Data channel switch in FPGA application

[zwfjames]

Dear all,

I am using cRIO 9074 and NI 9234 to collect vibration data with FPGA mode. However, I found the data I collected has some channels switched. For example, I have two NI9234 modules (8 channels). I found first channel sometimes have the data value which should be in channel 8. It happens randomly. And also, the channel swich happens between thounsands of data points collected (sometimes around 50000). I think the switch happens in the Real time VI when I read FPGA data out from FIFO. I used network stream structure to stream data from Real time controller to Host computer. In the host computer vi, I can save the data when I need to. 

The screen shot I attached is the among 62123 samples, there are 3 samples shows the other channels' value. 

I attached my programs. Hoping anyone here can help me out. Thank you.

[bjoa]

Hi.

I think I've the same problem. The data switching change constantly during execution time? Try to read elements in number that is divisibly by the number of channels.

BJoa.

[zwfjames]

Hi, Thank you for the reply. In the real time VI, I read the elements number equals the samples per channel times the channel numbers. It did not happens periodically actually. 

[bjoa]

Then you've the same problem? http://forums.ni.com/t5/LabVIEW/myRIO-FIFO-Data-Switching/td-p/3083894

I tried everything and didn't figure out the reason.

BJoa.

[zwfjames]

My channel switch is almost invisible from the the screen. You can only find it on the data that was collected. However, when you watch the indicator, the value jumped to other channel in less than one second, then it jumped back. 

[bjoa]

Can you make a VI Snippet where you read the buffer? I solve that problem reading the number of elements divisible by 4 (I've 4 channels) and leaving the remainder on the buffer.

BJoa.

[zwfjames]

Here it is. I will try your way to read the data. See what will happen. 

[bjoa · Accepted solution]

Think see your problem. The "Greater or Equal" don´t ensure that the number of elements is divisible by 8. The way I will doing this is check the number the elements on the buffer, divide by 8 and verify if the reamainder is not equal zero, if true read the number of elements minus reminder, if false read the number of elements per se.

[zwfjames]

Yes, I think your way solved the previous problem. However, the same problem you posted in the link also happened to me. The channel constantly switched. 

[JÞB]

Why do you need the compare or select?  Just subtract the remainder from the dividend.  (what happens when you subtract 0 from a number?)