Converter Step/Dir to RC PWM

In one of my latest projects for making a clone of AxiDraw (see links below) I used Arduino Uno GRBL (GRBL set to CoreXY) to drive the machine.

http://www.thingiverse.com/thing:1796957
http://www.thingiverse.com/thing:1808184

When it comes to drive the Z axis the servo motors are the best choice - they are lightweight and fast compared to steppers. Unfortunately GRBL has no PWM output to control the Z axis so I decided to make a small project to convert the STEP and DIRECTION signals