Hi,
that is correct, the FPGA on the previous model USB SmoothSteppers is smaller and somewhat less capable than the ESS.
Warp9 hope to release a plugin for the USB version to run Mach4 but are focused on completeing the ESS plugin.
The ESS has been on the market for some years, do you not suppose that there would have been a major backlash had
it not been capable of controlling a spindle? It does in fact control a spindle perfectly adequately, depending on the breakout
board you've selected to partner it, and is one of the most popular external motion controllers for either Mach3 or Mach4.
Craig