input impedance of PCI-6255?

[TimL77]

I am hitting some impedance issues on a PCI-6255 on the counter inputs, are these inputs (essentially all PFI and DIO lines) fairly low impedance on this card? I can't actually find any specs on this. However I tested with a different card PCI-6713 which seems to have a very high input impedance on the same lines.

Basically I am trying to measure frequency of a digital signal of fairly large impedance ~47k, and am seeing large voltage drops on the signal at the input on the 6255 but not the 6713.

[Eric-E]

Hello,

I believe that the impedance is not listed because it is typically not an issue for digital logic. Some initial thoughts include sending the signal through an op amp so that the impedance is no longer an issue. Also, are these signals high frequency? Just out of curiosity, what is outputting a 47K Ohm impedance? That level is a bit unusual. Also, if you could elaborate or send a screenshot of what "large voltage drops on the signal" actually looks like or means it might help troubleshoot the issue.

Thanks,

Eric E

[John_P1]

According to the specifications, there is a pull-down resistor on the digital input lines of the 6255 (including the PFI lines used for counter inputs).  The value of this pulldown resistor is at least 20 kOhm, but more tpyically 50 kOhm (see here).

The 6713 on the other-hand, uses a weak pull-up resistor according to the user manual (the user manual is correct, although the information is missing from the specificaitons for some reason).

Best Regards,

[TimL77]

Thanks for the replies!

The signal is a 5V sqaure wave that I am tapping out of an existing circuit. Frequency depends on speed of the motor but goes up to about 3kHz.

It seems that this square wave is generated by way of a 47k pull-up resistor (to 5V). once connected to the DAQ input the signal drops to about 1.8V. Clearly this is just creating a voltage divider with the pull-down resistor in the 6255.

[John_P1]

Yup--that would make the value of the DAQ pull down resistor ~26.4 kOhms, which meets the >20 kOhm specification.  You'll have to make some external hardware changes to get the signal into the DAQ card.

Best Regards,