TOPICS

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TOPICS

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File Input and Output

A. Understanding Resources

B. File I/O

C. File Formats

D. Creating File Paths

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A. Understanding Resources

Definition of Resources

Accessing Resources in LabVIEW

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What are Resources?

• LabVIEW includes VIs and functions to

allow you to access your resources.

• Resources are known to the system by a

path, name, port, or other identifier.

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Resource

– An addressable file,

hardware device, object, or network
connection available on your system

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Accessing Resources in LabVIEW

A typical resource operation involves the following

process:

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Open,

Initialize

or Create

Read

and/or

Write

Close

Check for

Errors

• Specify the path or device name.
• LabVIEW creates a refnum as a

unique identifier to the resource.

• A refnum is a temporary pointer to the

resource.

• The refnum becomes obsolete.

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Accessing Resources in LabVIEW

Sample refnum wires:

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B. File I/O

Understanding File I/O

File Formats

High-Level and Low-Level I/O

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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 for

Errors

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File Formats

LabVIEW can use or create the following file formats:
• Binary—Efficient, compact, and allows random access

reading but not a human-readable file. Commonly used
for high-speed and multi-channel DAQ.

• ASCII (text)—Human-readable text file where data is

represented as strings. Commonly used for low-speed
DAQ.

• LVM—Format built on ASCII, the LabVIEW measurement

data file (.lvm) is a tab-delimited text file you can open
with a spreadsheet application or a text-editing
application.

• TDMS—An NI-specific binary file format that contains

data and stores properties about the data.

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High-Level File I/O

• High-level

File I/O functions:

− 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

File I/O functions:

− Provide individual functions for

each step.

− Are efficient for writing to a file

in a loop.

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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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Understanding Low-Level File I/O
Functions

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DISCUSSION

Exercise:
Logging Data

What would happen if you used the Write to
Measurement File Express VI inside a While loop?

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C. File Formats

Text (ASCII) Files

Binary Files

TDMS Files

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Text (ASCII) Files

• Use ASCII 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 do not need to perform random access reads

or writes.

− Numeric precision is not important.

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Binary Files: Writing

Boolean

•

LabVIEW represents

Boolean values as 8-bit
values in a binary file

•

Eight zeroes represents
False : [00000000]

•

Any other value represents
True: [00000001],
[01000110], [11111111],
etc.

Integers

Binary Value U8 Value
00000000

0

00000001

1

00000010

2

11111111

255

Strings

• series of unsigned 8-bit integers, each of which has

a value in the ASCII Character Code Equivalents
Table

Arrays

• represented as a sequential list of the elements

(representation dependent on data type of element)

• a header contains a 32-bit integer representing the

size of each dimension

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Binary Files: Reading

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

Random

Sequential

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TDMS Files
(Technical Data Management
Streaming)

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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File I/O: TDMS

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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File I/O: TDMS

Properties

• You can assign properties to

• entire file

• Channel Groups

• individual Channels

• Examples:

• File: Date/Time of logging, user signed into system

• Channel Groups: Location of sensor group (top of the

bridge), types of sensors (strain, temperature, etc.)

• Channel: Sensor ID, max/min readings

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File I/O: TDMS Functions

•Use the Express VIs :

•

quick, but little control of organization

--- Write to Measurement File

--- Read from Measurement File

•Use the TDM Streaming API

• Total control over organization, more involved set-up

•Use the TDM Excel Add-In Tool

• Allows you to read TDMS files in Microsoft Excel

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D. Creating File Paths

Methods of Creating Paths

Creating Relative Paths

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Methods of Creating File and Folder
Paths

Hard-coded paths

− Useful for quick prototypes.

− Not recommended for applications.

File Dialog

− Allow user to specify the path to a file or directory.

− Customize dialog options to limit file types (*.txt).

Programmatic creation

− Create consistent filenames and extensions.

• Example: testdata_001.txt, testdata_002.txt, etc.

− Specify a consistent location.

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Creating Relative Paths

•

Relative paths set paths relative to the application or
system directory.

•

Paths differ based on the operating system and user.

•

Look up these functions and subpalette on File I/O
functions palette:

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Homework:
Write Spreadsheet File

Use a high-level file I/O VI to write
to a spreadsheet-readable file.

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Homework:
Logging Data

Modify a VI to create an ASCII file using disk
streaming.

Save (create) the file in a path relative to the VI or
project.

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Homework:
Read Spreadsheet File

Use file I/O VIs or functions to read spreadsheet
files created in the previous homeworks.