Changing the PXI-6624 Counter Output rate on the fly

I'm programming an application in VB.net 2005 to set several counter output channels (each with its own task) on the PXI-6624 card. I'd like to know if there is a way to change the frequency of the output without stopping the task, changing the frequency and restarting the task. I'm using the PXI-6624 card to simulate encoders and speed sensors.
Thank you very much,
JonS101

Hi,
Yes, you can change the frequency on the fly. Here is a knowledgebase that will give you an insight of how you should approach this matter. How Can I Change the Duty Cycle on My Continuous Pulse Train? To find this property node and how to use it refer to the DAQmx .NET reference: Where Can I Find NI-DAQmx Property Node Listings and Help?
I hope it helps
Jaime Hoffiz
National Instruments
Product Expert
Digital Multimeters and LCR Meters

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How to Immediately Change Counter Output Rate?

I have a piece of code that largely works like this example: http://zone.ni.com/devzone/cda/epd/p/id/5493
In other words, I set up the Counter Output with some initial frequency and duty cycle, but then during the main loop of my program I continuously change the frequency to a new value based on other criteria.
I'm using an M-series PXI card and LabVIEW RT.
The problem I'm having is that the card waits for the next edge before changing the counter output rate. For instance, lets say it is going at a low frequency and I am upping to a high frequency. If the command arrives in the middle of the current pulse, it will wait to complete the low-rate pulse before starting the high frequency output. Is there a way to make it interrupt the current count and immediately start counting at the new rate?
Thanks,
Isaac

Hi Isaac,
I posted the code in LV 8.2 so you should be able to open it now (it sometimes takes several minutes to upload).
There are a few limitations to using the digital lines instead of the counters:
1. The digital lines are updated off of a sample clock which will be much slower than a timebase. For example, on the 6221 the maximum update rate is 1 MHz, while the counter output has a max timebase of 80 MHz. As a result, the number of frequencies you can generate are going to be more restricted (divide down from 1 MHz vs. 80 MHz).
2. On M series devices, the digital lines must be clocked from an external source. This could be generated from a counter
3. You have to build the digital waveform, which is a bit tricky (I think the example code should help out with that but I haven't had time to thoroughly test it).
4. If you are generating digital lines at fast rates, you will need to write quite a few samples at a time to the output buffer to ensure the data does not underflow. If the buffer includes multiple periods of the digital signal, you would have the case that using the counter output would still update more immediately.
Again, to determine the best course of action it would be useful to know what frequencies you want to generate and which exact hardware you are using. I just mentioned the digital lines as an alternative to the counters, but it might not be ideal for your situation.
-John
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Change default terminal for Counter Output

hello,
I am using a DAQ-6251. I am trying to change the default output terminal (CTR0) for a counter signal but am having trouble with this because I am extremely new to labview. Could anyone tell me how to do this?
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Hello Bilog,
Us the DAQmx Channel Property Node, and your property of interest would be Counter Output,Pulse,Output Terminal
Regards,
Glenn
Message Edited by ManLongAtNI on 06-18-2009 04:07 PM
Regards,
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PXI-6229 counter output

Hi,
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Thanks
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Shiladitya,
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Counter Output/Counter Input PXI Signals Behaving Erratically

