How to use internal counter of cDAQ 9172 to measure PWM and generate frequency ?

[John_P1]

Hi Galli,

The 9403 is SW-timed only (as you have found out).  Thus, the software-timed approach that you are trying is the only option without purchasing other hardware.  The 9172 provides support for clocked digital outputs (using a HW-timed module in slot 1-4) or for the access to two counters (using a HW-timed module in slot 5-6).

Now... there are quite a few optimizations you can make in your software.  For one, I would suggest using a producer/consumer architecture to remove as many steps as possible from your update loop.  You also might consider using the lower-level DAQmx VIs to have more explicit control over your tasks.

That being said, the communication to the 9172 is over USB which is going to have a much higher latency than a PCI card would (for example).  There is not really anything you can do to reduce this latency, it is simply an inherent limitation with USB.  In a feedback-type application, the latency actually hurts you twice (you must transfer data both from and back onto the 9172).

What kind of latency numbers are you seeing?  Although there are a few optimizations you could make in your code, I'd imagine the bulk of the latency mostly due to the latency of USB transfers.

Best Regards,

John

Message Edited by John P on 02-10-2010 11:45 AM

[Galli]

Hi John

Thanks to your explanation. I'll try to work with my code to still working with th SW timer.

When you say about the counter in the slot 5 or 6, are you saying that is possible to use this counter to control my PWM? In this way I tested my 9403 in the slot 5 and in the "I/O name filtering" I check the box to the "Internal Channels". Now I can see the cDAQ1/_ctr0, cDAQ1/_ctr1, cDAQ1/_freqout but how can I transfer this counter to a physical output of the 9403 card? I'm configuring the internal counter of the 9172 chassi but I don't understand how to put this counter to an output of the 9403 in the slot 5.

Regards

Galli

[John_P1]

Hi Galli,

Sorry I think my last post was a little unclear.  The 9172 chassis itself has the 2 counters (+ freqout).  To route these externally, you need to use a HW-timed module in slot 5 or 6.  Since the 9403 is a SW-timed module you cannot use it to route the counters.

The following has a list of our cDAQ modules: 

C Series Support in NI-DAQmx

Any of the digital modules that say "Hardware Timed" should work for routing out counters in slot 5 or 6, or for performing correlated digital I/O in slots 1-4.

The cDAQ1/_ctr0 syntax indicates that the counter is internal on the chassis.  You can use the counters internally if desired in this way.

Best Regards,

[Galli]

Hello John

Thanks again. First, I would like to try the possibility to use the internal counter and the 9403 in the slot 5. The problem is that I don't know how can I do this.

How can I connect the cDAQ1/_ctr0 in the software to send the PWM to an output of the 9403???

Please, let me know if you have some news.

Regards

Galli

[John_P1]

Like I mentioned last post, since the 9403 is a SW-timed module you cannot use it to route the counter outputs.

[Galli]

Hi John

I understood everything now. I will work to improve my VI to SW timed.

Thanks to support.

Regards

Galli