Saturday, September 25, 2010

Error Clusters

Summary: In this section, you will learn about LabVIEW Error Clusters.
No matter how confident you are in the VI you create, you cannot predict every problem a user might encounter. Without a mechanism to check for errors, you know only that the VI does not work properly. Error checking tells you why and where errors occur.
When you perform any kind of I/O, consider the possibility that errors will occur. Almost all I/O functions return error information. Include error checking in VIs, especially for I/O operations such as file, serial, instrumentation, data acquisition, and communication operations, and provide a mechanism to handle errors appropriately.
Checking for errors in VIs can help you identify the following problems:
  • You initialized communications incorrectly or wrote improper data to an external device.
  • An external device lost power, is broken, or is not working properly.
  • You upgraded the operating system software, which changed the path to a file or the functionality of a VI or library. You might notice a problem in a VI or a system program.

Error Handling

By default, LabVIEW automatically handles any error that occurs when a VI runs by suspending execution, highlighting the subVI or function where the error occurred, and displaying a dialog box. You can choose other error handling methods. For example, if an I/O VI on the block diagram times out, you might not want the entire application to stop. You also might want the VI to retry for a certain period of time. In LabVIEW, you can make these error handling decisions on the block diagram of the VI.
VIs and functions return errors in one of two ways-with numeric error codes or with an error cluster. Typically, functions use numeric error codes, and VIs use an error cluster, usually with error inputs and outputs.
Error handling in LabVIEW follows the dataflow model. Just as data flow through a VI, so can error information. Wire the error information from the beginning of the VI to the end. Include an error handler VI at the end of the VI to determine if the VI ran without errors. Use the error in and error out clusters in each VI you use or build to pass error information through the VI.
As the VI runs, LabVIEW tests for errors at each execution node. If LabVIEW does not find any errors, the node executes normally. If LabVIEW detects an error, the node passes the error to the next node without executing. The next node does the same thing, and so on. Use the Simple Error Handler VI, shown in Figure 1, to handle the error at the end of the execution flow. The Simple Error Handler VI is located on the Functions>>All Functions>>Time & Dialog palette. Wire the error cluster to the error in input.
Figure 1
Figure 1 (SimErrorHand.png)

Error Clusters

The error clusters located on the Functions>>All Functions>>Array & Cluster palette include the components of information shown in Figure 2.
Figure 2
Figure 2 (errorcluster.png)
  • status is a Boolean value that reports True if an error occurred. Most VIs, functions, and structures that accept Boolean data also recognize this parameter. For example, you can wire an error cluster to the Boolean inputs of the StopQuit LabVIEW, or Select functions. If an error occurs, the error cluster passes a True value to the function.
  • code is a 32-bit signed integer that identifies the error numerically. A non-zero error code coupled with a status of False signals a warning rather than a fatal error.
  • source is a string that identifies where the error occurred.
Use the error cluster controls and indicators to create error inputs and outputs in subVIs.

Explain Error

When an error occurs, right-click within the cluster border and select Explain Error from the shortcut menu to open the Explain Error dialog box. The Explain Error dialog box contains information about the error. The shortcut menu includes an Explain Warning option if the VI contains warnings but no errors.
You also can access the Explain Error dialog box from the Help>>Explain Error menu.

Using While Loops for Error Handling

You can wire an error cluster to the conditional terminal of a While Loop to stop the iteration of the While Loop (Figure 3). When you wire the error cluster to the conditional terminal, only the True or False value of the status parameter of the error cluster is passed to the terminal. When an error occurs, the While Loop stops.
Figure 3
Figure 3 (errorwhile.png)
When an error cluster is wired to the conditional terminal, the shortcut menu items Stop if True and Continue if True change to Stop on Error and Continue while Error.

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