This configuration drives a standard 4-axis system using the PRU for
high speed step/dir pulse generation and a Xylotex BBB interface board.  The
interface board sets up the STEP/DIR/Limit as:XS-2, XD-3, XL-11, YS-4,
YD-5, YL-12, ZS-6, ZD-7, ZL-13, AS-8, AD-9, AL-15

The STOP Input is Pin 10 and the Enable Input is Pin 1

There are 3 additional Outputs, Pin 14 Spindle On, Pin 16 Flood Coolant,
Pin 17 Mist Coolant

Current axis settings:

max vel   =   1.0 in/s
max accel =  15.0 in/s/s
scale     =  2000 steps/in

Adjust as required for your machine.  Make scale negative to invert any
axis that is moving 'backwards'.
