What does the VFD show for the frequency when you have linuxcnc running? It should read whatever your minimum speed is set to in the vfdmod ini file - I have my minimum speed set to 4800 rpm / 80 hz frequency. The physical VFD displays 80.0 hz as expected as soon as linuxcnc is started. If yours shows anything other than your minimum speed, and it's showing connected without errors, then the frequency register address is probably wrong. If the frequency changes / displays correctly but the spindle won't start, check the control register address.
