250 ex squarewave?

what kind of square wave does the 250 ex have?

I would have thought the ans: would have been obvious, Advanced Square wave, original square wave had no frequency control.

Its true squarewave design. The "advanced" square wave is marketing speak by Miller trying to distinquish their machines from other units on the market, trying to make them seem superior, when inverter based square waves were relatively new. In reality, its just an inverter square wave that doesn't use or require an HF overlay. A square wave still may have a blip or two in it depending upon how the mfgr, sets it up and the way it switches.

Through my own personal research over the past couple of years my findings would force me to concur with mark. Different Mfg's seem to have their own definition of what "advanced" squarewave control is. As has been said, some sources seem to indicate that it is the added feature of frequency control in AC, but as time has progressed, the term "advanced squarewave control" has been used to describe newer features, such as the ability to modify amperage levels on either side of the zero current line independently.