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Square Root VI

Module by: National Instruments

Summary: In this exercise, the objective is to use the Case structure in a VI. Complete the following steps to build a VI that checks whether a number is positive. If the number is positive, the VI calculates the square root of the number. Otherwise, the VI returns an error message.

Problem 1

1.a) Front Panel

  1. Open a blank VI and build the front panel shown in Figure 1.
    Figure 1

1.b) Block Diagram

  1. Build the block diagram shown in Figure 2.
    Figure 2
    1. Place the Case structure, located on the Functions>>Execution Control palette, on the block diagram. Click the decrement or increment button to select the False case.
    2. Place the Greater or Equal to 0? function, located on the Functions>>Arithmetic & Comparison>>Express Comparison palette, on the block diagram. This function returns True if Number is greater than or equal to 0.
    3. Right-click the numeric constant and select Properties from the shortcut menu. Select the Format and Precision tab. Set Digits of precision to 1, select Floating point notation, and click the OK button to ensure there is no data conversion between the constant and the numeric indicator outside the Case structure.
    4. Place the One Button Dialog function, located on the Functions>> All Functions>>Time & Dialog palette, on the block diagram. This function displays a dialog box that contains the message Error...Negative Number.
    5. Right-click the message terminal of the One Button Dialog function, select Create>>Constant from the shortcut menu, type Error...Negative Number in the constant, and click the Enter button on the toolbar or click outside the control. Refer to the lesson on Strings and File I/O for more information about strings.
    6. Complete the diagram as shown in Figure 2.
  2. Select the True case of the Case structure. Place the Square Root function, located on the Functions>>Arithmetic & Comparison>>Express Numeric palette, on the block diagram. This function returns the square root of Number. Wire the function as shown in Figure 3.
    Figure 3
  3. Save the VI as Square Root.vi in the C:\Exercises\LabVIEW Basics I directory.

1.c) Run the VI

  1. Display the front panel and run the VI.
    caution: Do not run this VI continuously. Under certain circumstances, continuously running this VI could result in an endless loop.
    If Number is positive, the VI executes the True case and returns the square root of Number. If Number is negative, the VI executes the False case, returns -99999, and displays a dialog box with the message Error...Negative Number.
  2. Close the VI.

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