With PC's you want to remove them from the real time aspect of things - nevertheless with a P4 running Windows you have quite a bit of computing horsepower.You can always have the PC pre calculate the positioning stuff beforehand and then output the precalculated data to a micro better suited to the positioning task at hand. You could even have the 8051 do the calcs beforehand! Another solution is to use another o/s on the PC that is more suited to real time stuff.