Why does the Data Acquisition Toolbox “read” function return faster than the specified “span” in MATLAB R2025b?

I am using the "read" function from the Data Acquisition Toolbox in MATLAB R2025b to read data from a device. I want to collect data for 3 seconds. The documentation describes a "span" parameter, which can be specified as a duration. When I call "read(dq, seconds(3))", and I notice the call executes for less than 3 seconds when timed using "tic" and "toc". How is this possible?

Answer 1

[MathWorks Support Team]

It’s possible for “read” to execute for a time shorter than the specified "span" if the data buffer already has data when “read” is called. The example below illustrates this behavior.

dq = daq("ni");
addinput(dq, "Dev1", "ai0", "Voltage");

start(dq, "continuous");  
pause(1);   

tic; 
data = read(dq, seconds(3));
elapsed = toc();
stop(dq);

dataSpan = seconds(max(data.Time) - min(data.Time));

fprintf("Recorded elapsed time: %.3f s\n", elapsed); % ~2.000 s
fprintf("Data time span: %.3f s\n", dataSpan);       % ~3.000 s