Saturday, September 25, 2010

Digital Example VI

Summary: In this exercise, the objective is to control the digital I/O lines on the DAQ device. Complete the following steps to complete a VI that turns on the LEDs of Port 0 on the DAQ Signal Accessory based on the digital value set on the front panel. Each LED is wired to a digital line on the DAQ device. The lines are numbered 0, 1, 2, and 3, starting with the LED on the right.



The LEDs use negative logic. That is, writing a 1 to the LED digital line turns off the LED. Writing a 0 to the LED digital line turns on the LED.
  1. Open the Digital Example VI, located in the C:\Exercises\LabVIEW Basics I directory, and modify the block diagram as shown in Figure 1.
    Figure 1
    Figure 1 (digitalexamplebd.png)
    DAQAssistantIcon.png Place the DAQ Assistant Express VI, located on the Functions>>Input palette, in the While Loop. Complete the following steps to configure the counter to perform event counting.
    1. Select Digital I/O>>Port Output for the measurement to make.
    2. Select Dev1>>port0 for the physical channel and click the Finish button.
    3. In the Digital Output Port Task Configuration dialog box that appears, select Invert All Lines In Port because the LEDs use negative logic.
    4. Click the OK button to close the configuration dialog box. All of the settings specified for the task are saved internally in the DAQ Assistant VI.
    BuildArrayicon.png Place the Build Array function, located on the Functions>>All Functions>>Array palette, on the block diagram. Wire the Build Array function to the DAQ Assistant. Complete the wiring of the block diagram. The Boolean buttons on the front panel are stored in an array to simplify the code. The Array Subset function extracts only the first four elements in the array. The output of the array subset needs to be reversed because element 0 of the array is the most significant bit. The array is then converted to a number with the Boolean Array to Number function, and converted into an array of one element. This value is passed to the DAQ Assistant Express VI to write that value to the port.
  2. Save the VI.
  3. Display the front panel and run the VI. Turn the Boolean LEDs on and off and observe the changes on the DAQ Signal Accessory.
  4. Stop and close the VI.

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