I have a 4 axis cnc router running on mach3. It uses 2 steppers on the Y axis. I have been using it as a 3 axis machine with success. I built the electronics box with a g540 running the A axis slaved to the Y axis. I also have installed a Chinese breakout board with a g251x in my electronics box with the intention of running the 4th axis off it. This works fine as far as being able to manually jog the 4th axis. The first problem came when I realized that the gcode from rhinocam wanted to control the 4th axis with the A axis in mach3. I then switched things around to run the 4th axis on the A axis of the g540 and slaved the B axis on the Chinese breakout board to the Y axis. In this configuration I can't get the Chinese board to play nice being slaved to the G540.
Is there a way to run the 4th axis from the B axis so I can leave the four X,Y, and Z stepper motors on the g540 since they all play nice that way?