problems reading cmos levels?

[henry99]

I'm using a PCI-6221 card on a digital input.  I hooked up a CMOS line to it and the following happens:

1) If I measure the voltage with a voltmeter, it says 3.3V

2) As soon as I connect the wire to the daq card port 0 line 0, it goes from 3.3V to 1.6V

3) When I run my program, it always says FALSE on the DIO line

4) If I connect a jumper between port 0 line 0 to +5V, it says TRUE

Is there any way I can get the cmos logic to work here without adding too much componentry (or using a different port)?  I'm guessing the weak pull down is dividing my voltage in half and it's not working.  Can I disable the internal pull down somehow?  Interestingly, I hooked up a quadrature counter to CTR0 on lines PFI8 and PFI10 and it seems to work fine without problems with the same logic!

I also have a PCI-6202 that I haven't hooked up yet.  Would this be any different?

I tried it with my Labjack and it works fine at 3.3V.  Any ideas what I can do to get Labview and my pci 6221 reading these 3.3V levels without dropping them 1.6V?

Also I'm using a SCB-6868 if that makes any difference.

Thanks for any help.

[LabJackSupport]

Looks like the PCI-6221 has 50k pull-downs on the digital inputs.  That would mean your signal has a source impedance of about 50k since the 3.3V is dropped to 1.6V.  That sounds to me like your signal is not actually driving an output-high, but rather is just an input pulled up to 3.3 volts with a 50k resistor.  Is this a tri-state line such that you can set it to ouput-high, output-low, or input?

Which LabJack did you test with?

[henry99]

I have a Labjack U3 (which works great by the way) and it worked fine.  Yes I think they are open collector outputs that are pulled high.  They are from an IR transmitter/receiver pair and I'm interfacing to another circuit.

Can way to get it the PCI 6221 card working too?  Thanks a bunch.

[LabJackSupport]

So the problem is that your sensor has pull-ups and the PCI 6221 has pull-downs of similar value.  The U3 has pull-ups, that is why it works.

You should be able to just add a stronger pull-up.  You need to pull the floating voltage over 2.2 volts, and yet make sure the voltage when your switch closes is below 0.8 volts.  I would try 10k from the signal to +5 volts.