Problems with continuos sampling and writing to disk PXI-6071E

[agolovit]

Robert,

The code runs for about a minute or less. The backlog builds immediately. Nothing is running on the PXI-8176 machine other than that code. I've tried anything from 1MB to 50MB buffers the only thing that changes is how quickly the code errors out due to buffer overruns. This behaviour occurs even without a disk write, but since I need to stream the data for at least 1/2 hour (which turns out to be approximately 3.6GB << 20GB harddrive) that is not an issue since I can get no more than 175 seconds into a buffer of 350 MB RAM buffer.
It seems that the spec of continuous streaming to HD at 1.2MS/s or even 1.0MS/s is not achievable with this hardware (i.e., 1.2GHz, 512Mb RAM, 20Gb HDD, PXI-8176 and PXI-6508).


Alex

[Robert M]

Alex,

I'm still not convinced. Have you tried the High Speed Data Logger example? It has some important differences from the code you sent me, even though it is more complex. How fast does its backlog backup? I�m going to set up the same hardware you have and see if I can reproduce your situation. Cheers.

Robert M
Applications Engineer
National Instruments

[agolovit]

Robert,

Functionally, I do not see major difference between my code and the High Speed Data Logger.vi. However, I did try it and it exhibits the exact same behavior as my code. I asked it to record 30MS (30 seconds of data). I set the buffer to 4M, Scan rate: 1M, File write (5k, 100k, 500k). All failed. What settings do you recommend for this to work?

Alex Golovitser

[Robert M]

Alex,

I agree, for what you are doing, your code and the High Speed Data Logger and pretty much functionally equivalent, but wanted to rule out any possible discrepancies.

I have set up the same hardware you are using and am attempting to reproduce your situation. The 8176 I am using only has a 10 GB hard drive and 128 MB of RAM. It is running Windows 2000. The code you gave me was able to run for about 45 min, and only stopped when my file size hit 4 GB and I ran out of space.

I was also able to sample at 1.25 MS/s for a while, as long as I wasn�t doing anything else.

I did notice that if I tried to run other applications in the background, the backlog crept up and sometimes ov
erran. Even unplugging my network cable caused the backlog to ramp up to 2 MB before going back down. Having anything else running in the background (i.e. antivirus scan) may be causing problems.

You mentioned having a PXI-6508. What are you using this for? Is it running something at the same time? Could it be interfering somehow?

I strongly suspect something else in your system/setup is causing the problem here, since I�m able to max out the 6071E�s sampling frequency with only a fourth of the RAM and half the hard drive. You may want to try reformatting/reinstalling your system/software.

I hope we get to the bottom of this. Let me know what you find. Thanks!

Robert M
Applications Engineer
National Instruments