TOPICS
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File Input and Output
TOPICS
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?
Resource  An addressable file,
hardware device, object, or network
connection available on your system
" 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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Accessing Resources in LabVIEW
A typical resource operation involves the following
process:
Open, Read
Check for
Initialize and/or Close
Errors
or Create Write
" Specify the path or device name.
" The refnum becomes obsolete.
" LabVIEW creates a refnum as a
unique identifier to the resource.
" A refnum is a temporary pointer to the
resource.
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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:
Open/ Read
Close Check for
Create/ and/or
File Errors
Replace File Write to File
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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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Exercise:
Logging Data
What would happen if you used the Write to
Measurement File Express VI inside a While loop?
DISCUSSION
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 Strings
" LabVIEW represents " series of unsigned 8-bit integers, each of which has
Boolean values as 8-bit a value in the ASCII Character Code Equivalents
values in a binary file Table
" Eight zeroes represents Arrays
False : [00000000] " represented as a sequential list of the elements
" Any other value represents (representation dependent on data type of element)
True: [00000001], " a header contains a 32-bit integer representing the
[01000110], [11111111], size of each dimension
etc.
Integers
Binary Value U8 Value
00000000 0
00000001 1
00000010 2
11111111 255
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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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