SCXI Problem with bad TC, RTD, and voltage readings

[usaplanb]

Hi,

I have a data acquision program used to monitor and record various analog signals, and one frequency signal (counter input).  We are using a SCXI-1000 controlled by a PCI-6220.  The device has an 1102 card (with a 1303) for voltage and thermocouple signals, as well as a 1503 (with a 1306) for RTD's.  About 50% of the time, the system works fine and generates good data.  The other 50% of the times we run the system, we get garbage out on various channels.  It seems to be random which channels work and which don't.  Usually it's our temperature channels that give erroneous data, which will often show temperatures around -242 C or at the other extreme, 400 - 1000 C (this is at a time when they should be reading a bit above room temperature).

It's particularly frustrating because the problem will sometimes go away after restarting our computer or the SCXI chassis, or will return after simply stopping and restarting our DAQ VI, so it's difficult to know immediately the effects of any troubleshooting we do.  I use a DAQ Assistant task to configure all the analog inputs on the SCXI chassis. More recently, I've been able to (usually) fix the strange behavior by setting all my thermocouple inputs to have a "Constant" CJC Source.  Ideally I'd like to be able to use the built-in CJC source.  Any thoughts or insight into this behaviour would be greatly appreciated.

Thanks,

Sam

[jschwartz]

Hi Sam,

What changes from when your chassis measurements work and when they don't?  Are you changing any settings?  When does the VI stop measuring correct data?  How many DAQ Assistants do you have in your program?  Does changing the CJC to constant fix the readings for good?  

[usaplanb]

Hi Jim,

Thanks for the response.

Nothing appears to change on my chassis when the measurements work/don't work.  If it's working, sometimes I'll stop the VI (which erases any DAQmx tasks) and then start it again within a minute and it may start behaving strangely.  Sometimes a computer and chassis restart will fix the problem.  For the past couple days, I've been running it with all the thermocouple inputs set to have a fixed CJC value, and during this time I have not had the bad measurments show up at all.  If this sort of behavior continues, it seems that the use of the built in CJC would be causing the problem.  If this turns out to be the case, is there some way to implement a working form of non-constnat CJC, since the temperature where our SCXI chassis is located can vary quite a bit over the course of a day.

The VI has one actual DAQ Assistant in it which is used for an analog out signal on a separate USB device.  All the AI (thermocouples, RTD's, and voltage signals) are handled by a DAQ Assistant task defined and stored within the project, and then DAQmx task control VI's are wired to a task constant on the block diagram, which references the tasks from the project.  There's a total of 3 tasks being used in the project (1 SCXI AI task, 1 counter frequency measuremnt task, and an AI input task for a separate USB device).

Thanks,

Sam

[jschwartz]

Hi Sam,

Is there anyway that we could run the test with only the thermocouple module and the built in CJC to narrow this down to just one module?  You also might try switching PCI slots with your 6220.  I would also try switching SCXI slots for the 1102.  Do you get any errors when your data is junk?  Also, what kind of environment is this system in?  Could you provide some screenshots of when the data is good and when the data is bad?