I've been working with an IDEC FT1A SmartAXIS controller http://www.clrwtr.com/PDF/IDEC/IDEC-FT1A-12.pdf to interface with Mach3. My issue is that ModBus seems a bit random in its ability to work.  I'm also controlling an AC Tech SMVector VFD and it works perfectly over Modbus via ESVZAR0 http://shop.actechdrives.com/AC-Tech-Lenze-ESVZAR0-p/esvzar0.htm.  I've tried three different ModBus converters, all with the same basic results - no response, random responses, etc.