cheers for the support.

i had another play around. i kept motor rated speed at 24,000rpm, and set the line speed display to 24,000 rpm. this gives me what i would expect (10 hz = 600rpm, 20 hz = 1200 rpm, 100hz = 6,000 rpm and so on) but once it gets above 10,512 rpm, the rpm readout starts getting a bit funny (goes back to zero and starts counting up again)(although spindle itself rpm carries on increasing nicely all way up to 400hz). so quirky rpm readout aside, i think its all working properly. i am jut using the frequency readout at the minute. i have ordered a cheap laser tachometer which should arrive on monday. then i can tell if i am where i'm supposed to be with regards to rpm and frequencies.

all good fun...