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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