I populated the machine breakout PCB this morning.  This board will actually be installed inside of the CNC milling machine, and the various homing and index sensors will wire to it via the mini-sized green screw terminals.  The 40 pin header adapts to a Centronics (old school printer) style plug, and connects back to the main board via a 6′ cable.  My design has a significant noise immunity advantage to any other CNC breakout boards that I’ve seen on the market, in that I chose to carry the signals over the interconnect cable using differential signaling.  This is a good thing, since the stepper drives/cables are quite noisy.  On the (minor) down side, it seems the chips I chose for converting to/from differential are the ones responsible for high current draw.  I didn’t catch that the MC3486 receiver chips draw 85mA max, each chip (and I have a bunch in the design).  I figured that would only be while switching, but they have a very high quiescent draw as well.  They’re a really old design, so that’s probably why they’re so inefficient.  At least it’s not a concern in this design since I’ll have a big honking power supply available.