Thanks, Clive - there's a pointer in there that ends up with a few Mach3 tutorial videos which gave some background. However, I also had a hunt around for something about the HY inverters and Modbus, as the HY manual mentions it in passing but gives no specific information on what you can control or monitor. Apparently, HY have used their own very non-standard interpretation of the Modbus standard and although someone has produced a Mach3 plugin which gives access to some of the HY inverter features, I have not yet found the kind of information needed to use with a Mach3 brain.

Still, it has all thrown a bit of light on the "brain" feature, and I'm now wondering if that could be used to, for example, provide a custom Mach3 facility for master/slave homing which my CSMIO-IP/M does not do. Too many distractions from actually getting on and finishing the router...