Data Acquisition Toolbox

Examples: Using Callback Properties and Functions

This section provides examples that show you how to create callback functions and configure callback properties.

Displaying Event Information with a Callback Function

This example illustrates how callback functions allow you to easily display event information. The example uses daqcallback to display information for trigger, run-time error, and stop events. The default SampleRate and SamplesPerTrigger values are used, which results in a 1-second acquisition for each trigger executed.

You can run this example by typing daqdoc5_6 at the MATLAB command line.

1. Create a device object -- Create the analog input object AI for a sound card. The installed adaptors and hardware IDs are found with daqhwinfo.

• AI = analoginput('winsound');
  %AI = analoginput('nidaq',1);
  %AI = analoginput('mcc',1);
  

2. Add channels -- Add one hardware channel to AI.

• chan = addchannel(AI,1);
  %chan = addchannel(AI,0); % For NI and MCC
  

3. Configure property values -- Repeat the trigger three times, find the time for the acquisition to complete, and define daqcallback as the M-file to execute when a trigger, run-time error, or stop event occurs.

• set(AI,'TriggerRepeat',3)
  time = (AI.SamplesPerTrig/AI.SampleRate)*(AI.TriggerRepeat+1);
  set(AI,'TriggerFcn',@daqcallback)
  set(AI,'RuntimeErrorFcn',@daqcallback)
  set(AI,'StopFcn',@daqcallback)
  

4. Acquire data -- Start AI and wait for it to stop running. The waittilstop function blocks the MATLAB command line, and waits for AI to stop running.

• start(AI)
  waittilstop(AI,time)
  

5. Clean up -- When you no longer need AI, you should remove it from memory and from the MATLAB workspace.

• delete(AI)
  clear AI
  

Passing Additional Parameters to a Callback Function

This example illustrates how additional arguments are passed to the callback function. Timer events are generated every 0.5 second to display data using the local callback function daqdoc5_7plot (not shown below).

You can run this example by typing daqdoc5_7 at the MATLAB command line.

1. Create a device object -- Create the analog input object AI for a sound card. The installed adaptors and hardware IDs are found with daqhwinfo.

• AI = analoginput('winsound');
  %AI = analoginput('nidaq',1);
  %AI = analoginput('mcc',1);
  

2. Add channels -- Add one hardware channel to AI.

• chan = addchannel(AI,1);
  %chan = addchannel(AI,0); % For NI and MCC
  

3. Configure property values -- Define a 10-second acquisition and execute the M-file daqdoc5_7plot every 0.5 seconds. Note that the variables bsize, P, and T are passed to the callback function.

• duration = 10; % Ten second duration
  set(AI,'SampleRate',22050)
  ActualRate = get(AI,'SampleRate');
  set(AI,'SamplesPerTrigger',duration*ActualRate)
  set(AI,'TimerPeriod',0.5)
  bsize = (AI.SampleRate)*(AI.TimerPeriod);
  figure
  P = plot(zeros(bsize,1));
  T = title(['Number of callback function calls: ', num2str(0)]);
  xlabel('Samples'), ylabel('Signal (Volts)')
  grid on
  set(gcf,'doublebuffer','on')
  set(AI,'TimerFcn',{@daqdoc5_7plot,bsize,P,T})
  

4. Acquire data -- Start AI. The drawnow command in daqdoc5_7plot forces MATLAB to update the display. The waittilstop function blocks the MATLAB command line, and waits for AI to stop running.

• start(AI)
  waittilstop(AI,duration)
  

5. Clean up -- When you no longer need AI, you should remove it from memory and from the MATLAB workspace.

• delete(AI)
  clear AI
  

Creating and Executing Callback Functions

Linearly Scaling the Data: Engineering Units