What does hysteresis actually do in the "Voltage - Continuous Input.vi" example?

[ASharma]

I've been working with DAQmx long enough that I feel silly I don't understand "hysteresis" in relation to analog triggering as well as I should.

For example, looking at the very basic "Voltage - Continuous Input.vi" example that ships with LabVIEW 2013 and newer:

When the Trigger Type is "Analog Start" what does the Start >> Analog Edge >> Hysteresis property actually do?  The Help for the property states "If Start.AnlgEdge.Slope is Rising, the trigger does not deassert until the source signal passes below Start.AnlgEdge.Lvl minus the hysteresis. If Start.AnlgEdge.Slope is Falling, the trigger does not deassert until the source signal passes above Start.AnlgEdge.Lvl plus the hysteresis."

Very simply put what does "deassert" mean for a Start Trigger? I mean once sampling has started what do I care if a deassertion happens or not? What am I missing?

I've attached the very example that ships with LabVIEW for ease of access. Look at the "Analog Start" case to the left of DAQmx Start Task.vi.

Thanks!

Akash

[RichieTheBard]

For that particular example, I agree with you with the fact that it doesn't serve much of a purporse, since once the sampling begins taking place the histerisys levels don't seem to matter, and neither does the deassertion of the trigger, so I'd seem that apart from cases where the noise levels could provoke an undesired trigger, these propoerty don't serve much of a fucntion for this particular VI.

[ASharma]

Thanks! Can you give me an example of where hysteresis would make sense to use? Mainly I am wondering if you need to make the trigger retriggerable for the property to be useful.

Thanks again!

[Dennis_Knutson]

You would use hysteresis when the trigger source has some noise on it. A scope manual should have an explanation and example or just Google it. It's a pretty common technique.

[ASharma]

Thanks. I understand how hysteresis works. My curiosity is how to implement it with DAQmx. I don't think it makes sense with a Start trigger (as in the "Voltage - Continuous Input.vi" example) unless the task is RETRIGGERABLE. Or does it actually act like a Pause trigger instead of a Start trigger when hysteresis is applied?

Thanks again.

[ASharma]

Apologies: I should have stated in my original post that I understand conceptually what hysteresis is about. But not how to implement it in DAQmx. It makes sense to me with Pause triggers. Not Start triggers unless the Start trigger is retriggerable. Am I wrong?