I observe something strange with the following HOME switches settings:

in Mach 3, USB attached to a motion controller connected via LPT to a BoB:

pin 10 associated to Home X
pin 11 associated to Home Y
pin 12 associated to Home Z

There are three identical inductive sensors, wired to provide a CLOSED CIRCUIT when NOT actuated. Thus, when a individual sensor trips, the V difference sensed by each pin increases to some 4.2 V, a value well within the TTL range.

In operation, Mach 3 homes every axis individually (as well as ref them all) without fault. Can work.

However, on the Diagnostic Panel provided by Mach 3 there is something strange: if I actuate only of the three Homes, the states of all three input channels monitored by Mach 3 flip (i.e. turn yeloow on the GUI). This is only true when all three switches are connected and operational. If I perform this test with ONE sensor connected at the time, the diagnostic panel displays the expect and single state change only.

Even if this situation does not preclude safe homing operation, I feel it as sufficiently strange to seek for an explanation.   

The voltage change measured at each channel upon sensor actuation is coherent with expectation and TTL value range. 

Has anyone observed similar "strange" behavior ?
Thank you for any comment you may provide.

Regards, Philippe

The answer to my above question is simple; it is a matter of point of view.

The 5V level provided by the BoB is common to all input channels; the input pins sense a change in NEGATIVE direction.
I had (naively) assumed the opposite trend, 0V close to ground as reference and the active pins would sense a POSITIVE difference of about 5V.

LESSON 1: trust the info given by the DIAGNOSTIC screen of Mach3.

ACTION: Simply modified the signal conditioning circuitry to match to the "inverse" logic and Mach 3 behaves as before and the diagnostic screen gives coherent information.

Enjoy your evening, Philippe