slow data acquisition with DAQ-Pad USB

[Arno Euteneuer]

Hi together,
I'm using one of NI's DAQ-Pads 6020-E for USB (because my computer runs out
of PCI slots) to acquire several more-or-less DC signals. The LabView
version is 6.0.2 full development and the computer (still) runs under
Windows 98 SE.

I want to use one of the analog inputs and one of the analog ouputs to
realize a software PID loop. So, I don't need the 100kSamples/sec available
in buffered mode but simply want to get one value at a time. In principle
this works fine but somehow the maximum sampling rate I get is always in the
few-Hz range only. I expected to get at least several ten, maybe several
hundred Hz with a while loop only containing the standard "AI sample
channel.vi". Is there maybe a hidden VI which is better s
uited for my
application, optimized for single value acquisition?
USB should not be the bottleneck....

Did anybody else make the same experience? And does anybody else have a
solution for a faster sampling?

Thanks to everbody thinking about my problem.

Arno

[Filipe A.]

Hello;

The loop rate limitation you are seeing there is not due to the NI hardware/software, but to the fact that Windows manages all tasks running on your computer. Since Windows is far from a Real Time OS, you won't be able to achieve fast rates.

To accomplish that task you definitely need a Real Time OS working together with a PCI DAQ board.

Regards
Filipe A.
Applications Engineer
National Instruments

[Arno Euteneuer]

Hi Filipe,

Thanks for your reply.

But even under Windows it should easily be possible to retrieve more than
maybe 5 signals per second from a DAQ board, may it be a PCI version or USB
version. Even if I readout complete frames from a USB-webcam with LabView I
get more frames per second than I get

single analog values per second from the DAQ-Pad. With a self-made AD-PCB,
controlled via parallel port, I get rates much higher then ten times those
from the DAQ-pad. So, until now I cannot believe that Windows is the
bottleneck.

A rate of 10-15 values per second (needs not really to be a constant rate)
would most probably already be enough for my application. I think this must
be possible without realtime OS.

Regards

Arno

"Filipe A." schrieb im Ne
wsbeitrag
news:506500000005000000CFB90000-1031838699000@exchange.ni.com...
> Hello;
>
> The loop rate limitation you are seeing there is not due to the NI
> hardware/software, but to the fact that Windows manages all tasks
> running on your computer. Since Windows is far from a Real Time OS,
> you won't be able to achieve fast rates.
>
> To accomplish that task you definitely need a Real Time OS working
> together with a PCI DAQ board.
>
> Regards
> Filipe A.
> Applications Engineer
> National Instruments

[Bob Hopf]

Was the second part of this thread ever answered? I have the same (similar) question on USB/DAQ response.

[bjorn1985]

I am using a DAQPad-6020E as well with Labview 8.0.

I am using AI Acquire Waveforms in a parallel VI to sample up to 32 channels at a time. With this VI, you can sample multiple channels multiple times. So sample 1000 samples at a 1000hz sample rate for example. When the parallel VI is finished, it saves the results in a global variable so my timed loop can read it and put them in a chart.

[bjorn1985]

In addition to my previous reply,

Remember that the USB port IS the bottleneck because it receives packets of information. I have tried getting reaction times via the USB port, the delay through the port was over 20ms. This is because the data is buffered by Windows before Labview can read the data.

[Jesse_O]

Hello Arno,

When performing a single sample of your DAQPad-6020E, the DAQ driver/device does not just sample the data.  For example the device has to be configured to the setting you specify in the AI (one point) Sample Channel vi.  As a result the number of samples you get using this method is very low.  If you want to get a higher data transfer rate, you should consider using buffered data acquisition.

Regards,

Jesse O.
Applications Engineering
National Instruments