Gruppo di Misure Meccaniche e Termiche UNIBS - DIMI A. File Formats At their lowest level, all files...

Gruppo di Misure Meccaniche e TermicheUNIBS - DIMI

A. File Formats

At their lowest level, all files written to your computer’s hard drive are a series of bits

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Binary

TDMS

ASCII


A. File FormatsASCII TDMS Direct Binary

Numeric Precision

Good Best Best

Share data

Best (Any program easily) Better (NI Programs easily)

Good (only with detailed metadata)

Efficiency Good Best Best

Ideal Use Share data with other programs when file space and numeric precision are not important

Share data with programs when storing simple array data and metadata

Store numeric data compactly with ability to random access


A. Understanding File I/O

• File I/O writes to or reads from a file• A typical file I/O operation involves the following process:

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Open/Create/

Replace File

Read and/or

Write to File

Close File

Check forErrors


A. Understanding File I/O – File Formats

LabVIEW can use or create the following file formats:• Binary—Underlying file format of all other file formats• ASCII—Specific type of binary file that is a standard used by

most programs• TDMS—Type of binary file created for NI products consisting

of two separate files: a binary file and a binary index file

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A. Understanding File I/O – File Formats

• Use text files in the following situations: − You want to access the file from another application− Disk space and file I/O speed are not crucial− You must not perform random access reads or writes− Numeric precision is not important

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B. Understanding High-level File I/O

• High-level VIs− Perform all three steps (open,

read/write, close) for common file I/O operations

− Might not be as efficient as the functions configured or designed for individual operations

• Low-level VIs− Individual VI for each step− If you are writing to a file in a loop, use

low-level file I/O functions

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B. Understanding High-Level File I/O

Write to Spreadsheet File• Converts an array of double-precision numbers to a text string and

writes the string to an ASCII file

Read From Spreadsheet File• Reads a specified number of lines or rows

from a numeric text file and outputs a 2D array of double-precision numbers

Write to/Read from Measurement File• Express VIs that write data to or read data

from an LVM or TDMS file format

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B. Binary Files—Sequential/Random Access

Two methods of accessing data: • Sequential Access—Read each item in order, starting at

the beginning of a file• Random Access—Access data at an arbitrary point within

the file

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B. Binary Files—Sequential Access

• To sequentially access all of the data in a file, you can call the Get File Size function and use the result to calculate the number of items in the file, based upon the size of each item and the layout of the file• You can then wire the number of items to the count terminal

of the Read from Binary File function

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B. Binary Files—Random Access

• Use the Set File Position function to set the read offset to the point in the file you want to begin reading

• The offset is in bytes; therefore, you must calculate the offset based upon the layout of the file

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C. TDMS Files

TDMS• Technical Data Management Streaming

Use TDMS files for the following purposes: • To store test or measurement data• To create a structure for grouping your data• To store information about your data• To read and write data at high speeds

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C. TDMS Files—File Format

• TDMS file− Binary file (.tdms) that contains data and stores properties

about the data• TDMS_Index file

− Binary index file (*.tdms_index) that provides consolidated information on all the attributes and pointers in the TDMS file

− Speeds up access to the data while reading− Automatically regenerated if lost• TDMS file format internal structure is publicly documented

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C. TDMS Files—Data Hierarchy

• Channel − Stores measurement signals or raw data in a TDMS file− Each channel can have properties describing the data− The data stored in the signal is stored as binary data on disk to

conserve disk space and improve efficiency

• Channel Group− Segment of a TDMS file that contains properties and one or more

channels− Use channel groups to organize your data and to store information

that applies to multiple channels

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C. TDMS Files—Accessing TDMS Files

• Use the Write to Measurement File and Read from Measurement File Express VIs − Allows you to quickly save and retrieve data from TDMS format − Very little control over your data grouping and properties• Use the TDM Streaming API

− Set of functions for opening, writing to, reading from, and closing TDMS files

− Allows you to organize your data into channel groups and channels• Use the TDM Excel Add-In Tool

− Allows you to read TDMS files in Microsoft Excel

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C. TDMS Files—TDM Streaming API

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C. TDMS Files—Write Data

• Streams data to the specified TDMS file

• Data subset to write is determined by group name and channel name(s) inputs

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C. TDMS Files—Read Data

Reads the specified TDMS file and returns data from the specified channel and/or channel group

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C. TDMS Files—Set Properties

Sets the properties of the TDMS file, channel group, or channel

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C. TDMS Files—Get Properties

Returns the properties of the TDMS file, channel group, or channel

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C. TDMS Files—File Viewer

Opens TDMS file and presents the file data in the TDMS File Viewer dialog box

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Practice

Generate a set of waveforms and export them as a single CSV file.

Compute the waveforms PSDs and export them as a single CSV file.

Open both files in excel and check the results.

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