time stamp daqmx

[Mark Levine]

So I have a DAQmx Read in a while loop waiting to acquire 1600 samples at 16,000 KS/s.

So it loops on the order of 100ms.

There are no timers in the while loop, I'm just depending on the continuous Read to keep things going.

I want to time tag the first sample in the data every time the loop executes.

I put a subroutine right before and then right after the Read.

When I chart out the difference between the current and previous reading I get variations on the order of +-100ms peak.

I would have expected to have variations of less than +-1ms.

I think put the Read in timed loop.  While the time looks great there appears to be a couple of second lag after changes to the input signal source frequency or amplitude.  Not sure what going on there either.

So I need an accurate +-1ms time stamp for this data.

Any ideas?

Thanks,

Mark

[Mark Levine]

Just to qualify this.  The DAQ is an Ethernet NI-9234.

[Norbert_B]

Those peaks can have many different reasons:

- Parallel running threads preemting the CPU

- "Clock uncertainty": Drift between OCX on DAQ device vs. "system time"

- Depending on the way you acquire the "time stamp": Possibly you jitter within the loops iteration so in iteration N, you take it as the first action done in N, in N+1 as last action. This results to somewhere about 200ms (+100). N+2 is again first action, resulting in about 0 ms (-100).

You should consider to read the data as waveform which contains a t0 and delta t.

hope this helps,

Norbert

[Ben]

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@Norbert B wrote:

Those peaks can have many different reasons:

- Parallel running threads preemting the CPU

- "Clock uncertainty": Drift between OCX on DAQ device vs. "system time"

- Depending on the way you acquire the "time stamp": Possibly you jitter within the loops iteration so in iteration N, you take it as the first action done in N, in N+1 as last action. This results to somewhere about 200ms (+100). N+2 is again first action, resulting in about 0 ms (-100).

You should consider to read the data as waveform which contains a t0 and delta t.

hope this helps,

Norbert

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Ditto that!

It is rare that i do not use the Waveform data type.

Ben