Is there a way I can get a DMM subvi to run faster?

[dj143]

I cannot get the attached subvi to run in a timed loop faster than about 400mSec.  All I need out of this is a four-wire resistance measurement.  The task cluster is defined in an initialization loop and it is set at 6-1/2 digit resolution.  It may be that I’m expecting to run this too fast at this resolution.  Is there a way I can increase the loop rate for this?  It’d be nice to run it at 100mSec.  I’m using LV 8.5 with Win-XP.  The DMM card is a PCI-4065.  The system is running on a dual core processor, but one processor is dedicated to run a critical DAQmx loop.

It may be a difficult question to answer, but please let me know if there is a way to increase the loop rate with this subvi.

Thanks,

Dave J.

[A_Ryan]

dj143,

I have a couple of suggestions to speed up the subVI:
1.  Do you have to check the range every time you enter the case structure?  If you do check the range every time, the DMM is going to need some time to settle proir to taking a measurement.

2.  Does it make any difference if the NI-DMM Over Range.VI is present?

3.  If you expand the property node for niDMM, and configure the property node for aperture time, what is the resulting value?

[dj143]

The range is set to a fixed value.

I feel silly for not recognizing the Over Range vi output not connected (or needed).  I removed this from the Read DMM TS1 vi.  However, this does not make any measureable change in the timing.

I do not understand aperture time as applied to the property node.

Dave

[PBear]

Hi Dave,

To find information about aperture time vs. measurement cycle vs. digits of precision, there is a great deal of information in the NI-DMM help file located in the Start Menu under National Instruments/NI-DMM/Documentation. In general, apeture time is directly proportional to the digits of precistion, thus the longer the apeture time, the greater the digits of precision. There you can find under NI Digital Multimeters Help/Devices/NI 4065/DMM Measurements information about this correlation. Below is a diagram of the time it takes a measurement and what steps are involved. Many are adjustable and again have great documentation in the Help file.



I would also suggest using a shipping example to show this in real time. Run Maximizing DC Reading Rate.vi (in example finder available from the help menu) and you can adjust aspects of the measurement cycle and see how long these readings took. 6 1/2 Digits take a great deal longer than 5 1/2 digit readings.



Message Edited by Patrick_Ba on 03-11-2008 05:38 PM