Brian
That's good of you. Thank you, it will be a big help if we can get this to work as is. Primarily this is my development machine for Ezilathe, and I do need to get back to mach3 periodically, just to check I haven't stuffed it up, so less hardware changes the better.
Managed to get a simple PWM - voltage board on ebay, cheaper than buying the components here, not much of a change to the hardware if required.
My spindle drive is a Chinese AC servo motor (very good, inexpensive, using them a lot these days) but no Modbus in mine (Option only). This gives me the capacity to use spindle as a Drive "C". Very handy.
The mention of RS-232 these days brings back too many memories of days spent fiddling with cables and protocols on old PLC's. Probably will consider Modbus, but not yet.