Skipped seconds in time stamp loop

Question

Hi all,
I have a two 6024E cards which I am synchronizing via a RTSI cable. I
have an 860mHz Dell dimension with 512 MB of RAM. I am simply logging
analog signals from the cards to a text file on a 1s timebases. The
problem is that the output data file shows a time stamp is missed every
once and a while. I have tried adjusting the rate of sampling and
number of samples, but there always appears to be a hiccup in the
timestamping. I even tried hardware timing with a timed loop and one of
the on-board counters. The hiccups were even more frequent then. Maybe
the code needs to be more efficient, but I am stumped. I attached the
data logger vi. I appreciate any input.
Thanks,
dewey
Attachments:
acquire_ver5a.zip ‏1433 KB

Answer 1

Hello Dewey,
A couple recommendations for your program:
You are manually generating a timestamp for your signals using an
elaborate algorithm that I haven't really taken the time to
understand.  I might be easier, more efficient, and more accurate
to just acquire your data as a Waveform data type from the DAQmx Read
Analog VI instead of acquiring data as an array of doubles.  The
Waveform data type includes a built in timestamp that you could parse
out and log to file instead of calculating your own timestamp using the
Tick Count (ms) VI.  Take a look at the LabVIEW Help and there is
a section under File I/O about Writing Waveform Data to a File Using
Storage VIs.
I would also highly recommend you remove any Wait or Wait for Next ms
Mutlitple VIs from your program.  You are already configuring the
sampling rate of your continuous acquisition earlier in your program
using the DAQmx Timing (Sample Clock) VI.  This is configuring the
DAQ tasks to use the hardware clock to use the onboard sample clock of
Dev1 for your acquisition.  With the sample clock timing
configured, the NI-DAQmx driver will automatically take care of the
speed of execution of your while loop based on the rate you specify in
the DAQmx Timing VI.  By manually including waits in your while
loop, you are conflicting with the timing parameters you set up in your
DAQ task.  This could lead to buffer overflows, missed samples,
and otherwise non-deterministic sample timing.  I would suggest
taking a look at the LabVIEW DAQmx shipping example Multi-Device
Synch-Analog Input-Continuous Acquisition.VI which can be found in the
NI Example Finder (Help >> Find Examples) in the following
category: Hardware Input and Output >> DAQmx >>
Synchronization >> Multi-Device.
I hope these suggestions help!
Regards,
Travis G.
Applications Engineering
National Instruments
www.ni.com/support