DAQ with NI USB card - Continuous output with 20KHz update rate in SIMULINK?

Hello everybody,

I want to use use my National Instruments USB DAQ card (NI-USB 6356) to generate a continuous sinusoidal output in Simulink. In particular, a sinus with 200Hz Frequency, at a sampling rate of 20000 Hz. According to NI, the board should be able to cope with 1.25 Megasamples/sec. However, when I generate a 200 Hz wave in Simulink using the discrete-time wave generator, and connect this block to aDAQ toolbox analog output, the output signal at the board is not continuous at all: sometimes it manages to output an entire period, but most of the time the signal is erratic and anything but continuous... could this be a buffering problem? When I insert a buffer block before the analog output, the output wave stops at regular intervals ( ca. 1sec), stops for 0.2 seconds and and restarts again...

[Markus]

Ironically, if I add a buffer block in order to queue samples before handing them over to the output, the break time scales linearly with the buffer size. Using a large buffer, I get a few seconds of continuous signal output and then one or more seconds of "nothing"....

Answer 1

[Walter Roberson]

As a general statement, unless programmed otherwise at the driver level, virtual serial ports over USB have a 0.2 second buffering time -- a data packet will not be transmitted until it is full or until 0.2 seconds have elapsed with no further data.

USB driver code can override this and request that a partial packet be sent as soon as feasible, but MATLAB does not provide an explicit interface to that function.

I have no information as to whether the NI interface knows to use that "push" mode or under what circumstances: I am speaking of the way USB was designed to operate by default.