Slow FPGA-RT Transfer with sbRIO-9636

[drtdogger]

I have run into a problem using the sbRIO-9636. When transferring data between the FPGA and RT using the front panal read/write controls the data transfer rate is very slow. I have attached the example test code that I am running. So when I am doing just three variable writes, and one variable read the loop run time here is about 3.6 ms. When I run a similar test on the sbRIO-9606 board the loop runs at about 0.5 ms. Has anyone else run into this issue?

[MatthewL.]

Drtdogger,

Have you tried using DMA FIFOs to communicate between the RT and the FPGA? This is the preferred method to communicate between the RT and the FPGA and it has less overhead. I have attached a couple of links that should be helpful.

http://www.ni.com/tutorial/4534/en/

http://digital.ni.com/public.nsf/allkb/583DDFF1829F51C1862575AA007AC792

Matthew

[drtdogger]

Thanks Matthew-

I have use DMA FIFOs in certain instances, but for my current application I just want to transmit the "latest value" and don't need the queue. If needed I could use it here, but it would require quite a bit more programing to make it work (I'm transmitting about 12 values back and forth), and based on the information I've found online the FPGA front panel communication should be able to support rates much greater than 1 kHz. In your experince have you found the FPGA front panel to RT communication to be this slow? 

[Intaris]

Just one small comment:

The Input for opening up your bitfile is a network URL.  Is the device really remote or is it actually int he same device.

I have found that opening the reference to a bitfile with a simply ALIAS (i.e. RIO0) will perform significantly faster than using a URL.  The drivers don't seem to realise this is a local reference and get some network stacks involved int he communications which can really slow things down.

I have outlined my (unexpected at the time) findings (along with another unrelated topic) HERE.

[drtdogger]

Thanks, that fixed it! Yes, the FPGA was on the same device; I was switching between running on the computer and the RT, but now I know that when migrating the code to RT to make sure to remove the network URL!

[ch_novice]

The answer is fine ! ! ! thanks you  ! ! !