31 TDMS

TDM (Technical Data Management) is a data model and file format from National Instruments that defines how test and measurement data should be organized. TDMS (Technical Data Management Streaming) is a newer, more efficient file format that implements this model by storing both the data and descriptive metadata in a single, structured binary file.

Quick View

The in LabVIEW built-in TDMS File Viewer can be used to quickly have a look at .tdms files, without Excel or Diadem.

Flush

Think about usage of the TDMS Flush function instead of opening and closing the file constantly, depending on your usecase.

Attributes

Misc
Name	Level	Example	Required?	Description
`datetime`	File		no	Start DateTime of the whole TDMS file
`unit_string`	Channel		no	String representation of the channel unit
`NI_ChannelName`	Channel	“temp_1”	no	the name that appears in the plot legend of a LabVIEW chart/graph

Standard Waveform Property Names [1]
Name	Level	Example	Required?	Description
`wf_xname`	Channel	Time	yes	Name of the x-axis quantity
`wf_xunit_string`	Channel	s	yes	Unit of the x-axis quantity
`wf_start_offset`	Channel	0	yes	Start offset value of the x-axis
`wf_increment`	Channel	0.001	yes	Increment value of the x-axis
`wf_start_time`	Channel		no	Start DateTime value of the time axis
`wf_samples`	Channel	1000	yes	Number of values of the x-axis

XY Channel Relationships [2]
Name	Level	Example	Required?	Description
`wf_xcolumns`	Group	“one”	no	the first channel in that group is the time channel
`xchannel`	Group	“Time”	no	name of the time channel in that channel group

Properties to Improve Loading Speed in NI DIAdem
Name	Level	Example	Required?	Description
`minimum`	Channel	-3.14	no	The minimum value of the channel
`maximum`	Channel	3.14	no	The maximum value of the channel
`monotony`	Channel	Not monotone	no	If the channel is monotone rising or falling
`novaluekey`	Channel	No	no	If any NaN values are in the channel

[1] Note that the property names are case-sensitive and must be in lowercase. The wf_xname and wf_xunit_string properties are not set by default in NI LabVIEW— you need to add those properties yourself to every waveform channel in the TDMS file.

[2] See “Non-Equidistant Data” below.

Non-Equidistant Data

Acquired data channels must have associated timing information. For constant sampling rates (equidistant samples), timing information is contained implicitly via t0 and dt. For non-equidistant samples (acquired in a non-deterministic way or without constant sampling rate), timing information must be provided explicitly.

The clearest and simplest approach is to always have only one explicit time channel inside each channel group and to always position the explicit time channel as the first channel in that channel group. This leads to two common cases: one explicit time channel plus one acquired data channel in each channel group (XY) or one explicit time channel plus multiple acquired data channels in each channel group (XYYY).

If you adopt this suggestion to have only one time channel in each channel group, then you should set the wf_xcolumns property on the channel group level to have the value “one”, which will tell all NI software that the first channel in that group is the time channel. If for some reason your time channel is not first, you can set the xchannel property on the channel group level and fill it with the name of the time channel in that channel group.

TDMS files offer a native datetime data type for both properties and data channels. When you save datetime information, make sure to always use the built-in datetime option. Writing a numeric value of elapsed seconds is not sufficient to record a datetime, because different applications that write and read TDMS files have different conventions for the starting datetime value and even the increment metric (seconds as opposed to days). For NI LabVIEW programs, you should always wire a brown datetime wire directly to the property value or channel data input.

(Source: http://www.ni.com/white-paper/14252/en/)

Test of TDMS write performance

All test are executed with the LV2016 project “TDMS-Perf-Test” from the GitLab repo https://code.hampel-soft.com/hse-labs/proofs-of-concepts.

Windows

Tested in a Win 7 VM (2 cores, 6GB RAM, SSD) on a Dell XPS13 notebook.

Conditions: TDMS Write data layout: decimated, TDMS Flush after every write. Runtime 2 minutes.

Block size	Block rate (ms)	# Channels	kBytes per block	Ø Write time (s)	Ø Write rate (kB/s)	File size (kB)	Index size (kB)
100	100	16	12,50	0,043	109	7.491	4
250	250	16	31,25	0,047	740	7.488	4
500	500	16	62,50	0,046	1.534	7.504	4
1000	1000	16	125,0	0,053	2.636	7.504	4

Linux RT

Tested with a cRIO 9030 (firmware version 4.0).

Conditions: TDMS Write data layout: decimated. Runtime 2 minutes.

Block size	Block rate (ms)	# Channels	kBytes per block	Ø Write time (s)	Ø Write rate (kB/s)	File size (kB)	Index size (kB)
100	100	16	12,50	0,002	6.059	7.404	4
250	250	16	31,25	0,002	14.314	7.457	4
500	500	16	62,50	0,002	26.886	7.473	4
1000	1000	16	125,0	0,003	49.591	7.504	4