5v output on PFI

[Plato]

I am trying to use PFI0 on a 6015 as a digital trigger, but am having some trouble. I have read that on device reset, these ports are supposed to behave as inputs, but I am measuring 5 volts between this pin and the digital ground. Is there a specific physical connection that I have to make to this port to bring it low and wait for my digital signal? Or is there a way to software re-wire the port as an input? Thanks in advance.

[Brian F.]

Hello Plato.

Thank you for posting on the NI Discussion Forums. 

The 6015 PFI lines, when reset, default to tristate mode.  When you set a line to tristate, it is not being driven high or low, but instead to a high-impedance floating state.  When the output is floating, it is putting no load on the circuit, and it is impossible to determine what the state of the line will be when measured.  It can be driven low or high depending on the device used to measure the state of the line.  What tristate is useful for is if you have two sources driving the same signal, but only want one to drive it at a time.

In order to best answer your question, I would like to know which 6015 device that you have.  Do you have the NI 6015, the NI DAQPad-6015 (BNC), or the NI DAQPad-6015 (Mass Termination)?  Also, even though you are reading 5V, what happens when you actually run your VI?  Does it perform incorrectly?  Can you describe this performance to me as well as elaborate on exactly what your application is trying to accomplish?  I would be happy to help you, but I need just a little more information to help you in the best way possible. 

Have a great day and let us know how we can help you!

Brian F
Applications Engineer
National Instruments

[Plato]

Brian,

Thanks for the reply! Since posting the message, I have spent about 5 hours looking around the NI website for my answer. At long last, I found an application note describing my problem, and prescribing a 4.7k pull-down resistor attached between the PFI pin and the DGND pin (I have a screw-terminal 6015). This worked like a charm. There was still an output signal of ~0.6V, but well below the trigger level of digital logic. I was then able to route my digital signal into the PFI port and successfully trigger an analog acquisition.

If I could voice two requests for a "DAQ wish list," they would be: 1. Add better documentation to the users guide with respect to physical connections of various signals to the board. 2. Develop the "connection diagram" for the 6015 device to aid in the connection of said signals.

If my approach is inappropriate, or you can think of a better one, please post it.

Thanks!