If anyone is interested in an RS485 (Modbus) Huanyang GT series (yes, it's really Modbus compliant) spindle controller for Mach4, I will be writing a step by step set of instructions in the next few days.
This will consist of a set of instructions on setting up the VFD, the Modbus in Mach4, and the LUA code necessary to tie it all together.
The VFD responds to the normal M3, M4, M5 commands and the speed input from the S command. It uses an off the shelf USB to RS485 converter.
I have used a RS485 based VFD on my Mach3 machine for about 5 years and love it. But I really like some of the features in Mach4 much better.
This setup is easily modified to work with other (Modbus compliant) FVDs.