Generate Continuous and Background Signals Using NI Devices

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This example shows how to generate analog output data using non-blocking commands. This allows you to continue working in the MATLAB® command window during the generation. This is called background generation. Use foreground generation to cause MATLAB to wait for the entire data generation to complete before you can execute your next command.

Create DataAcquisition Object and Add Analog Output Channels

Use the daq function to create a DataAcquisition object and the addoutput function to add output channels. Set the data scan rate to 10000 samples per second. This example uses an NI 9263 module in National Instruments® CompactDAQ Chassis NI cDAQ-9178. This is module 2 in the chassis.

dq = daq("ni");
dq.Rate = 10000;
addoutput(dq, "cDAQ1Mod2", 0:2, "Voltage");

Create Synchronized Signals

Generate output signals by creating a pattern of data that is repeatedly written to the output device. The data for each channel is column based and the output signals are synchronized to a common clock.

Create 3 waveforms:

  • data0: 1 cycle of a sine wave

  • data1: 1 cycle of a sine wave with a 45 degree phase lag

  • data2: 10 cycles of a sine wave

data0 = sin(linspace(0, 2*pi, 1001))';
data1 = sin(linspace(0, 2*pi, 1001) + pi/4)';
data2 = sin(linspace(0, 2*pi*10, 1001))';

The above waveform contains sin(0) and sin(2*pi). To repeat the waveform coherently, omit the final point.

data0(end) = [];
data1(end) = [];
data2(end) = [];

At a generation rate of 10000 scans per second, you can expect to observe data0 and data1 as 10 Hz sine waves and data2 as a 100 Hz sine wave.

subplot(3,1,1)
plot(data0)
title('data0')
grid on
subplot(3,1,2)
plot(data1)
title('data1')
grid on;
subplot(3,1,3)
plot(data2)
title('data2')
grid on;

Queue Output Data and Start Background Generation

To start continuous background generation immediately, preload half a second of data. This example preloads 5000 scans of data because the device scan rate is defined as 10000 scans per second. Use start to initiate the generation and return control to the command line immediately, allowing you to do other operations in MATLAB while the generation is running in the background.

preload(dq,repmat([data0, data1, data2], 5, 1));
start(dq, "repeatoutput")

Generate Output Data Dynamically Using MATLAB Functions

To dynamically generate the output data using a MATLAB function, assign the function to the ScansRequiredFcn of the DataAcquisition object. The code below is functionally equivalent to "repeatoutput".

dq.ScansRequiredFcn = @(src,evt) write(src, repmat([data0, data1, data2], 5, 1));

preload(dq, zeros(dq.Rate/2, 3));

start(dq, "continuous")

Monitor Data Generation

Use pause in a loop to monitor the number of scans output by the hardware for the duration of the generation.

t = tic;
while toc(t) < 1.0
    pause(0.1)
    fprintf("Scans output by hardware = %d\n", dq.NumScansOutputByHardware)
end

fprintf("Generation has terminated with %d scans output by hardware\n", dq.NumScansOutputByHardware);

Scans output by hardware = 1109
Scans output by hardware = 2089
Scans output by hardware = 3100
Scans output by hardware = 4095
Scans output by hardware = 5093
Scans output by hardware = 6094
Scans output by hardware = 7082
Scans output by hardware = 8082
Scans output by hardware = 9088
Scans output by hardware = 10099
Generation has terminated with 10099 scans output by hardware

Stop Continuous Background Generation

Background generation runs simultaneously with other operations in MATLAB. Explicitly call stop to end the background generation.

stop(dq)