Zmin won't toggle - help! (1 reply)

I am trying to troubleshoot a friend's system - a kossel mini with RAMPS 1.4 and Marlin. He wants to use a touch probe microswitch into the ZMIN line on the ramps.

If I jumper ZMIN to +5 (verified with a DVM), an M119 in Marlin/Repetier shows "open." Other wise if Zmin is either open or grounded, it also reports "open."

Zmin in pins.h is defined to be 18 (which I think is correct - the motherboard is defined to be 33.)

ZMIN_ENDSTOP_INVERTING is true in configuration.h, but if I invert it, it reports TRIGGERED under all conditions above (still will not toggle.)

#define ENDSTOPPULLUPS is uncommented in configuration.h

//#define DISABLE_MAX_ENDSTOPS is commented out in configuration.h
//#define DISABLE_MIN_ENDSTOPS is commented out in configuration.h

Also, I have swapped out both the mega and RAMPS card with new ones, one at a time with the same report, so I think it must be something in Marlin, but can't see what I am missing.

Any ideas why I can't get Zmin to toggle in a M119??? I would really appreciate the advice!