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

From: Naoko Kurahashi <naokok_at_stanford.edu> · Date: Tue, 15 Jun 2004 15:31:52 -0700

Hi,

Thank you for your advice. I adventured my way through the sourcecode
ni_mio_common.c. (I'm still confused about the difference between high and
low data which are divided after ni_readl)
I've been trying XOR-ing as you suggested. Since 'data' is 16bits, I tried
data^0xffff and this changes negative values to positive values and gives
something between 2048 - 4096 (upper half of my 4096 resolution, not lower),
but I still don't know how to change it to the comedi equivalent of negative
voltages (0-2047 in my case). also, if I do this operation to 'data', if the
input was truely positive, the value of 'data' again shoots up to the
63000's. Is there a good way around this? Did I understand your advice right? 

I don't have a formal background in signal processing or engineering for
that matter, which means I'm misunderstanding many things, but I thought 2's
complement data format means that I have to subtract 1 before I invert the
1's and 0's. Is this still the case here?

Thank you for your patience in advising me. 
Naoko

Quoting Bernd Porr <BerndPorr_at_f2s.com>:

> 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/
>          http://www.linux-usb-daq.co.uk/
> Mobile: +44 (0)7769 506746
> Work:   +44 (0)141 330 5237
>          Department of Electronics & Electrical Engineering
>          Room 519, Rankine Building, Oakfield Avenue,
>          Glasgow, G12 8LT
> 
> 
> 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
> > 
> > _______________________________________________
> > comedi mailing list
> > comedi_at_comedi.org
> > https://cvs.comedi.org/cgi-bin/mailman/listinfo/comedi
> 