M-series: setting power-up direction of digital ports

[CDancer]

I'm using a PCI-6221.

I'd like to know how to definine the power-up direction of the digital ports, and whether the ports have pull-up and/or pull-down resistors.

In measurement and automation explorer I can set the reset port state to be logic 0, logic 1 (which imply direction = out), or tri-state.

This article: 38685CGP refers to tri-state in the context of port direction,  so does "tri-state" mean "input"? If so, why not just call it "input"? What about the resistors?

Chris

[Wayne.C]

Chris,

The term 'tri-state' does not always imply an input.  There is such a thing as a tri-stated output.  Tri-state will usually be associated with high impedance.  Have not used this card myself but found this in docs for card.

At system startup and reset, the hardware sets all PFI and DIO lines to high-impedance inputs by default. The DAQ device does not drive the

signal high or low. Each line has a weak pull-down resistor connected to it, as described in the specifications document for your device.

NI-DAQmx 7.4 and later supports programmable power-up states for PFI and DIO lines. Software can program any value at power up to the P0, P1,

or P2 lines. The PFI and DIO lines can be set as:

• A high-impedance input with a weak pull-down resistor (default) 

• An output driving a 0

• An output driving a 1

Refer to the

NI-DAQmx Help or the LabVIEW Help in version 8.0 or later for more information about setting power-up states in NI-DAQmx or MAX.

[CDancer]

Thanks, that is all the information I needed! Well done finding it - the "detailed specification" that I downloaded for the PCI-622x devices does not have that information in it.

I should have explained that I do actually know the normal meaning of the term "tri-state": it refers to the functionality of an output that can be put into a high-impedance state, usually so that another device can drive the same bus line. NI have used it incorrectly to refer to the state of a port line when it is defined as an input.

Chris

[Wayne.C]

Chris,

Glad that answered your question.  You did by the way pass the tri-state quiz.   

[Brad_K]

Hi Chris,

Centerbolt is correct. NI-DAQmx has some advanced digital I/O properties that give you more control over when digital lines are tristated or not, but this also means that input and output aren't always mutually exclusive. Digital output channels don't always drive their lines: you can set the DO.Tristate property to true to set the lines to high impedance. Likewise, digital input channels don't always tristate their lines: if you set the DI.Tristate property to false, you can "snoop" on the values that a digital output task is outputting, allowing you to input and output from the same line simultaneously.

By the way, I think your definition of "tri-state" describes how NI-DAQmx is using the term. "Input" vs. "output" is sort of a point of reference for a digital I/O task, whereas "tri-state" describes the state of the digital I/O hardware (whether the line is being driven or not). I hope this doesn't further confuse the issue. 🙂

Brad