Question for all your LabVIEW guru's out there,
I am running a frequency loopback test using the NI PXI 6229 MIO DAQ card. I am generating a "Counter Output" pulse train signal which feeds through my device under test and then back out of my device under test and back into the PXI 6229 for a "Counter Input" frequency measurement. Both the "Counter Output" and the "Counter Input" are assigned different PFI lines using DAQmx in LabVIEW.
I have 4 lines to test on my DUT. All four lines run this same frequency measurement but with different PFI lines on the PXI 6229. Each line is test independently.
This is my setup for the 4 lines:
Path 1: P2.0 (Counter Output - Pulse Train) -> DUT (Device Under Test) -> P2.1 (Counter Input - Frequency Measurement)
Path 2: P2.2 (Counter Output - Pulse Train) -> DUT (Device Under Test) -> P2.3 (Counter Input - Frequency Measurement)
Path 3: P2.4 (Counter Output - Pulse Train) -> DUT (Device Under Test) -> P2.5 (Counter Input - Frequency Measurement)
Path 4: P2.6 (Counter Output - Pulse Train) -> DUT (Device Under Test) -> P2.7 (Counter Input - Frequency Measurement)
where:
P2.0 = PFI8
P2.1 = PFI9
P2.2 = PFI10
P2.3 = PFI11
P2.4 = PFI12
P2.5 = PFI13
P2.6 = PFI14
P2.7 = PFI15
I have a LabVIEW VI which generates the "Counter Output" and reads the "Counter Input" frequency. I am seeing weird behavior from the PXI 6229 card. I can test "Path 1" and "Path 2" and the frequency I read is what I generated. No issue there. However, when I test "Path 3" and "Path 4" the frequency measurement is erratic. The readings are two different frequencies repeated over and over again and none of those frequencies are the expected frequency which was generated out of the "Counter Output." If I reset the card, and start by testing "Path 3" and "Path 4" the frequency readings are correct and the erratic behavior is gone. However, when I try to then test "Path 2" and "Path 1" now those lines have the erratic frequency issue. I can continue resetting the card and see same issue. The PFI lines that I test first will always pass.
To summarize:
Steps Taken:
1. Test Path 1 = SUCCESS
2. Test Path 2 = SUCCESS
3. Test Path 3 = Erratic Frequency (Two Frequencies repeated over and over again in my frequency results array)
4. Test Path 4 = Erratic Frequency (Two Frequencies repeated over and over again in my frequency results array)
5. Reset the PXI 6229 Card
6. Test Path 3 = SUCCESS
7. Test Path 4 = SUCCESS
8. Test Path 3 = Erratic Frequency (Two Frequencies repeated over and over again in my frequency results array)
9. Test Path 4 = Erratic Frequency (Two Frequencies repeated over and over again in my frequency results array)
I am wondering if Port 2 (P2.0-P2.7) on the 6229 card has certain dependecies and this is why I am seeing issues. I am trying to get around this issue so that I don't have to always reset the card.
Are P2.0-P2.3 (PFI8-PFI11) and P2.4-P2.7 (PFI12-PFI15) treated differently or require different setup? How do I resolve this issue?
Thanks so much!

I have a theory...
The DAQ card follows a policy called "lazy uncommit" wherein the terminal used for the output will continue to be connected to the counter even after the task has completed (until the terminal is needed for something else). So as you run more tests, the counter output will end up driving more lines. This behavior should be easy enough to confirm.
As the DAQ card drives more lines, I'd imagine this affects the actual signal. You could scope it to check, but it sounds like either the rise/fall times are becoming longer or some extra noise is being introduced on the line.
The readings are two different frequencies repeated over and over again and none of those frequencies are the expected frequency which was generated out of the "Counter Output."
This implies you are picking up an extra edge during transitions--this isn't too uncommon if the signal is noisy since there is no built-in hysteresis on the DAQ card. I would expect the measured frequencies to have periods that sum to either the full period or the semi-period of your actual signal (depending on how many duplicate edges are detected).
Suggestions are as follows:
To stop the DAQ card from driving multiple PFI lines, it would probably be easiest to just programmatically reset the device in between your tests (using DAQmx Reset Device). If you can't reset the device (e.g. because you are running some other task that can't be interrupted) then you can instead configure a dummy task that uses the PFI line in question as an input.
To stop the DAQ card from picking up multiple edges during transitions, you should configure a digital filter on the input terminals. If you reset the device it sounds like this might not be necessary... it's up to you if you want to configure this or not.
Best Regards,
John Passiak

