Wish to output a ramp voltage USB 6008 with Signal Express?

[Dave Sb]

I have just purchased NI USB 6008 hardware being run by Signal Express.  What I want to do is output a ramp voltage (zero to 5 V from say time t = 0 to t = 1 sec).  I am entirely a novice with regards to Signal Express software and so far have not been able to do this.  Does anyone have a recipe I could follow?  (Note that the 6008 samples only at 150 S/s and appears to be limited in other areas as well).

[Aaron_W.]

Dave,

Thank you for posting on the NI Forums. If I understand correctly you are wanting to output just a single Ramp Function from 0V-5V, with a total of 150 Samples. I have included a couple pictures of the task I have set up to accomplish this task. Basically, you just need to generate a signal and then output that signal with a DAQmx Analog Output task. The picture shows under "Sampling Conditions" a sample rate of 150 S/s, which is just the rate you want to output, the Block size (samples) is the set to 150 currently so only one period of the signal will be output. You could change this to add more periods if you need. This should give you a good place to start from, let me know if you have other questions. 

[Dave Sb]

Arron,

Thanks for the suggestion, but...

when I ran your configuration I got a DAQ Assitant Error - 200077" Requested value is not a supported value for this property"

Property = Samp TimingType

You have requested = Sample Clock

You can select = On Demand.

So I switched N samples to On Demand and got a "missing inputs" error.

Any other ideas? 

[Jordan F]

Hi Dave,

The USB-6008/9 AO lines are software timed only so that is why you are seeing the Error -200077.  This means that you can not specify a sample clock for your device to output because the lines are updated when the software is ready.  The AO lines can update at a maximum analog output rate of 150S/s (non deterministic).  I attached the specifications manual (page 19)  and a knowledgebase article that explains this in more detail.    

USB-6008/9 User Guide and Specifications (page 19)

http://www.ni.com/pdf/manuals/371303l.pdf 

USB-6008/6009 Continuous Analog Output

http://digital.ni.com/public.nsf/allkb/6F2C2B49A89D685C8625711D007BDD64

To output a ramp out of your device in signal express you need to specify On demand for the sample timing.  Then you need to use a sweep step to set a Value to Write for the USB-6008/9 (it will update the AO lines on sample at a time).  I attached a screenshot of how to set this up in signal express.

[Dave Sb]

Jordan,

Thanks.  That did work.  The only problem is that the ramp takes about 70 seconds to complete (i.e., about 0.7 sec per write).  Any idea on how the rate could be increased by say a factor of ten (to just 7 seconds)?

Thanks again 

[John_P1]

Hi Dave Sb,

I ran the sweep on my computer and it took about 25 seconds.  I also ran it on a colleagues and it took about 15.  The loop rate is going to be completely system dependant, but SignalExpress does introduce quite a bit of overhead to the DAQmx function calls.  I'm not immediately sure of anything we can do to help optimize the rate, but you could always reduce the number of points you are sweeping through (although there would be bigger "jumps" in voltage with each update).

-John