【正文】 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 the block diagram to configure channels and tasks for use with NIDAQmx for data is a programming interface for municating with data acquisition devices. You can use the DAQ Assistant Express VI to control devices supported in NIDAQmx. Refer to the Taking an NIDAQmx Measurement in LabVIEW help tutorial for information about the DAQ Assistant. To launch this help tutorial, select Help187。 利用空模板創(chuàng)建新的 VI 在下面的練習(xí)中，您將要打開一個(gè)空的 VI，并在程序框圖中添加 Express VIs 和結(jié)構(gòu)，來(lái)創(chuàng)建一個(gè)新的 VI 程序。 圖 31 Reduce Samples VI 的前面板 打開一個(gè)空 VI 如果沒有模板適合您需要實(shí)現(xiàn)的任務(wù)，您可以從一個(gè)新 VI 開始，添加 Express VIs 來(lái)實(shí)現(xiàn)指定的任務(wù)。 3 注意 在 New 對(duì)話框的 Create new 列表內(nèi)單擊 Blank VI，或者從前面板或程序框圖的菜單欄中單擊 FileNew VI，都可以打開一個(gè)空 VI。 添加一個(gè)模擬信號(hào)的 Express VI 完成下面的步驟，查找您想要使用的 Express VI，并將其添加到程序框圖中。 注意， Context Help 窗口中顯示每個(gè) Express VI 的功能的信息。 6. 配置 Simulate Signal Express VI，以產(chǎn)生一個(gè)頻率為 、振幅為 2的正弦波。注意，此時(shí) Context Help 窗口內(nèi)顯示出 Simulate Signal Express VI的配置。HowTo Help，打開 LabVIEW Help。 4. 熟悉該 Express VI 的描述后，點(diǎn)擊在左邊 Place on the block diagram 按鈕，來(lái)選擇該 Express VI。 7. 配置 Sample Compression Express VI，指定平均因子為 25，來(lái)減少信號(hào)的樣本點(diǎn)數(shù)。您也可以在程序框圖中添加控制件和顯示件。 3. 右擊 Enable 輸入端，單擊 CreateControl 來(lái)創(chuàng)建 Enable 開關(guān)控件。 5. 利用連線工具，將 Sample Compression Express VI 的 Mean 輸出端連接到 Sine 控件終端。 5 圖 32 Reduce Samples VI 的程序框圖 7． 顯示前面板。為了在某條件滿足前 VI 可一直運(yùn)行，可在程序框圖中加入 While 循環(huán) (While Loop)。 3. 移動(dòng)光標(biāo)至程序框圖的左上角，在此放置 While Loop 邊框的 左上角。 6 5. 顯示前面板，運(yùn)行 VI。 圖 33 放置環(huán)繞 Express VIs 的 While Loop 注意， While Loop，如左圖所示，顯示有一個(gè) STOP 按鈕， 與條件 (Condition Terminal)相連線。 注意 VI 運(yùn)行后就停止，且沒有 STOP 按鈕。 8． 保存此 VI 程序。 6. 排列程序框圖中的對(duì)象，如圖 32 所示。 注意，您可以從程序框圖中創(chuàng)建控制件和顯示件。 1. 右擊 Smaple Compression VI 的 Mean 輸出，單擊 CreateNumeric Indicator 來(lái)創(chuàng)建一個(gè)數(shù)字顯示件。 9. 利用連線工具，將 Simulate Signal Express VI 的 Sine 輸出端連接到 Sample Compression Express VI 的 Signals 輸入端。 注意， LabVIEW 使 Sample Compression Express VI 跟隨著光標(biāo) 移動(dòng)。 4 注意， 該單詞的選擇反映出您需要 Express VI 壓縮還是減少信號(hào)采樣點(diǎn)。 調(diào)整信號(hào) 完成下面的步驟，利用 LabVIEW Help，查找可以使信號(hào)采樣點(diǎn)減少的 Express VI。 8. 關(guān)閉 Configure Simulate Signal 對(duì)話框。 4. 選擇 Express VI 并將其放置到程序框圖內(nèi)，出現(xiàn) Configure Simulate Signal 對(duì)話框。您也可以按 Show Context Help Window（如左圖所示），打開 Context Help 窗口。點(diǎn)擊在 Functions 選板左上方的圖釘圖標(biāo)，如左圖所示，則把該模板放置在屏幕上。 1. 在 LabVIEW 對(duì)話框，點(diǎn)擊 New 按鈕上的箭頭，從快捷菜單中單擊 Blank VI 或快捷鍵 CtrlN，來(lái)打開一個(gè) 空 VI。當(dāng)您完成練習(xí)時(shí)， VI 程序的前面板如圖 31 所示。然而 ,在某些情況下模板并不適用于您需要?jiǎng)?chuàng)建的 VI程序。Graph Indicator. Notice that you can create controls and indicators from the block diagram. When you reate controls and indicators using this method, LabVIEW automatically creates terminals that are labeled and formatted correctly. 5. Using the Wiring tool, wire the Mean output in the Sample Compression Express VI to the Sine terminal. Notice that the Merge Signals function appears. 6. Arrange the objects on the block diagram so that they appear similar to Figure 32. Tip You can rightclick any wire and select Clean Up Wire from the shortcut menu to have LabVIEW automatically arrange the wires for you. Figure 32. Block Diagram for the Reduce Samples VI 7. Display the front panel. Notice that the controls and indicators you added automatically appear on the front panel with labels that correspond to their function. 8. Save this VI. 5 Configuring the VI to Run Continuously Until the User Stops It In the current state, the VI runs once, generates one signal, then stops executing. To run the VI until a condition is met, you can add a While Loop to the block diagram. Complete the following steps to add a While Loop. 1. Display the front panel and run the VI. Notice how the VI runs once and then stops. Also notice how there is no STOP button. 2. Display the block diagram and select the While Loop on the Execution Control palette. 3. Move the cursor to the upper left c