Monday, July 30, 2007

Pop Quiz Answer

Nobody answered my pop quiz!

Pop Quiz: Default data on a front panel control is useful, for example, when the control is on the connector pane, but isn't wired in the caller's diagram. The subVI runs with the default value in that case. When is default data on an indicator useful?

Read more for the answer...

We pass data out of subVIs through its indicators that are on the connector pane. But what if an indicator doesn't receive any data while the subVI runs? In that case, we use the indicator's default data and pass that out to the caller.

A picture helps. Here are the two frames of a case structure...

This is an example of a conditional indicator. The indicator is only updated in one frame. If the VI never executes that frame, no data ever reaches the terminal for the indicator. When the indicator's data is passed out of the connector pane, the default data for the indicator is used in that case.

So in the example above, I made the default data for "My Conditional Indicator" the value 456. I put the "Case?" Boolean and the "My Conditional Indicator" on the connector pane and saved the VI. In the calling VI, if I pass True, I get the result "123". If I pass False, I get "456". Make sense?

Why did I bring this up in a posting about performance? Because it affects memory usage. LabVIEW has to account for two different ways that a conditional indicator can be updated (through a wire or by copying the default data). This interferes with the in-place algorithm and means that LabVIEW can't be as efficient with memory usage.

Conditional indicators aren't typically needed. I could have achieved the same effect with the following diagram...

(Or, for simple things like this, I could have used the Select function.

So, you might want to look at your own code for places you are using conditional indicators to see if you can improve your memory usage. I wouldn't worry much about scalars and other small data, but if you have large arrays or strings, this can make a difference.

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Thursday, July 26, 2007

NIWeek 2007

NIWeek 2007 is fast approaching on August 7-9. If you use NI products (or are thinking of using them), this is an awesome event. Dozens of technical sessions, amazing keynotes, and scores of exhibitors. In addition, we have special summits for Graphical System Design, RF and Wireless Communications, Sound and Vibration, and Vision applications. Register now at

Staying informed during NIWeek

  • Whether or not you are attending NIWeek, watch the NIWeek Blog by Michael Aivaliotis. Videos and stories throughout NIWeek.

  • The official NIWeek Twitter link can keep you informed of late-breaking NIWeek news. Or maybe you can just twitter each other during my presentation about how great it is. ;-)

Speaking of my presentations, I have three this year...

  • Software Engineering - The NI Way

  • Using LabVIEW in an LXI-Based Test System

  • Head-to-Head High-Speed Bus Comparison: GPIB, PCI, PCI Express, USB, and Ethernet/LAN

I'll post more information on these as we get closer.

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