Stacked Sequence Structure

[Lao wan]

I use Stacked Sequence Structure to produce some digital control signal,but the output frequency is too low to work ,I want to known how can I boot the output frequency. The hardware I use is DAQCard-6062E! Thanks very much!

[Dennis_Knutson]

I don't have the Elvis software so I don't know what's in the subVIs but the 6062 has static digital I/O and that means the output is strictly software controlled and will vary depending on the type of pc and OS you are running. What kind of rate do you need and what rate are you seeing?

[Lao wan]

The digital output signal frequency I need is about 1KHz, but now I can only get a signal about 42Hz, I can't understand it!

[Jennifer O]

Hi Lao,

Dennis is correct that the digital timing on your E-Series board (60xx) is controlled by your computer (software timed).  The outputs will only be able to update as fast as your computer can run the code. You can use timing functions in LabVIEW to measure the time it takes to run each of the digital writes.  Also note that the speed is highly dependent on the processes running on your computer.  For example, if a virus scanner is running and the user is interacting with the mouse, this will take up processor time and the application will output slower. This means that the time between writes will not remain constant.

If you need correlated (hardware timed) or faster digital output I would reccomend working with an M-Series board.  In case you have access to an M-Series board (62xx), I will include a link to an example of timed DIO with ELVIS: [example no longer available]

Regards,

Jennifer O.
Applications Engineer
National Instruments

[Jennifer O]

Hi Lao,

I thought I would include one extra suggestion in case you do not have an M-Series board. 

With NI-ELVIS, the digital write must go through 2 steps and can slow down the speed at which you can output.  You can bypass one step and write directly with DAQmx VIs.  This will still be slower than the timing you can achieve with M-Series and will not be deterministic (timing will vary as explained above). 

Open the example linked before.  This demonstrates how to bypass the ELVIS portion and then performs timed DIO.  You can remove the DAQmx Timing (sample clock) from the code and add more DAQmx writes to update the digital outputs.

For readability I recommend using the flat sequence instead of the stacked sequence.  To control timing you can add a wait function in each frame of the sequence.  I expect that you will be able to approach the 1kHz frequency, but you may not be able to meet it (system dependent).

Regards,

Jennifer O.

[Lao wan]

Hi Jennifer,

Thank you for your reply.

The following is my working enviroument, could you help me to analyse whether the output frequency is too low or reasonable? Thanks a lot.

Hardware specification:

CPU: P4 2.8G

RAM : 1G

Hard Drive: 80G, 7200RPM, 40G free space

OS:Windows XP SP2

software enviroument: Office, Acrobater Reader, IE7, Symantec Antivirus

Is the output frequency for my computer reasonable?

Thanks again for your help.

Lao Wan

[Lao wan]

Did you know what the  utmost digital ouput frequency of the DAQCard-6026E is? Thanks very much!!

[Jennifer O]

.Hi Lao,

As you can see from the DIO benchmarks for the boards listed, the data rate can easily double between different processors.  Also, the bus type is a large factor.  PCI and PXI devices are much faster than the USB device.  The comparison is on the order of 50-100kHz while the USB device does not reach 1kHz on the systems tested.  For this reason you will have to test the card on your own system to determine how fast it can output.

You can find the specifications for the 6062E here. Under the Digital I/O specifications you will see that the theoretical maximum is 50kwords/s and 10kwords/s sustainable (system dependent).

If you just want to output a pulse at 1kHz, you may want to investigate using a counter.

Regards,

Jennifer O

[Lao wan]

I think the choke point of the output frequency is the NI ELVIS DIO-Write.vi, but not the DAQCard's specifications feature. It also means that the write operation from the DAQCard to the NI ELVIS to output the signal need too much time, so this is the point that slowdown the output frequency! Is this rigth? thanks, very much!

[Lao wan]

Hi Jennifer:

I have solved the problem by your advice. Through bypass the elvis I can got a 2kHz digital output by using the DAQmax Write!

Thanks very much!!