Make sure to hit the enter key after entering the number.this is unpredictable some times it lets me enter a minimum some times not (I have been unable to determine what causes success or failure)
Mach3 defaults to 0RPM. You have to tell it what RPM you want for it to work.
A simple way to add a default RPM is to add it to the Initialization string in General Config. Just put S8000 in there.