Ni9421

[Niners]

I am trying to connect the NPN output of my flow sensor with the below wiring asignment to the NI9421. My flow sensor has an NPN output with its Red wire being the +24VDC, the Black wire is the 0VDC and the White wire is my NPN output. Could you please let me know the way this should be wired.

[GiantDeathRobot]

Generally speaking, 0 VDC is considered ground, so it would be wired to COM. The +24 VDC is probably an excitation voltage, which the NI-9421 does not provide, so that would have to be connected to an external source. The NPN output is most likely the signal of interest, so that would be connected to the digital input channel of your choice. 

If you could post the model of flow meter, that would be helpful in determining which connections go where.

[Niners]

Thanks for the feedback. That is also how I would think it should be wired but the customer is not getting a pulse.

The sensor is Burkert's 8071 sensor. Please see my original emai attachment and current attachment for the wiring.

[GiantDeathRobot]

Then it looks like the wiring should be correct. How do you have the excitation set up, and how are you testing for a pulse?

[Niners]

When I hook the 8071 sensor's NPN output to one of the DI inputs and connect the 0VDC to common, it damages the transistor inside the 8071 sensor. It's happened 3 times.

The only way I have been succesful at communication with the 9421 is to have the 24+ connected at DI0 and the positive 24V+ on the sensor with the sinking output connected to common and 0-V connected to 8071's -0 VDC terminal.

It reads the output but I wouldn't be able to use the other 7 inputs.

I've had success wiring a PNP output to the 9421 as shown in the attached.

Does the NI 9421 require a sinking or sourcing output device? 

[GiantDeathRobot]

The 9421 is a sinking digital input devices. Pages 8-11 of the user manual indicate that, yes, this module does require a sourcing output device. (The manual is available here: http://www.ni.com/pdf/manuals/373504f.pdf)