【正文】 g box. 7. In the New Name text box, enter First Voltage Reading, and click the OK button. 8. Click the OK button to apply this configuration and return to the block diagram. 9. Display the front panel and run the VI. Notice that First Voltage Reading appears in the waveform graph plot legend. 10. Save this VI. 44 12 Editing an NIDAQmx Task You can add a channel to the task so that you can pare two separate voltage readings. You also can customize the task to acquire the voltage readings continuously. Complete the following steps to add a new channel to the task and acquire data continuously. 1. Display the block diagram and doubleclick the DAQ Assistant Express VI to add a new channel. 2. Click the Add Step button, shown at left, to open the Add Channels To Task dialog box. 3. Select any unused physical channel in the My Physical Channels listbox. 4. Click the OK button to return to the DAQ Assistant. 5. Rename the channel Second Voltage Reading. 6. In the Task Timing tab, select the Acquire Continuously you set timing and triggering options in the DAQ Assistant, these options apply to all the channels in the Channel List. 7. Click the OK button to apply this configuration and return to the block diagram. 8. Place a While Loop around the DAQ Assistant Express VI and the graph indicator wired to the data output. The block diagram should appear similar to Figure 42. Figure 42. Block Diagram for the Read Voltage VI 13 Visually Comparing Two Voltage Readings Because you have two voltage readings displayed on a graph, you can customize the plots to distinguish between the two. Complete the following steps to customize the plot color on the waveform graph. 1. On the front panel, expand the plot legend to include two plots. 2. Run the VI. Notice two plots now appear in the graph and the plot legend automatically updates to include both plot names. 3． Rightclick First Voltage Reading and select Color from the shortcut menu. Using the Color picker, select a color, such as yellow, so that the plot is easy to read. Change the plot color of Second Voltage Reading. 4. Save this VI. Communicating with an Instrument Instrument drivers simplify instrument control and reduce test program development time by eliminating the need to learn the programming protocol for each instrument. Use an instrument driver for instrument control when possible. National Instruments provides instrument drivers for a wide variety of instruments. Visit the NI Instrument Driver Network at If a driver is not available for your instrument, you can use the Instrument I/O Assistant Express VI to municate with your instrument. Complete the following exercises to municate with an instrument. Selecting an Instrument Before municating with an instrument, you must select the instrument with which you want to municate. Complete the following steps to select an instrument using the Instrument I/O Assistant Express VI. 1. Make sure you turn on the instrument you want to use. The instrume nt must be powered on to use the Instrument I/O Assistant Express VI. 2. Select the Instrument I/O Assistant Express VI on the Input palette and place it on the 14 block diagram. 3. Click the Show Help button, shown at left, in the upper right corner of the Instrument I/O Assistant dialog box. Notice how the Show Help button displays the help to the right of the dialog box. The help window on top contains procedural information about using the Instrument I/O Assistant. The help window below provides contextsensitive help about various controls and indicators in the dialog box. 4. Follow the procedures in the help window on top to select the instrument with which you want to municate. 5. If necessary, configure the properties of the instrument. 6. Click the Hide Help button, shown at left, in the upper right corner of the Instrument I/O Assistant dialog box to minimize the help window. Acquiring and Parsing Information for an Instrument After selecting the instrument, you can send mands to the instrument to retrieve data. In this exercise, you will learn to use the Instrument I/O Assistant Express VI to acquire and parse identification information for an instrument. Complete the following steps to municate with your instrument. 1. Click the Add Step button, and select Query and Parse. 2. Enter *IDN? in the Enter a mand text box. *IDN? is a query that most instruments recognize. The response is an identification number string that describes the instrument. If the instrument does not accept this mand, refer to the reference manual for the instrument for a list of mands the instrument does understand. 3. Click the Run Sequence button. The Instrument I/O Assistant sends the mand to the instrument, and the instrument returns its identification information. 4. Parse the instrument name as an ASCII string. You also can use Instrument I/O Assistant to parse ASCII numbers and binary data. 5. Click the Parsing help button, shown at left, in the Instrument I/O Assistant dialog box for more information about parsing data. 6. Assign a name to the token in the Token name text box. A token is a parsed data selection. 7. Click the OK button to return to the block diagram. Notice that the name that you entered in the Token name text box is the output of the Instrument I/O Assistant Express VI, shown at left. Summary The following topics are a summary of the main concepts you learned in this chapter. DAQ Assistant Express VI You can use the DAQ Assistant Express VI to graphically configure channels or mon measurement tasks. Using the DAQ Assistant Express VI, you can interactively build a measurement channel or task. Place the DAQ Assistant Express VI on t