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As you know it is hard to pin point the source of the noise occuring on a line between a device and the PC. It sounds like you have taken the right steps to start to reduce the noise by using a shorter cable as well as assested some of the envirnomental variables. These new pieces of equipment in the facility could very well be causing the spradic noise that you are seeing. To futher reduce this noise I would recommmend, if possible, changing your wiring from reference single ended to floating differential (as shown on page 12 of the manual). Other tips to reduce noise can be found in the "Solving Noise Problems in Measurement Setups" section of the Field Wiring and Noise Considerations document linked below:
Hopefully this points you in the right direction to reduce your noise and let me know if you have any questions/comments.
Ok, thanks. The shorter cable is doing the trick for noise, so I am not going to change anything else there. I am more concerned with the 5.9% DC offset that appeared all of a sudden, and then disappeared the next day. It might be indicative of a 9215 module about to go bad, but I don't have any hard evidence for that guess - the modules seem pretty bullet-proof. I have taken some voltage readings at various places throughout the system case this happens again. I will re-post if any new information comes my way when I find out for sure what the problem is/was.
It looks like the failures were a result of a connected PXI-6704, not the cRIO module itself.
The error came back for the 4th time in about 2 years but this time did not clear. We went to swap out the PXI-6704 board (input to cRIO module) with a known good one and saw some pretty spectacular board-level damage. It is clear that a small area of the PXI board was on fire at one point, and there are other areas of probable heat/current damage.
The damage appears to be centered around a blown diode, but we are not sure of the exact cause. There does not appear to be any damage to the PXI chassis itself or the adjacent cards. We are forced to assume the board just gave up the ghost. It is amazing that the only indication that there was a problem of this magnitude during operations was a very rarely incorrect voltage output from the card.
I will post again if this DC offset error ever comes back once the card has been replaced.