This project was born out of another project which used hardware based counters to track position for both PPI and engraving.  Since the speeds are much lower with vector cutting and there are quite a few people out there who use Retina engrave and/or a DSP which currently do not do PPI, there was a lot of interest in having a simple hardware add on to enable that kind of control.  This code will work on a single Atmega 328 chip to do PPI and will be put on a custom board that can plug right into a 25 pin parallel port.  The 328 is fast enough to handle PPI at almost any reasonable cutting speed for low powered home built CNC lasers.  The ardware will work with any stepper based controller out there and will pulse a laser at sub milisecond pulse widths (however most dc discharge lasers won't fully turn on in less than 2ms).  It works by listening to the step and direction pulses of the x/y axis and counts them only when the laser is on for a cut.  A bit of testing is needed to find out how fast this controller can go, but it should work fine with any cut speed less than 800mm/min.