This example shows how to save data acquired in the background to a file. Use the session-based interface and acquires analog input data using non-blocking commands. If you are using the legacy interface, refer to "Logging Data To Disk" in the Analog Input examples list
Create a Session with Analog Input Channels
Create a session and add two analog input channels with
Voltage measurement type. For this example use National Instruments® X Series data acquisition device, NI PCIe-6363 card with ID
d = daq.getDevices; s = daq.createSession('ni'); addAnalogInputChannel(s,'Dev1',0:1, 'Voltage'); s
s = Data acquisition session using National Instruments hardware: Will run for 1 second (1000 scans) at 1000 scans/second. Number of channels: 2 index Type Device Channel MeasurementType Range Name ----- ---- ------ ------- --------------- ---------------- ---- 1 ai Dev1 ai0 Voltage (Diff) -10 to +10 Volts 2 ai Dev1 ai1 Voltage (Diff) -10 to +10 Volts
Create a Log File
Create the file
log.bin and open it. You will write the acquired data to this file in binary format. Save the file identifier in the variable fid1.
fid1 = fopen('log.bin','w');
Add a Listener
When you acquire data in the background, you can provide the acquisition session directions to handle the incoming data, using listeners and events. A
DataAvailable event occurs when a specific amount of data is available to the session. A listener can respond to that event and initiate specified function.
addlistener to add an anonymous function to the session. This function is called every time the
DataAvailable event occurs, and logs the acquired data to a file. By default this listener is called 10 times per second.
The anonymous function
logData requires three inputs:
src- source of the event
event- name of the event
fid1- file identifier for logging data
When you add a listener, a handle to the listener is returned. Save the handle in the variable
lh and delete it later.
lh = addlistener(s,'DataAvailable',@(src, event)logData(src, event, fid1));
Acquire Data in the Background
Acquire data continuously in a non-blocking mode.
s.IsContinuous = true; s.startBackground;
Continuous background acquisition runs in parallel with other operations on MATLAB®. You can execute MATLAB commands while the acquisition is running. For the purpose of this example, simulate a long computation by calling
Stop the Continuous Acquisition and Close the Log File
Explicitly call stop after 5 seconds to end the background acquisition and delete the listener.
Close the log file.
s.stop; delete(lh); fclose(fid1);
Load Data from the Log File
Read the log file.
Load file contents as a 3 column matrix into
Close the log file.
fid2 = fopen('log.bin','r'); [data,count] = fread(fid2,[3,inf],'double'); fclose(fid2);
Plot the Data
Assign each column of
data to the following variables:
ch- channel 0 and channel 1
t = data(1,:); ch = data(2:3,:); plot(t, ch);
Write a Function to Log the Data.
Create a multi-line function
logData and store in a separate MATLAB file.
function logData(src, evt, fid) % Add the time stamp and the data values to data. To write data sequentially, % transpose the matrix. % Copyright 2011 The MathWorks, Inc. data = [evt.TimeStamps, evt.Data]' ; fwrite(fid,data,'double'); end