Perhaps you are confusing "active high" with "normally closed".
These are related, but not the same concept.
It is desirable to use a normally closed circuit to sense home or limit
switches because a failure of connectors or wiring will result in the
system stopping in a safe fashion. Normally open sensors will also
work, but the wiring failure fail safe advantage is lost.
The inputs of the PMDX-424 interface board is internally pulled to +5
volts by a resistor. If you connect a normally closed switch between
the input signal and ground, the signal is normally low. Therefore
you would configure Mach for "active high" operation. The is the
default setup with a red X in the box labeled active low, indicating
that active low operation is not being requested. Yea, I know, it
is confusing.
If you have proximity sensors things can be more complicated. These
sensors can be normally open (most common) or normally closed,
and they can be NPN (ok for the PMDX-424) or PNP (requires use of a
PMDX-105 for level conversion).
