What is the Input Resistance of My NI 6509?

Issue Details

I am using a voltage divider to bring 10V signals down to 5V for my NI 6509. The 6509 is configured to use the 47K pull downs and I use the same resistance in my divider.

What is the input resistance of the 6509 if it is not configured with pull downs? I am looking to evaluate a fault conditions.

Is there any difference for PCI/ PXI variation of the 6509?

Solution

It is important to have the interface version of the NI-6509. The PCIe version differs from the others (the front end of PXIe and PCI is almost the same). This is also noted in the PCI/PXI/PCIe-6509 User Guide.

If we are speaking about the PCIe-6509 then Figure 5 from the User Guide reveals more information about the internal structure of the DIO. We have a weak (high resistance) pull-down and a user-controlled strong (low resistance) pull-up. When the pull-up is off, we have a weak pull-down DIO pin configuration. The pull-down is always there, you can't disable it.

However, since it is a weak pull-down you can build your circuitry outside of PCIe-6509. You can have the 47 kΩ in mind, and calculate voltage divider values. Here you should keep the pull-up disabled.

The PCI and PXI versions don't have any pull-up/down resistors, and you can still go with the voltage divider.