usb-6501 cannot capture digital signal

[fazr]

Hi,

I am setting up a liquid monitoring system which is comprised of a fiber optic sensor and an amplifier. When the light intensity on the laser changes, the amplifier trigger an on/off signal (0v ~ 5v). I wired the amplifier to a DC power source and the data line to the USB-6501 port0/line0.

I launched LabView and setup a task so that I can try to intercept the signal that comes out of the amplifier.

I am not an electrical engineer so this concept is fairly new to me. I basically need to be able to capture the digital signal from the amplifier using the DAQ device.

When the liquid level drops on the bottle the light intensity value on the amplifier changes as well and the signal is generated. However, I am unable to capture that signal using the USB-6501.

All I know about the amplifier (from the vendor) is that it is a sinking 0v ~ 5v (on/off) 100mA.

This will eventually have to be integrated into an application which send email alerts when a signal is captured.

Any help will be greatly appreciated.

[SnowMule]

I'm assuming you've got a ground connection to the 6501 as well as the signal line.

There's a pull-up resistor in the DIO that pulls the line high with nothing on it, so when your output is driven low, the DIO should see that. 

Have you tried looking for a change in Test Panels?

Also try measuring the output lines with a multimeter, see that it makes that transition from +V to ground and back, then hook it up to the DIO.

[Kevin_F]

Hey fazr,

The best place to start with debugging your DIO is using the Test Panels in Measurement and Automation Explorer.  This Knowledge Base Article gives instructions on how this can be done: http://zone.ni.com/devzone/cda/tut/p/id/4638.  You will probably want to start by looking at your signal on one of the Analog Inputs and if it seems correct you can verify using the Digital Input tab that you are getting the changes in state that you expect.  Using another device to verify the behavior of your hardware is another good step to determine where the issue with your system is.  Please post back with any additional questions.

[fazr]

Hi ShowMule,

Now that you mentioned it, I do not think I have a ground connection to the 6501. I think I did use the Test Panels but got nothing. I will try to follow the directions from the engineer as well to see if I do get any signal.

Just to clarify: The amplifier has 3 lines. pos, neg and signal. I connected the pos and the neg to the DC power source and the data line to the 6501 at port0/line0.

I've noticed that there are terminals labled GND. Should I connect another line from GND to the ground terminal on the AC/DC power source??? Would that take care of the ground connection?

Thanks

Rob

[Tom Sedlack]

You would connect the 6501 gnd connection to the negative line of the power supply.  The ground connection on the power supply is (probably) the chassis ground of the unit.  Most power supplies now have the outputs floating so conning the 6501 to the chassis ground won't fix your problem.

[fazr]

Thank you all for your prompt replies.

I was able to solve the issue by grounding the DAQ device to my power supply negative pole.

The next step is figure out how to programatically intercept the signal so I can write a tool/program that sends email alerts.

I think NI supply some code sample in C to get you started? is that correct? Is there a code repository where I can search for similar code. Maybe someone has already done something similar so I don't have to reinvent the wheel.

Thanks again for the help.

Rob