Automatic probing

###GRBL supports probing (check linuxcnc.org for further information).

###Problem If the probing-switch is triggered, GRBL throws a position message and stops movement (check GRBL feedback messages), but the final stop position is not the position where the switch was triggered because of movement during deceleration of the CNC.

###Solution My solution is to store the position thrown with the position message internally in the controlling software. Following variables are available:
gcodeVariable.Add("PRBX", 0.0); // Probing coordinates
gcodeVariable.Add("PRBY", 0.0);
gcodeVariable.Add("PRBZ", 0.0);
gcodeVariable.Add("PRDX", 0.0); // Probing delta coordinates
gcodeVariable.Add("PRDY", 0.0); // delta = actual - last
gcodeVariable.Add("PRDZ", 0.0);

Each variable will be replaced by it's stored value if found in the GCode.
E.g. write Z@PRBZ to set the Z value to the last Z-position thrown with the probing message.

My GCode for tool length compensation:
(Move to stationary length sensor at machine coordinates (HOMING NEEDED!))
(Move down 50, stop when switch triggers, set new z-coordinate)
G90 G0 (absolute distance mode, rapid move)
G53 Z-8 (move in machine coordinates to save height)
G53 X-146.6 Y-74 (move in machine coordinates to stationary switch/tool length sensor)
G91 G1 F500 (relative distance mode, feed mode)
G38.3 Z-50 (probe toward tool length sensor, stop on contact)
G43.1 Z@PRBZ (Offset Tool)
G53 G0 Z-8 (move in machine coordinates to save height)
G53 Y-85
G90 G1 F2000