Hi Hood , thanks for the reply looks like on the CSlabs posts you have the bull by the horns and your knowledge is appreciated , my project is retrofitting a very large cnc , its mass is around 8 ton and the x axis gantry about 3 ton so which is exciting when the P gain goes into a 3 ton oscillator ! i think the problem i have is that the AC brushless drives are very large and their is a huge amount of EMF and harmonics from the 3 phase 425 v supply which is stuffing up the pc and CS labs system , I have experience with many retrofits ( Linux EMC and Fanuc ) but tuning this one has been a night mere. I am not sure like like the idea of 3 stages of PID gain / command positioning and acceleration , 1 in the servo drive , 2 in CS labs , 3 in Mach3 each one has an influence on each other , and if you dont start with a snappy well tuned servo drive its points less continuing. i am looking at big front end filters and decoupling the supply rails . thanks for your help . Peter