Signal Conditioning

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Bad REadings

I found out that the virtual channel system is kind of problematic with my system. One of the NI engineers recomended me to use those, specially because they perform CJC on every reading automatically, and I did. But I wasted a lot of effort just to discover that the AI Read VI doesn't update the CJC on every reading, what is done only in the setup.
Here is the spec of my systgem:
- SCXI-1000
- SCXI-1200 connected in the parallel port and controling the chassis, with connector block 1302, and using its counter resources.
- SCXI-1163, controling a few switches with connector block 1326
- SCXI-1120, reading T thermocouples, with connector block 1327.
- LabVIEW 5.1f1
- NI-DAQ - 6.9.1f28
- IBM Personal Computer 300 GL, with Pentium II 266 Mhz.

I dicover
ed that the Virtual Channels weren't that good using the "SCXI Scan VI", provided with the package. I configured a single virtual channel and started an acquisition, returning me NaN. Then, using the SCXI Scan VI and configuring everything manually, I could get some results. I have tried so many things to get it working that the possibilities are almost over.

Anyway, the error I get most is code 10800. I tried to slow down the Scan Rate, increase the samples/channel, increased the time limit, checked all jumpers, reduced the gain, set the same gain for all channels, connecting and not connecting "-" channel to ground, tried to split the daq into 2 readings of 4 channels instead of 1 of 8, (and maybe some other things I can't remeber now) and nothing seems to work. The other modules work well. Also, it's important to say that I got the same DAQ problems with the VIs provided with LabVIEW. A funny thing is that some times, the DAQ works, sometime, it doesn't. Of course, I am comparing
in exactly the same conditions, i.e., turning everything ON, then first loading LabVIEW and starting a acquisition. This is really strange.

I hope anyone can help me!


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Since you are getting a time out error (10800) it would worth to check that the SCXI-1200 is responding. In the measurement and Automation Explorer, select the SCXI-1200 under devices an interfaces, right-click and go to properties. Click on "test resources" and you must get "The device has passed the test" If not that could be one possible reason of the problem.

Can you get readings directly from the SCXI-1120 by using the SCXI string? This does not mean necessarily the CJC sensor:

"You need to use SCXI channel strings to communicate with other modules in the SCXI chassis. The general format of the SCXI channel string is OBz!SCx!MDy!a, where z is the device number of the DAQ board connected to the chassis, x is the SCXI chassis number, y is the module nu
mber, and a is the channel number (use a:b to scan multiple channels). The SCXI channel string is not used for the SCXI-1200 because it is its own DAQ board."

The Virtual channels should not represent a problem, that is why is important to make sure that teh system is working fin without them too.

Thanks for contacting National Instruments!

Arturo Q.
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