Chapter 5 Introduction to Data Acquisition in LabVIEW

If the limit settings cluster array has fewer elements than the channel string array, LabVIEW assigns any remaining channels the limit settings contained in the last entry of the limit settings cluster array. Figure 5-13 illustrates this case.

Figure 5-13. Limit Settings, Case 2

In this example, channels 0, 1, 2, and 3 have limits of 10.00 to –10.00. There are more channels left, but the limit settings cluster array is exhausted. Therefore, the remaining channels (4, 5, 6, and 7) are also assigned limits of 10.00 to –10.00.

The Easy Analog Input VIs have only one pair of input limits. This pair forms a single cluster element. If you specify the default limit settings, all channels scanned with these VIs have identical limit settings. The Easy Analog Output VIs do not have limit settings. All the Intermediate VIs, both analog input and output, have the channels array and the limit settings (or input limits) cluster array on the same VI. Assignment of limits to channels works exactly as described above.

In analog applications, you not only specify the range of the signal, you also must specify the range and the polarity of the device. A unipolar range is a range containing either positive or negative values, but never both.

A bipolar range is a range that has both positive and negative values. When a device uses jumpers or DIP switches to select its range and polarity, you must enter the correct jumper setting in the configuration utility.

In DAQ hardware manuals and in the configuration utility, you may find reference to the concept of gain. Gain is the amplification or attenuation of a signal. Most National Instruments DAQ devices have programmable gains (no jumpers), but some SCXI modules require the use of jumpers or DIP switches. Limit settings determine the gain for all DAQ devices used

Chapter 5 Introduction to Data Acquisition in LabVIEW

with LabVIEW. However, for some SCXI modules, you must enter the gain in the configuration utility.

Other DAQ VI Parameters

The device input on analog I/O, digital I/O, and counter VIs specifies the number the DAQ configuration software assigned to your DAQ device. Your software assigns a unique number to each DAQ device. The device parameter usually appears as an input to the configuration VIs. Another common configuration VI parameter, task ID, assigns your specific

I/O operation and device a unique number that identifies it throughout your program flow.

Some DAQ VIs perform either the device configuration or the

I/O operation, while other DAQ VIs perform both configuration and the operation. Some of the VIs that handle both functions have an iteration input. When your VI has the iteration set to 0, LabVIEW configures the DAQ device and then performs the specific I/O operation. For iteration values greater than 0, LabVIEW uses the existing configuration to perform the I/O operation. You can improve the performance of your application by not configuring the DAQ device every time an I/O operation occurs. Typically, you should wire the iteration input to an iteration terminal in a loop as shown in Figure 5-14.

Figure 5-14. Wiring the iteration Input

Wiring the iteration input this way means the device is configured only on the first I/O operation. Subsequent I/O operations use the existing configuration.