Has anyone seen lag when using a DAQPad-6015?

[phartwell]

I have been using a DAQCARD-6062e and just got a DAQPAD-6015.

I am using these to continuously collect samples from a sensor. Using two copies of the same VI in Labview 7.1 on XP, I can start 2 tasks, one for each device. I am monitoring the output of 2 sensors, one on each device, at the same time. I am sampling 10 channels of AI at 3kS/s. The DAQPAD is 16bit, the DAQCARD is 12 bit.

The signals start out synchronized. But if I let the application run for about 5 minutes, there is a lag on the DAQPAD relative to the DAQCARD. This lag can be from 2 to 4 seconds! I am not missing any data from the DAQPAD, its just showing up late. I am sure it is not in my VI programming since it works fine with the DAQCARD.

It this just what I should expect for a USB based DAQ system relative to my PCMCIA based DAQCARD?
Since this is a new product, perhaps there are driver improvements pending to increase the performance?

Anyone got any ideas??

Thanks,

pete.

[sergesl]

Hello Phartwell

The delay you are having is due to time drift between the two devices. Each device has an internal timebase from which it derives its sample clock. Since the two time bases are not synchronized and the sample clock derivation process is proper to each device, there is a drift between the two acquisition processes. The drift is very small and that is why you cannot notice it at the beginning. But once you let the programs run for a long time, the drift increases and becomes noticeable.

To avoid this, you can synchronize both operations by sharing a sample clock for example. That way, you are sure that they will acquire always at the same time. For example, you could route the sample clock of one device to be used as the sample clock of the other device using a PFI line. If you use LabVIEW, there are shipping examples that deal with DAQmx synchronization. If you are programing in Traditional NI-DAQ, you can search our developper exchange for examples.

Hope this helps. Have a good weekend.

[phartwell]

Thanks for the advice, but Im pretty sure that isnt a timing thing.

I am running DAQmx as that is the only thing supported by the DAQPAD.

If I run the DAQPAD by itself, I get the same delay. What I mean is, I have an accelerometer connected to the AI channel. When I start the VI, if I tap the accelerometer, I see the signal immediately on my graph. After about 5 minutes, when I tap the accelerometer, it takes 2 to 4 SECONDS for the signal to appear on the graph. If I do a series of taps, they all show up, eventually, so I know I am not losing data, it is just arriving late.

I used an identical sensor with my DAQCARD and the same VI and there is never a delay. Running both at the same time, you can see they are syncronized at first and then the DAQPAD slowly falls behind.

Ill try the sync thing just to see what happens, but im not optomistic.

pete.

[nickbike]

Hello Pete,
I know you and I have been working on this issue for sometime over phone support. I wanted to post the solution we found up here so if others found this post, they could find their answer. I have replicated what Pete was saying about the lag in data points coming back to the screen as time went by. After 10 minutes I was seeing about a 2 second delay. I searched and could find no documentation on what could be causing this issue. With the release of NI-DAQ 7.4 during the week of March 14th, 2005 we decided to try to upgrade from version 7.3.1. I have confirmed that this fixed the issue. There should now be no problems doing a long term continuous acquisition with a DAQPad 6015 or 6016 after an upgrade to NI-DAQ 7.4.

I hope this helps someone down the road!

Nicholas C
National Instruments
Applications Engineering

Message Edited by Nick C on 03-30-2005 10:53 AM