Simultaneous PWM generation and Digital I/O, NI cDAQ9174 + NI 9375

[CraigA]

I've found this post that describes my situation, but naturally there's no answer. I've monkeyed with the various waveform generation examples built into LabView 2021 and they allegedly generate my PWM signal without issue. The problem stems from when I attempt to output the PWM signal (that is being sent to an internal counter I presume) or when I add a separate Digital Output task in the same VI.

I'm looking for a simple example that shows how PWM and plain old Digital I/O are supposed to exist on the same cDAQ I/O card.

[Kevin_Price]

I don't know cDAQ exhaustively, but it's pretty common that you can't have more than 1 hardware-timed task of a given I/O type running on the same module.  Modules that support multiple I/O types can support as much as 1 each, until the cDAQ chassis runs out of clock resources to provide.  FWIW, DI and DO will typically count as distinct types, though some modules are input-only and others are output-only.

Anyhow, when you talk about using "waveform generation" to generate PWM, it sounds to me like you're using a hardware-timed DO task for it.  I dunno what kind of "plain old Digital I/O" you're trying to do at the same time, but I suspect at least one of the following:

- it's also meant to be hardware-timed, and thus running into the 1-per-module restriction

- it uses a specific digital i/o line that's part of a port that the waveform task has reserved for its generation.  Some devices can treat each DIO line as independent channels, others may group them in 4's or 8's so that using any one of them in one task reserves all 4 or 8 and prevents other tasks from having access.

The answer: generate your PWM with a counter output task rather than a digital output task.  The chassis has counters and you can route i/o signals to/from them with most digital modules.  It's possible that the 2nd item above could still come into play, so you'd just need to choose output lines that allow independent access rather than ones that are part of a hardware group.

Hopefully someone with more familiarity with your particular hardware can offer more detailed guidance.

-Kevin P

[CraigA]

Most of my confusion stems from a lack of familiarity with the architecture. I understand the conepts in isolation, but networking them together is where I get lost; the wide range of capabilities/incompatibilities are not doing me any favors either.

To elaborate on my specific situation a bit, here are the goals I'm trying to achieve:

• I need to generate a 200hz pwm signal, no crazy resolution required, 4-bit would be plenty. Its a low frequency and low resolution, so naturally the first attempt was using a for loop in labview and manually toggling the output. As one would expect, there's excessive jitter that makes the signal useless by the time it gets to the physical wire.
• for the plain digital I/O I mentioned, I'm wanting to check if an input is on or off periodically. Timing is of no concern, as long as I can check it a couple times a second at least.
• In a later stage of this project I'd like to also measure the duty cycle of another pwm signal running at a similar speed, but this is outside the scope of this thread.
• Naturally I'm attempting to drive this pwm and check that input on the same NI module

[Kevin_Price]

For experimenting and learning purposes, I'd recommend you open up 2 of the shipping examples:

• Counter - Continuous Output.vi
• Digital - SW-Timed Input.vi

To measure pwm duty cycle, you can try the example:

• Counter - Read Pulse Duty Cycle and Frequency (Continuous).vi     

If your DAQ device doesn't seem to support "pulse measurement mode" (which handles several low-level details for you), you'll still be able to get freq & duty cycle info by running in "semi-period measurement mode".  You just have to do a little extra work to break out alternating high and low times from the data array and use them to calculate duty cycle and frequency.

Between those examples and details you can find in the manuals for your specific devices, I think you're pretty likely to be able to find suitable DO and DI lines on your module that let you run all these tasks at the same time.  

-Kevin P

[CraigA]

I have tried looking into those examples especially the Counter - Continuous Output.vi but I am still not able to get our cDAQ-9174 to output on our NI-9375 properly. Inside the example, I want to be able to select the appropriate output terminal to be a DO line from my NI-9375 module. I am not able to do so and as you can see in the screenshot below, I can only select certain trigger options but nothing of I/O lines. Can you help me figure out how to select our DO line instead of a trigger source?

Also, I've attempted creating our own code using the counter output along with turning on a DO output but when I try to use another DO on a digital output channel, I get the 50103 error code - "The Specified Resource is Reserved". Can you help explain why this may occur?