RE: Comedi with SC-2345 and SCC-TC0x modules

Hi Brad,
 
Thanks for responding.  I have already done everything you suggested.
The PCI-602x are 12-bit boards yet when I read the binary values, I get
values greater than 2048 and less than -2048 !?!?  So far, it would seem
that comedi is the problem since it is reporting values beyond what the
board should output.  I am using the suite of VIs from the NI website
that use CINs (Comedi60waveformVIs.tgz).  If you can think of any other
things I can try, please let me know.

Ching-Hwa Yu
Software Engineer
San Francisco Industrial Software
v: 415.743.9350 x145
f: 415.743.9351
e: cyu_at_sfis.com 

	-----Original Message-----
	From: brad.keryan_at_ni.com [mailto:brad.keryan_at_ni.com] 
	Sent: Monday, September 22, 2003 6:34 PM
	To: Ching-Hwa Yu
	Cc: comedi_at_comedi.org
	Subject: Re: Comedi with SC-2345 and SCC-TC0x modules
	
	

	There might be a tiny bit of magic, but not much: it acquires
from the SCCs in NRSE (nonreferenced single ended) mode and scales the
data. If you try to acquire from an SCC-TC0x in differential mode you
will be treating the thermocouple voltage as the positive terminal and
the cold junction sensor voltage as the negative terminal, which is not
what you want. Using NRSE mode will treat the thermocouple voltage and
the cold junction sensor voltage as separate channels (ACH(X) and
ACH(X+8), respectively). To make sense of the data you also have to
scale to compensate for the 100x gain on the SCC-TC0x, scale voltages
into temperature, apply cold junction compensation, etc. 
	
	       Brad Keryan 
	        Software Engineer 
	        National Instruments 
	
	
	
	"Ching-Hwa Yu" <cyu_at_SFIS.com> 
Sent by: comedi-admin_at_comedi.org 

09/18/2003 07:15 PM 


        
        To:        <comedi_at_comedi.org> 
        cc:         
        Subject:        Comedi with SC-2345 and SCC-TC0x modules



	Hi,
	
	Has anyone used comedi to read in thermocouple voltages via a NI
	SCC-2345 with SCC-TC0x modules?  I have an NI PCI-6024E running
comedi
	0.7.66 and cannot acquire the proper thermocouple voltages.
Comedi
	seems to be working fine reading voltages off a terminal block
but the
	SCC-2345 sends out incorrect values.  Is there some "magic" that
happens
	in MAX under Windows that doesn't translate over to comedi?
	
	Also, setting different input limits results in varying voltages
... a
	difference of .5V in some cases.  Anyone have any ideas as to
what is
	going on here?!?
	
	Any help is appreciated.  Thanks.
	
	Ching-Hwa Yu
	Software Engineer
	San Francisco Industrial Software
	v: 415.743.9350 x145
	f: 415.743.9351
	e: cyu_at_sfis.com
	
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