How Hysteresis for variables works?

How Hysteresis for variables works?

Hi,
Now I am trying to directly set a Hysteresis for a variable. I have a variable given by a 61850 driver. In its properties, [Value calculation] -> [Hysteresis], I have set both the [negative] and [positive] to 20.
But it seems not work.. In my Numerical Value object of RT picture, this value changes all the times, even if a little variation occurs.
So, I want to ask how the hysteresis works? Or I forgot to change other settings to make it work?

This is a migrated post! Originally posted on 27.04.2009 by user lephis. Please be aware that information can be outdated.