PNP Proximity Sensors

[rexx42]

Having trouble getting the PMDX 422 SmartBOB to recognize PNP proximity sensors.  the sensors are powered by an external 12v power supply and I have a 10k resistor on the signal output line from the sensor into input "11".  I set Mach4  to read that pin, with "active low" checked it reads as on and when unchecked it reads as off.  However, it does not read the signal from the sensor at all.  My meter shows +5 v at the input when the sensor is off and +12 V when on.  Any help would be appreciated.

[Bob at PMDX]

The PMDX-422 does not support PNP-style sensors.  At least not in a way that I can think of without external active circuitry (an NPN transistor or some sort of inverting buffer).

The inputs on the PMDX-422 need to be either pulled or driven to GND, or pulled, driven or allowed  to float (using the PMDX-422's internal pull-up resistors) to +5V.  While the inputs can tolerate voltages above +5V, that still looks like +5V to the PMDX-422.  PNP sensors are typically either "open" (i.e. floating output) or pulled to the sensor power supply (+12V in your case).  The +5V that you see on your sensor output when the sensor is "off" (its output is floating) is generated by the PMDX-422's internal pull-up.

Bob

[it_guy]

Bob,
So is that the same for NPN?
Just curious, soft limits scare me and I would preffer for the actual limit switches to work...

V/R
Mike

[Steve Stallings]

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.pdf

PNP 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.

[rexx42]

Outstanding.  Exactly what I needed.  Just ordered one.