The conversion is not that simple, the PWM freg period for the input is only 50ms and also the input requires a series resistor for the opto as there is just a small value under 100ohms for the opto.
There is alot of info on the web for the nature of the input, I am just experimenting with a Picmicro interface for one right now.
If I get the time I would like to create a circuit to enable the Mach3 RS485 huanyang plug in to be used.
Al.