End stops

Trying the "Home" command (G28 in Pronterface text input box) caused havoc (this was not unexpected).  I had to rescue the printer from itself by cutting the power (it tried to do a 360 degree turn with the arms).

To make a long story short - I made two mistakes.  It seems that the Hall effect devices are sensitive to the polarity of the magnet.  Mount the magnet (or Hall effect endstop) the wrong way round and it does not work.  I figured this out by measuring the signal on the RAMPS board with my multimeter while manipulating the arms and bed manually.  This can be done in Pronterface as well (M119 I think, it echoes the endstop status).

Secondly, the RAMPS 1.4 board has six endstops, and I wired into Xmin, Ymin and Zmin respectively.  The software reads Xmax, Ymax and Zmax.  This can be set in software, I just relocated my wires to the pins next door.

Homing then worked as planned.  On my printer, the offset from the zero position was approximately 18mm in the X direction and 82mm in the Y direction.  This of course is dependent on the mounting of the big drive wheels, and where the end stops detect the magnets.  It should be different for each printer (these values are entered into the Configuration.h file in the Arduino IDE).