question about Val(Sgnl) property

[bmishoe]

unfortunately this plan did not pan out...I used the /= like we discussed, but the history indicator still wasn't lighting.  I hooked up probes to the Old Value and New Value parameters of the event structure.  They change from false to true as the value of the binary input from the FPGA comes in...but they change at the same time.  In other words Old Value and New Value always match each other.  How can this be?

[Eric1977]

I think we are going to have to see the entire VI that you have and any SubVIs you've created that are a apart of your project. Pictures can only tell us so much, but if we get your code, we can probably pinpoint the problem(s) that your having.

[0utlaw]

Hi Bmishoe,

It looks to me as though the old and new value are the same because you are also writing to the indicator directly - there's something of a race condition here because both the property node and indicator are being set by a branched wire, but since property nodes take a bit longer to execute I'm betting the direct wire to the control indicator wins out 99.9% of the time.

What I think is happening:

-new value comes in

-indicator gets set to new value

-property node also sets new value, (identical) value just sent to indicator is now the "old" value

-event structure sees two identical values.

You probably just need to remove the direct connection to the indicator.

Regards,

[bmishoe]

But where would I make the connection?  I was under the impression that you had to write to the Val(Sgnl) property of an indicator if you want to use said indicator in an event structure.  So don't I need to write to that property each time I write to the indicator itself?

[0utlaw]

Hi Bmishoe,

That is correct (when using indicators), my suggestion was that you don't write to the indicator, just the val(signaling) property.  This will set the indicator value and trigger your event.  The indicator doesn't need to be wired at all.

Regards,

[billko]

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@bmishoe wrote:

But where would I make the connection?  I was under the impression that you had to write to the Val(Sgnl) property of an indicator if you want to use said indicator in an event structure.  So don't I need to write to that property each time I write to the indicator itself?

---

By writing to that property, you are also writing to the control.  By writing to the control itself also, you are writing to it twice.  The explanation for the old and new values being the same is because by the time the Val(Sgnl) event is fired, the direct write to the control has already overwritten the data, so what is new is old again.  😉

That's my take on the explanation anyway.  🙂

[bmishoe]

OK, you guys have been so much help, and this largely fixed my problem...BUT...it exposed something else that's related to what an originaly reply noted about how the value change structure is triggered just by the reference being written to, regardless of whether the value changed or not.

It would be difficult to explain here because it involves the conditions under which the FPGA sends me the value of the address, but anyway...what will fix it is a way for me to able to only write to the Val(Sngl) property *if* the value is different than it was the last time that address was read...is there a way to do this?  What I need is like a buffer that can store the previous update...can't really picture how to do that right now... Again it's a long, complex reason as to why..

Thanks for all the help you've been.

[RavensFan · Accepted solution]

A feedback node.  Feed the wire into that.  Then you will have the value from the previous iteration as well as the current iteration to compare.

[bmishoe]

Perfect!  It looks like it's all working now....thank you all so much!

[billko]

Excellent!  🙂