NI-PCI 8431

[alex62]

Hello,

I have a problem of writing frames with fixed frequency 200Hz(5mS) using the NI PCI-8431/2  and  LABVIEW 2013.

I cannot get frames with constant frequency 200Hz(5mS). The frequency is changed during communication.

Why?

Please open enclosed vi and scope picture from the port output of the NI-PCI-8431.

Thanks.

[Dennis_Knutson]

You have a baud rate of 1Mb. What do you mean by a frequency of 200 Hz?

[alex62]

I want send 16 bytes(in this case 16"1")  -  this is my frame - with constant period of 5mS (200 Hz).

Thanks.

[Dennis_Knutson]

Are you talking about the wait function in your loop? That is not at all deterministic on a Windows os. You would need a real-time os to send this.

[alex62]

Ok.

But the time cannot change so : 5mS,10mS,20mS... Please take a look to the picture from the scope.

I can change the While Loop to the Timed Loop,but  I will have the same result.

Thanks.

[Dennis_Knutson]

Of course it can change. The time between iterations will be a minimum of what you set because between iterations, the os will be doing whatever it wants to do. That is strictly a function of a multi-threaded, non-deterministic os. A timed loop is no better. It has nothing at all to do with LabVIEW. You will see the same uncertainty with any programming language. Either switch to LabVIEW real-time or live with the small amount of jitter.

[alex62]

OK.Thanks.

What do u mean :"LABVIEW real time" ? Do  I have to use some of  real time  NI  board ?

[Dennis_Knutson · Accepted solution]

No. It's an alternative operating system to Windows.

http://sine.ni.com/nips/cds/view/p/lang/en/nid/11766

[alex62 · Accepted solution]

Thanks a lot.

I will think about  real  time LABVIEW module.

[crossrulz]

For LabVIEW RealTime, you will nee an NI RT controller.  These are most common in cRIO or PXI.  So you may not be able to use the PCI-8431 anymore.  Possibly the PXI version of that card if you use a PXI RT controller.