn't matter if there are any lines of code between line 19 (output for the session configured for Analogue I/O) and line 20 (output for the session configured for Digital output), the execution always stops when it encounters the outputSingleScan(D,[0 0 1 0 0]); for session configured for Digital output and this line is number given as the error line in the command window. Also, as you can see in the above code if I execute the outputSingleScan(D,[0 0 0 1 0]); function for Digital output Session and then execute the outputSingleScan(A,0); function for Analogue I/O Session then it works fine. The error only occurs when I execute outputSingleScan(D,[0 0 0 1 0]); function for Digital output Session AFTER I have executed the outputSingleScan(A,0); function for Analogue I/O Session.

This problem has been driving me crazy from last 3 days. Do any of you know why this is happening? Thanks for your time and help in advance! :)

[Tom DeLonge]

I also observed similar bugs, but did not bother investigating further. I returned to Matlab 2018a

[Chaitanya Jha]

Hi Tom,

After writing this question here I reached out to Matlab, they checked it and it turns out it is indeed a bug in R2018b. They gave me an undocumented function clearSingleScanCache() to help with this issue. The way this function works is you call this function before calling outputSingleScan(). Example:

outputSingleScan(D,[0 0 0 1 0]);
queueOutputData(A,repmat(DataOut,20,1));
DataIn = startForeground(A);
clearSingleScanCache(A); % 1st call to undocumented function
outputSingleScan(A,0);
clearSingleScanCache(D); % 2nd call to undocumented function
outputSingleScan(D,[0 0 1 0 0]);

This will work even if you call the clearSingleScanCache() function only before calling the outputSingleScan() for Digital Session i.e. only the 2nd call in above code.

Hope this helps!

[Tom DeLonge]

Thank you for letting me know. Do you know whether this bug is fixed in 2019a?

[Chaitanya Jha]

I am not sure if it is fixed or not, I haven't yet started using 2019a.

[Steven Lord]

This sounds like it may be Bug Report 1935216 which is listed as fixed in release R2019a.

[Chaitanya Jha]

Indeed, it looks like Matlab has addressed this bug in R2019a. However, the fix is not what I hoped it would be as we still have to use the clearSingleScanCache function between successive calls to outputSingleScan or inputSingleScan. I have been doing this since last one year when I reported the bug, I was hoping for a better solution where we do not have to use clearSingleScanCache at all like in previous versions of Matlab. Hopefully in R2019b :)