Timed loop miss some items

Hi all,
I am using cRIO 9025 AND NI 9203 to sample from seven current channels at 1000 Hz. I wrote the data with timestamp into a file. However, it seems that the sampling is not continous.
As you can see from the text file attached, the time stamps for the fisrt sample and the second sample have a difference which is 3 ms. Then this would happen every 208 samples. If you open the file with Notepad++, you would see it clearly.
And I found that if I decrease the number of channels from 7 to 1, the problem would happen every 656 samples which is very wierd.
I try my best but still can't understand why this happens. Could you please help me have a look at the VI?
Thank you very much.
Attachments:
1KHzSample.vi ‏73 KB
7-Current Sep,27,2010 13h49m45sec.txt ‏1073 KB

From looking into the VI, i can only guess why the "time jumps" occur so "deterministic".
But it is quite obvious to me that some things can be optimized which maybe can already remove the issue:
- Eliminate redundant code (taking timestamp).
- Remove unnecessary computations (select function, feedback for dt and false-constants).
- Stick to one datatype (change the offset and scale to double).
- Never ever compare doubles as equal/not equal!
- Split up logging from acquisition into different loops. Use RT FIFOs instead of normal queues.
I assume that the "time jumps" are generated by finished late iterations and the setting to maintain the phase and to discard missed iterations....
hope this helps,
Norbert
CEO: What exactly is stopping us from doing this?
Expert: Geometry
Marketing Manager: Just ignore it.

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I have a camera control VI that waits for TTL information from the camera shutter and uses it as a timing source for a timed loop. In this timed loop the camera is readout and analyzed. The program should react as soon a possible to a new image and therefore I set the timed loop to 'Discard missed items' so that it doesnt try to catch up (the events im trying to detect are quite rare and last several frames).
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Solved!
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Dear j.win,
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Using an external timing source (your TTL for example) instead of an internal timing source does not change anything. You can use the "Period" (dt) input of the Timed Loop to specify when the loop is supposed to iterate (the unit is the "ticks" in the case of an external timing source). A value of "1" means that the Loop try to iterate at every ticks of the external timing source. If for some reason one iteration lasts longer than that, you will have a "late iteration". Also with a value of "1", it is not possible to modify the "phase", then the "Maintain Original Phase" parameter will just defined wether or not you skip the iteration. If "Maintain Original Phase" is false, the loop will execute immediatly after a late iteration to still execute it, but a bit latter. If the "Maintain Original Phase" is true, the loop will execute immediatly after a late iteraion if the "Discard missed item" is false otherwise the loop will skip the iteration.
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RT Loop Timing.png ‏8 KB
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Minimise front panel of vi containing timed loop = problem

Is this expected behaviour...
Run the attached VI.
All it is supposed to do is beep every 10 cycles.
When you minimise it and try to restore the front panel though... it locks up for a random amount of time.
I don't think it is supposed to do that. (I'm running it on XP Pro.)
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It took all day to track it down to this.
Hmmm... I just tried to save it back to a version 8.0 vi and it doesn't seem to have the same error!
Message Edited by Troy K on 10-03-2008 05:26 PM
Message Edited by Troy K on 10-03-2008 05:31 PM
Troy
CLDEach snowflake in an avalanche pleads not guilty. - Stanislaw J. Lec
I haven't failed, I've found 10,000 ways that don't work - Thomas Edison
Beware of the man who won't be bothered with details. - William Feather
The greatest of faults is to be conscious of none. - Thomas Carlyle
Attachments:
Timed Loop minimised.vi ‏85 KB
minimisetimedloop.png ‏7 KB

I'm using LabVIEW 8.5.1 on two PCs and it does it on both of them.
I just emailed the vi to a colleague and after a few minimise/maximises it locked up on him too.
I've just found that if I remove the left and right data nodes inside the timed loop the bug seems to go away.
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1. Minimise all windows (Windows+m)
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3. Minimise / Restore it rapidly by repeatedly clicking on the vi's Front Panel 'task bar button' (down the bottom).
After a while the beeping will stop when the front panel is minimised and it wont restore. Then after a while (with or without clicking on the task bar button) the front panel will restore and the missed periods will execute very quickly (as the timed loop does what it is configured to do).
I think because of the simplicity of the example vi the bug doesn't show up as often. When I ran it in the real application (too many vi's to post) it crashed every time. In the real application there isn't much going on in the timed loop, it is just used to queue items into a separate consumer loop. The problem is that when the bug occurs, ALL LABVIEW CODE stops executing, not just the code inside the timed loop. So another vi running in parallel (in a daemon) that retrieves messages from a device queue stops and the device buffer overflows.
Attached is a modified vi to demonstrate that code outside the timed loop stops executing too. The bug occurs here on three different PCs, the only common thing I can think of is LabVIEW 8.5.1.
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CLDEach snowflake in an avalanche pleads not guilty. - Stanislaw J. Lec
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Beware of the man who won't be bothered with details. - William Feather
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Attachments:
Timed Loop minimised 2.vi ‏89 KB

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