I wrote some code in App Designer several months ago. The code was working fine until yesterday when I started seeing the following warning:
Warning: only X scans available
Where X is some number between 1 and 4.
It is happening when my code switches between background and foreground data acquisition on a button push.
Here is a snippet of the code in question -
% Get DAQ info from pulldown
deviceIndex = app.Screen1.DAQDropDown.Value-1;
deviceID = app.Screen1.DevicesInfo(deviceIndex).ID;
vendor = app.Screen1.DevicesInfo(deviceIndex).Vendor.ID;
% Delete existing data acquisition object
delete(app.DAQ);
app.DAQ = [];
% Create a new data acquisition object
dq = daq(vendor);
app.DAQ=dq;
% Add channels
ch = addinput(app.DAQ, deviceID, 0:15,"Voltage");
for cnt=1:16
ch(cnt).TerminalConfig = 'SingleEndedNonReferenced';
end
% Change to background process DAQ settings and start background acquisition
app.Screen3.Screen2.DAQ.Rate = 5;
app.Screen3.Screen2.DAQ.ScansAvailableFcnCount=1;
app.Screen3.Screen2.DAQ.ScansAvailableFcn = @(obj,evt) WriteMyDataScan(app,obj,evt);
start(app.Screen3.Screen2.DAQ,"Continuous");
% Button pushed function: TakeScanButton
function TakeScanButtonPushed(app, event)
%Increase scan count
app.ScanCount=app.ScanCount+1;
%Stop the background DAQ process
stop(app.Screen3.Screen2.DAQ);
daq_dev=app.Screen3.Screen2.DAQ;
%Set sample rate and number of samples
daq_dev.Rate=25;
samples=50;
%read 50 samples
data=read(daq_dev,samples,'OutputFormat',"Matrix");
%Change back to background process DAQ settings and re-start background acquisition
app.Screen3.Screen2.DAQ.Rate = 5;
app.Screen3.Screen2.DAQ.ScansAvailableFcnCount=1;
app.Screen3.Screen2.DAQ.ScansAvailableFcn = @(obj,evt) WriteMyDataScan(app,obj,evt);
start(app.Screen3.Screen2.DAQ,"Continuous");
end
The read command in the TakeScanButtonPushed -
data=read(daq_dev,samples,'OutputFormat',"Matrix");
should be reading 50 samples.
...but sometimes upon stopping the background DAQ process, the NumScansAcquired field would be non-zero... and in that case instead of 50 scans, I would only get the number of scans listed in the NumScansAcquired field. If the NumScansAcquired field was zero, the read command gets all 50 scans.
The problem seems to be fixed by adding a flush between stopping the background acquisition and doing the read.
The mystery is - why did this suddenly start happening? The machine is not connected to the internet. The DAQ driver has not been updated. The OS (Windows 10) hasn't been updated. Matlab (R2020b) has not been updated. The code hasn't changed.
