If you already have PNP sensors or prefer them for some other reason, you can
use a PMDX-105 to convert the signals just before going into the PMDX-422. See
the examples on page 6 in the PMDX-105 user manual. While these diagrams
show a PMDX-132, the same approach works for a PMDX-422.
http://www.pmdx.com/Doc/PMDX-105_Manual_10.pdfPNP style sensors may be used directly with the PMDX-126 parallel port
breakout board and with the future PMDX-426 SmartBOB-Pro.
The PMDX-422, PMDX-410, and PMDX-411 inputs will work fine with NPN
style proximity sensors. Remember that if you intend to combine multiple
sensors into a single input, the sensor's output must be open when not
triggered. Most NPN sensors are "normally open" and will "close" to ground
when a metal object is detected. This is ideal in most cases because you
will most likely be looking for the presence of a metal target.
If you must instead look for a gap in the metal as the target, you would want
a "normally closed" NPN style sensor in order to be able to combine sensors
into a single input.
If you are not combining sensors, then either the more common "normally open"
or the "normally closed" sensors can be used. In no case can you mix "normally
open" and "normally closed" sensors connected to the same signal. They will
mask each other's output.