There is a config.h setting that enables or disables the “Safety Door” function in grbl.
Inventables released version 0.9j with the safety door option disabled.
Inventables released version 1.0c with the safety door option enabled.
When enabled the safety door option redefines the functionality of the Feed Hold pin (A1). In addition to the Feed Hold function the safety door function will stop the spindle and if “Parking” is configured move the spindle away from the work surface.
When Cycle Start (A2) is activated and the safety door option is configured (and the door is closed) the spindle will move back to the work surface (if “Parking” is configured) and restart the spindle.
For the Inventables builds tripped is A1 pulled to ground
Not tripped is A1 pull high (by the ATmega328).
For a normally set up machine with the safety door switch open ( = door closed = pulled high) then grbl will not execute the safety door open / feed hold function.
When the switch is closed (= door open = feed hold) grbl will stop until the door closes (switch open).
You can change the logic of the switch by compiling with the INVERT_CONTROL_PIN compile option.
Hope that clears things up.
Also see: