mach3turn spindle sensing problem

[GDoc]

hello, I'm new to mach3. i have set up a cnc lathe, both the z and x axis are working fine , my problem is with the spindle speed sensing. i have a single slot disc with optical sensor. the motor speed is controlled by an external dc drive with a pot of setting the speed. with mach3turn running and motor running the DRO does not indicate the true speed, it seems to top out at 160rpm when the true speed is a lot more. the confusing thing is that if i close down mach3turn and start up mach3mill with the same settings, i get true spindle speed!

what i am i doing wrong?