MPGs

Does anyone in the aftermarket offer a good quality MPG that will work with most any CNC
equipment? A lot of the MPGs I see offered with control upgrades seem to be in the $300-600
range. I'm not terribly clear on what is inside one of them, but that seems like a lot for what
appears to be a pretty simple device.

Every now and then there will be a used Fanuc MPG on eBay. Are MPGs sufficiently generic that
they can be swapped between controls, or do they need to be designed specifically for each
control?

Once electricity gets beyond the plumbing level (that's "if I pour electrons in this end of the pipe,
they should come out at the far end") I make no claims of expertise on the subject.

thanks,
Michael