Digital device to use to measure 3.3V logic with 5k pull-up and 10k series resistance

[Denn2]

I have a digital instrument with the setup that is shown below.  I wish to measure the logic state of the line with a National Instruments device.  Which one do you recommend?  

Trials and Failures: I tried using the usb-6501 but because of the 5V and 5k ohm pull-up on the insides of this device, I was getting bad readings and was told by my peers that this was the wrong instrument to use.  I also tried using the digital pins on the usb-8451 and received the incorrect measurements. I need to figure out why the usb-8451 seemed to have the input pins floating when the logic level I provided as an input was a LOW.

The figure below is a representation of my signal source:

 

[crossrulz]

Because NI (and many other manufacturers) like to have pull up or pull down resistors in their intruments, I would recommend adding a digital buffer chip between your circuit and the NI DIO.  That will provide some isolation between the circuit and the DIO's bias resistors.

[Denn2]

Thank you for the response.  Since my DUT is already assembled, it is a little late to add chips.  However, a coworker suggested adding a 0 ohm jumper on top fo the 10k resistor.  I will see if this works and provide an update. 

[johnsold]

Measure it with an analog input?

Lynn

[crossrulz]

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@Denn2 wrote:

Thank you for the response.  Since my DUT is already assembled, it is a little late to add chips.  However, a coworker suggested adding a 0 ohm jumper on top fo the 10k resistor.  I will see if this works and provide an update. 

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I was recommending adding the chip to your test circuitry, not the DUT.

[Denn2]

We purchased a couple of USB-6001 that have 13 DIO, 8AI, and 2AO and I will be connecting these to the lines to see if they are able to detect the correct LOW and HIGH voltages.  My guess is that because of the high-input impedance of the analog inputs (>1G Ohm), this will surely work.

I will come back and post an update once I have tested this.

Thank you all for your time.