labview >> trouble reading TDMS file

by Herbert Engels » Fri, 22 Jun 2007 02:10:08 GMT

You have chosen settings that will create a timestamp channel and one
or more data channels (double precision float) in the file. This is a
good choice if your values are not equally sampled. However, you cannot
read these values back with just one operation, because a timestamp is
128bit wide and a double is just 64bit wide.The conversion would be
lossy.

The best way of using the TDM Streaming functions is to use two "TDMS
Read". You wire the channel name for the time channel to one of
them and wire an array of time stamps to it's "data type" input. The
second "TDMS Read" should be typed to an array of doubles (since your data is
not equally sampled, a waveform wouldn't do you any good) and retrieve
the rest of your data. You would have to provide group/channel names
for both "TDMS Read" functions in order for that to work.

LabVIEW is shipping with examples (.\examples\file\plat-tdms\TDMS -
Write Event Data.vi and .\examples\file\plat-tdms\TDMS - Read Event
Data.vi) that show how to properly save and retrieve channels of
different types to/from the same channel group.

Hope that helps,
Herbert

labview >> trouble reading TDMS file

by abhiM » Fri, 22 Jun 2007 05:10:07 GMT


Based on what you suggested, I have set up my vi to read the TDMS file as shown in the screenshot(read.png)

I also used the TDMS viewer to look at the data contained in my TDMS file. A screen shot is attached(Data.png)

I get the feeling that I am not setting the right data type for my second TDMS reader.-abhiM

Message Edited by abhiM on 06-21-2007 03:51 PM


Data.PNG:
http://forums.ni.com/attachments/ni/170/254584/1/Data.PNG


reader.PNG:
http://forums.ni.com/attachments/ni/170/254584/2/reader.PNG

labview >> trouble reading TDMS file

by Herbert Engels » Fri, 22 Jun 2007 05:10:08 GMT

You can go with the waveform type, but I'd recommend an array of doubles,
because waveforms don't buy you anything here. The problem in your VI is that you
need to provide an array of channel names to the second function,
otherwise it will still try to load the timestamp channel (and fail converting it).

I am also wondering why you chose "One header per segment". That only
makes sense for values that cannot be appended (e.g. frequency-domain
data or histogram results) or triggered measurements without a time channel. Since you have
neither one, you might be better off with "One header only". You will
then have only one channel group with all data appended to the same set of channels. With that said, if your current settings
make it easier for you to post-process your data, obviously that would a good
reason to stick with it.

Herbert

labview >> trouble reading TDMS file

by Herbert Engels » Fri, 22 Jun 2007 05:10:09 GMT

An empty numeric array will do. See .\examples\file\plat-tdms\TDMS - Read data (events).vi.
Herbert

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Thanks MikeS81, helps a lot.... problem is solved.have a nice day !Alex