Hi julesjay, 
It looks like what we may be seeing here is that the DMA transfers from the FPGA to your RT target are taking priority of your controller's processor over the serial communication, resulting in a visa buffer overrun.  I would recommend taking a look at this knowledgebase article: http://digital.ni.com/public.nsf/websearch/D776A9D834B132AB86256ED2004E18EF?OpenDocument on setting the receive serial buffer size in the Measurement and Automation Explorer.  You can also take a look at this knowledgebase article: http://digital.ni.com/public.nsf/websearch/2D5F972CD550BBC386256F0B005E9A63?OpenDocument for more background information concerning this issue.  Hopefully this helps!

Hi Anna, 
 
I also think that the FIFO DMA  interrupts my VISA Read. But I've no idea how I can handle with this.
 
About the knowledgebase article:  
 
I cannot use Flow control because of the instrument I communicate with. 
And I've tried the change VISA FIFO buffer in measurement an dautomation explorer but no change either....  :-(
 
Since I'm back in ni-serial 3.0 I still have no error.
 
 

Hi julesjay, 
Hmm, it would appear that we could be looking at a bug in NI-Serial here.  In order to file a corrective action request to submit to R&D regarding this matter, I will need to reproduce the problem on my end first.  I was wondering if you could tell me what version of LabVIEW FPGA and what LabVIEW FPGA module you are working with, and if there is anything else specific to your set-up that you think would help me to replicate your system.  Thanks!

I'm working on labview FPGA 8.20 , Real time 8.2, RIO 2.1, Modulation toolkit 3.1, labview simulation 8.2, internet toolkit 6.0. 
I use a DMA FIFO of 8192  U32 that I read every 500 ms. I sent a message over RS232 periodically and the answer is about 170 bytes. 
 
Hardware PXI chassis 8196, pxi-7833R.

Hi Steven,
I've got some timed loops with different priority. But I've tried to set the highest priority for the RS232 loop and nothing changes.
regards