Axis: 'Touch Off' disabled

I am using a machine configuration derived from the 'gantry' simulator configuration.
I'd like to use the 'Touch Off' button in the manual control dialog of the Axis UI, but it remains disabled after homing. The 'touch off' button works for me with different (simulator) configurations (axis.ini for example). Like in my derived config, it is not usable in the unmodified gantry simulator configuration as shipped with linuxcnc.

So my question is how ini or HAL configuration determines whether the "Touch off" button in AXIS is available or not.

Thanks, that solved it