niDMM Fetch Multi Point.vi Slow First Acquisition

[sm07]

I have a PXI-4060 and am using niDMM Fetch Multi Point.vi to acquire my measurements. Currently I have initialization VI's outside of a while loop (including a zero sample count in the niDMM Configure Multi Point VI), and then I have the Fetch VI in the loop with 'samples to fetch at a time' set to 4. When I run the VI, it takes a good 5 or 6 seconds before the first measurements come through, and then they come in at a normal pace. Why is this? Is it possible to remove this delay or use another function which will not cause this delay to occur?

Thanks!

[sm07]

Furthermore, I've noticed that the DAQmx Switch Connect  (Multiple) VI is somewhat slow. I am using an SCXI-1335 on an SCXI-1129. i have a list of about 15 'channels' or relays to connect (along different rows). is there any way to speed this up as well?

[sm07]

Please disregard my second post. Only my first question is troubling me.

[Nick_F1]

Hi sm07,

Thank you for posting on the National Instruments forum. 

I’m not sure what it is causing the delay on your DMM acquisition, but there are a few things we can try to narrow it down.  One thing is to check if auto range is turned on and set to once.  On the Ividmm Configure Measurement.vi make sure you don’t have a -3 wired to the range pin.  Here is a knowledgebase article on how the Auto Range works.

I would also try to use the NI-DMM Soft Front Panel which you can find by clicking on Start >> Programs >> National Instruments >> NI-DMM >> DMM Soft Front Panel and see if you see the same delay there. 

If changing the auto range doesn’t fix your issue and the Soft Front Panel does not have the same delay, then you can try posting a screenshot of your application where you access your DMM and I can see if there is something there that is causing the delay.

[sm07]

For some strange reason I am also experiencing a delay in the soft front panel. For example, I am generating a signal on my PXI-5401 and am reading it on the 4060. As soon as i switch the signal mode on the 4060 front panel to AC Voltage Reading, there is a ~6 second delay before the DMM starts displaying/taking (?) the new readings. I am using 4.5 digit precision. I also do not believe I have auto-range turned on (please see screenshot) - it is set to 10.00 on the front panel. You will note that in the VI that I have created I am reading from two DMMs simultaneously.
Thanks!

Message Edited by sm07 on 07-19-2007 08:21 AM

Message Edited by sm07 on 07-19-2007 08:22 AM

[Mark_S1]

You are probably seeing the initial settling time required by the 4060.  This would also explain why you are seeing it in the Soft Front Panel as well.  You didn't mention what mode and range you were using but the amount of settling time is dependent on what mode and what range you pick.  AC modes are particularly slow because of the coupling capacitor. 

You can try making that time smaller by setting the Trigger Delay property/attribute to something other than -1 or setting it on the niDMMConfigureTrigger VI/function.  You have to be careful though and make sure that your data is still valid using your new, shorter trigger delay, because depending on your signal and the circuitry of the system, you may actually need that much settling time.

If you really need the speed, you may consider an NI 407X, which is significantly faster, or an NI 4065, which is somewhat faster.  If you have a recent version of NI-DMM (2.6 or later), the help file has the actual default settling times for the 407X and the 4065.

Mark

DMM Software Engineer

[Nick_F1]

Hi Sm07,

Here is a Knowledgebase article that helps explain this slowdown:

Slow First Measurement on the NI 4060

[sm07]

Thanks for your help. Unfortunately the delays i am experiencing are 5-6 seconds in length, which is quite strange. Would it have anything to do with the "Fetch Function." I am measuring a 3200Hz AC Voltage @ ~4V rms.

[Jennifer O]

Hi sm07,

Nick's linked KB is good for explaining some cases of the slow initial measurement, however it does not address the AC measurement case directly.

When taking an AC measurement there is some extra circuitry which must be primed before an accurate reading can be made. You can imagine if you are performing a running average of a set number of samples, you must build up a history first before your average would be accurate.  If you just began with a data set of zeros (or even old data) you would get a skewed average until all of the old samples or zeros were replaced by new data.  This analogy can be applied to the behavior of the NI-4060. 

It is expected behavior that the very first measurement can take roughly 6 seconds, but all subsequent reads of a multipoint fetch will return much faster.  You can set the initial delay to a shorter time as Mark suggests, but this would be adding error to your first couple of measurements (they would no longer be guaranteed to meet the specified accuracy of the card).  Alternatively, set up the DMM in advance and perform a read to "prime" the card.  Then when you are ready to take a measurement, the reading will be fast.

Regards,

Jennifer O.
Applications Engineer
National Instruments