POSITION_FILE and index

I would like to use the POSITION_FILE in combination with the index pulse
of the encoder to home my lathe. The idea is that you know approximately
where you are when starting up, and once you pass the first index pulse
you know the exact position. Has anyone done this... and if so can you
explain how?
 

OK, thanks. Time to start coding.

I've written the realtime component I needed. When linuxcnc is started
you can jog the axis and as soon as the index is passed, the position
is corrected using that index. The axis is then also homed.

If the encoder had already seen an index (for example, linuxcnc was
quit and then started again without turning off the encoders) The axis
is homed as soon as the amplifier is enabled.

After homing the distance between the actual position and what was in
the POSITION_FILE is made available on the "offset" pin.

This is tested using a Fanuc encoder which has an index every 65536
increments. This encoder resets the counter by itself as soon as it
encounters the index for the first time since powerup.

I've made the information if the index has been seen available using an
index pin, which is 0 if the index has not been seen and 1 if it has.
The first cycle with index==1 should present the last counter _before_
the reset. (This is how the Fanuc encoder works)

The component can be found at:

github.com/mark-v-d/motor_control/blob/f...l/absolute_index.cpp