Setting the Digital Output to Tri-State

[berry06]

I am using the sample code dio_write_tristate.vi to set a port to trisate, the resistance I measure from the port is about 20Kohm, does anyone know is it correct? or should it be higher like several Mohms?

Thank you.

[Ed W]

Hi,

Thank you for posting to the NI forums.  Which device are you using?  The resistance value that you’re measuring  for the tristate mode of your digital I/O lines is probably expected.  I have verified this behavior here using a PCI-6251, which also gives me approximately 20kOhm resistance during tristate output.

This information can be found in your device’s specifications sheet under Digital I/O.  For instance, the digital I/O/PFI pull-down resistor for the NI-625x series is at minimum 20kOhm, according to the NI 625x Specifications.

Keep in mind that this resistance is used to resist relatively small amounts of current from your device.  The digital I/O lines therefore have much less resistance than, for instance, the analog lines (these are measured in GOhms, like you expected).

I hope this helps.  Please post back if you have further questions.

Ed W.

Applications Engineer

National Instruments

[berry06]

Hello Ed,

Thanks for your reply, I am using PCI-6229 M series card, I tried power on tristate status, got 20Kohms resistance too, so I guess this is the right resistance. Is there any way to get a higher impedance like 20Mohms from those digital output?

[Ed W]

Hi,

The NI 6229 and, in fact, all of NI’s M Series devices have specific resistances listed in their specifications.  These are all configured during the design of the device.  Without adding your own external hardware, there is no way to increase the resistance of these digital lines.

Ed W.

Applications Engineer

National Instruments