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The Ramp node increases or decreases in a linear way the value of the Output connector until it reaches the value of the Input connector. The linear increment consists of adding (or subtracting) to the Output a value represented by the Step size property every time interval specified by the Time step property.
The value of the Output can't go beyond the Input value, so the nodes reduces the step to adapt the output to the input.

 Behaviour of Ramp