Axis machine position is not reported live when using an expansion board

[dc42]

Discovered on ToolChanger in which the Z axis is drive by an expansion board. To reproduce:

Command a Z move that takes several seconds to complete and watch the move.axes[2].machinePosition in the Object Model browser. The reported Z axis position doesn't change until the move completes. If the move also performs X movement then the X axis position does update during the move, and the reported Z position varies a little - looks like it's the original Z position adjusted by bed compensation for the new X coordinate.

Likely explanation is that the steps are generated on the expansion board, so the motor position in the DDA isn't updated until the move completes.

[dc42]

Fixed in 3.6.0-alpha.1 . The positions of axes driven by expansion boards are updated every 100ms.