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Hi!
I am trying to make high accuracy measurements with the PXI-6608.
I am using a 3rd party PXI backplane which has no StarTrigger slot that would override the PXI_CLK10 automatically with the PXI-6608 OCXO output PXI_CLK10_IN.
It appears that the PXI-6608's OCXO has no direct link to the time source of the counters but that instead the time source can "only" be PLL-synced to PXI_CLK10 signal of the backplane. Is this correct?
I tried to measure the high accuracy OCXO output on PXI_CLK10_IN but got nothing. What needs to have the PXI-6608 output its OCXO clock to PXI_CLK10_IN?
ThanXs for your help, Hezapeter

By connecting the OCXO to the PXI_CLK10_IN, you are overriding the PXI clock to make a more accurate reference clock. So anything that is using the PXI clock at a reference will become more accurate. I believe you have to set the source reference clock for the DAQmx tasks to the PXI clock in order to get this benefit.
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There are only two ways to tell somebody thanks: Kudos and Marked Solutions
Attachments:
Connect Clock.png ‏9 KB

How to use the PXI's 10MHz backplane clock to synchronize two or more 6031E boards?

Hi,
I'm trying to use two 6031E boards to acquire multiple channels Analog Inputs, and I'd like to sync them with the PXI backplane clock (not the RTSI bus). I found an example from NI, "Two E-Series Shared ScanClk.vi", and modified it a bit. But I'm wondering if the two boards must have the same acqusition rate? e.g. Can I have 1 board acquiring at 1kHz while the other one at 20Hz? and if yes, how do I do it?
Your input will be very much appreciated. Thanks!
Dan
Attachments:
mod_Two_E-Series_Shared_ScanClk.vi ‏148 KB

Unfortunately, E Series boards cannot synchronize with the 10 MHz PXI backplane clock. They do, however, have access to the PXI trigger lines, which are basically the RTSI lines. Except for line 6 which is the star trigger line. They can only read from this line.
The example program you have selected will definitely be part of your solution. It allows you to share the scan clock timing signal between two boards. If you would like the slave board to acquire at a slower scan rate, you could always route the master's scan clock to one of the slave's on-board counters which can act as a frequency divider. The output of this counter can become the new slower scan clock for the slave board. Use the Generate Pulse Train shipping example VI as your frequency d
ivider. Finally, make sure you route the signals appropriately and you'll be in business.
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Russell McMurtrey
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National Instruments
http://www.ni.com/support

Pulse Generation application with DAQmx and a PXI-6624 module?

What is the best implementation method for the following pulse generation application
using LabVIEW, DAQmx and a PXI-6624 counter\timer module?
I have two rising edge trigger signals (Trigger-1 and Trigger-2).
There is ample spacing between each trigger. They never occur at the same time.
I need to generate a single pulse (fixed width, variable delay) whenever Trigger-1 occurs and
a finite pulse train (fixed width, variable delay, N-pulses) whenever Trigger-2 occurs.
However, the output must appear on one counter output because this composite signal
will be used as a trigger source for another PXI module in the rack.
With DAQmx and a TIO counter\timer can I use both the GATE (for Trigger-1) and
AUX (for Trigger-2) at the same time on the same counter to gate out the desired pulses?
Trigger-1 would be wired to the GATE of CTR0. One Trigger-1 event would generate one pulse on the output of CTR0.
Trigger-2 would be wired to the GATE of CTR1 and the output of CTR1 would be routed to the AUX input of CTR0.
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Would DAQmx and the PXI-6624 TIO support this implementation?
What is the best way to accomplish the task at hand.
Thank You.
Best Regards,
Scooby

Hi Scooby,
I have looked into the application you have described and I see a potential problem with what you describe. In DAQmx, it is not possible to call the counters of the same DAQ device in the same task, so you cannot have the finite pulse train generation and the single pulse generation tasks running at the same time. What you can do, since you mention the triggers will not occur at the same time, is to stop one counter task while you are triggering another. The way I would suggest you merge the outputs is with a two input Or logic gate to avoid damaging your counters. Your signals will effectively be added together by this logic gate. I do not see any way to merge the outputs internal to the DAQ device.
Please let me know if I can be of additional assistance.
Laura

Changing the PXI-6624 Counter Output rate on the fly

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