PDA DAQ channels swapping

Hi everyone.
I'm experiencing problems with a PDA based app collecting analog data from 6 channels. All advice appreciated.
Hardware is a Juniper Allegro CX, running Windows CE.net 4.20, Intel ARM X-scale processor, 128Mb RAM, NI DAQCard 6024E in expansion pod.
Development platform is LabVIEW 8.6 with PDA module.
DAQmx base has 6 channel continuous acquisition set up, channels 0 to 5, 100Hz scan rate, 1000Hz conversion rate, as task 'readg' (see code)
Acquisition seems to be running perfectly well for a while, then channels switch around. e.g. all channels grounded, except for channel 2 with 0.1HZ sine input from a sig gen and channel 3 hardwired to 3V. After 30 seconds or so the channels all move around by 2 positions, Ch2 readings move to Ch0, Ch 3 to Ch1 etc. After another 30 seconds they move again by 2 steps, and after another 30 seconds or so they move two further steps (so they are back where they started). This continues until app is stopped.
This thread suggests something similar, but doesn't run to conclusion so no solution there... http://forums.ni.com/ni/board/message?board.id=250&message.id=12209&query.id=3315#M12209
(side issue - can anyone tell me how to post a thread in a neater way than this?)
Struggling here. 100Hz per channel is fastest required by my app, it's needs to run for up to 15 minutes, and channels need to stay put!
I'm posting code from a calibration routine that's doing this. The routine is part of a larger application. Upper loop is handling user interface and sitting in part 2 in the idle mode, so shouldn't be interfereing with anything.
Thanks for any assistance you can offer.
Bandit.
Attachments:
Do cal.vi ‏41 KB

Hi All
Sorry about the delay in getting back to you, I was out of office for a while. In the meantime though, a colleague of mine has been trying to replicate the issue using a PDA we have here at the office. Unfortunately though, whenever he tried taking measurements, he was unable to see any evidence of channels swapping in this manner.
Thanks a lot for posting, Mike. Funnily enough, Sacha still remembers that problem and I spoke to him about it as we sit nearby in the office. He described to me the way that your SCXI channels were swapping due to a nearby radar system and it does sound very similar to this issue. Bandit, could you be experiencing similar interference in your location? Are there any RF sources in your area? Could you try taking the PDA to another location and trying it out there to see whether that makes a difference. I would expect that what we are seeing is due to something in hardware, rather than software, since all the drivers do is assume that the first sample will be from channel 1, second from channel 2, third from channel 3 etc. Therefore, there must be some kind of hardware glitch that is causing this. Could you give us any information about the expansion module you are using to connect your DAQcard to the PDA? There may be a possibility that this interface is somehow causing a glitch in the data.
In terms of dealing with the channel swapping, one suggestion could be to pass a known reference voltage down one channel and detect when this reference voltage has been switched to another channel. This will tell you when the switching has occured so then you could tailor your other channels accordingly. I believe this is the type of thing that Mike did to get round the issue. Can you confirm whether this is the case, Mike?
Kind Regards
Jeremy T
Technical Marketing Engineer
National Instruments UK & Ireland

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Message Edited by SachaE on 03-23-2007 11:49 AM
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