Re: Problems with at-mio-16e-10

Hi Jim,

On Mon, Feb 13, 2006 at 01:28:07PM -0700, Jim Benson wrote:
> On Mon, 13 Feb 2006, Daniel Nilsson wrote:
> 
> > Based on the source for the kernel driver I come to the conclusion
> > that this is the FIFO buffer on the HW that is getting full. Since the
> > buffer is 512 bytes and I sample at 10kHz I calculated that the buffer
> > needs approximately 25.6ms to fill up (assuming 2 bytes per sample). I
> > seems like a long time to me taking into account that the HW will
> > interrupt the kernel to empty the buffer, which should take that long?
> 
> With these cards, I think when one says sampling at 10kHz,
> it means you are doing a scan of all channels you are sampling
> at 10 kHz. So if you are sampling 4 channels _at_ 10 kHz I think you get:
> 
> 10000 scans/sec * 4 channels/scan * 2 bytes/channel = 80000 bytes/sec
> So if your fifo is 512 bytes then:
> 80000 bytes/sec * 1 fifo/512 bytes = 156.25 fifo/sec
> or 1 sec/156.25 fifo = 0.00639 sec/fifo
> So there is only 6.3 ms to dump the fifo.

Well, I was wondering about this as well... From looking at the
command though, the command is actually adjusted so that the 
scan_begin timer is set to 400000ns (400us). Sampling at 10kHz gives a
100us interval between each sample and since I'm sampling 4 channels
the command is adjusted so that a new scan starts every 400us. Just to
convince myself I instrumented scope.c to measure how often a chunk of
20000 bytes was received and the answer is every second. That makes
since since 10kHz sampling gives 20000 bytes per second. So I'm pretty
sure my initial number of a full FIFO every 25ms is correct, do you agree?


> > Should I expect to be able to sample at 10kHz using this card? How can
> > I figure out why the FIFO is not emptied?
> > 
> 
> I don't know...though a few years ago I was running a
> NI AT-MIO-16E-1 (yup the 16E-1) with a 450 MHz machine,
> with RTAI. My scan rate was 200 kHz with 4 channels. The
> 16E-1 however has a 8196 byte fifo. When I switched to
> the PCI version of the card, which does only have
> a 512 byte fifo, my 3 GHz machine with RTAI
> was not fast enough, until I went to DMA.

Hmm, discouraging... I need a faster machine then, or sample slower
(which is actually a possibility for me). New HW is a little too
expensive though for this little project of mine...

Thanks
-- 
Daniel Nilsson

Received on 2006-02-13Z21:44:18