Workshop #4 File Input and Output

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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.


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