If anyone is interested, I wrote an AVR program for a ATMEGA48 mcu to read the Smoothstepper spindle speed PWM and convert it to my VFDs RS485 protocol.
It can read from 0 to max rpm with 50rpm increment.
It can easily be modified to fit other VFDs that accept rs485.

If you want it mail me to pamafe@gmail.com and I will send schematics and code.

Regards