Re: negative voltage reading problem with cmd.c and ni_pcimio

From: Bernd Porr <BerndPorr_at_f2s.com> · Date: Tue, 15 Jun 2004 10:15:51 +0100

Hi!

Seems so that the sign bit of the converter is not interpreted in the 
right way. Comedi uses unsigned data but some A/D converters give the 
data in two-complement form. The driver should "undo" this by XOR the 
appropriate bit(s):

comedi_buf_put(s->async,(this_usbduxsub->inBuffer[i])^0x800);

Might be a bug in the driver.

/Bernd

www:    http://www.berndporr.me.uk/
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Naoko Kurahashi wrote:
> Hi, 
> I've been battling a weird problem today of not being able to acquire
> negative voltage values with my NI PCI MIO card. When executing the cmd demo
> program, for example, every time the input voltage is < 0, the acquired
> value shoots up to 63000 or so (max res for my card is 4096). I have my
> range set to 3 (bipolar +/-1V). when I set it to unipolar 0-5V, the negative
> values just give 0 and everything looks fine.
> 
> Maybe my problem is hardware related, or I'm just missing something very
> simple because I'm very tired, but I am out of things to try. Please let me
> know if you have any suggestions or have experienced similar problems.
> 
> Thank you,
> Naoko Kurahashi
> 
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