Control.com - Nerds in Control

I need to run the stepper motor, the problem is that the Logic (MATLAB code (Simulink)) will be interfaced by the "dSpace (Control Desk NGV version 4)", which does not support the Stepper motor block, available in the Simulink Library.

In this case I can only use the "Pulse generator block".

What I need to do: Pulse generator will continuously keep generating the pulses and two (Constant Blocks) will take the input and generate the pulses unless the pulse reaches the second input block value, and then it should remain at zero.

Eg: I/P(1) = 0, I/P(2) = 20, so a Pulse generator will generate the pulses for 20 and then it should be stopped.

The basic problem is that all of the comparison block with whom I am familiar will make the decision by looking at the "Amplitude (values on Y axis)" and I need to compare or want to make a decision by Looking the values on the (x-axis).