Ethernet affecting measurements on Stand-alone compactRio

[iamnewhere]

Dear all,

I have CompactRio cRio 9024 with Modules 9205 and 9217. CompactRio is running independently without connecting to a PC.

I have a RTD resistive thermometer, it is a 2-wire PT1000. since 9217 does not support 2-wire RTD, so I use 9217 as a current source, and use jumpers to NI9205 to measure the voltage across the wires, therefore calcuate the resistance of the sensor. (Resistance = measured Voltage from 9205/ constant current from 9217 1mA)

Besides RTD, I also have other devices connected to NI9205, which differential voltage is measurred recorded. Everything works fine before PT1000 is connected. 

The problem is, somehow the Ethernet is affecting the all measurements on NI 9205. When Ethernet is connected to PC(PC does not necessary to be turned on, just simply plugged in), everything would work fine. when Ethernet is not connected, all voltage measurements would go down and even go to negatives. I don't understand how it could be possible, that Ethernet could affect the system.

Any ideas? Maybe there is another way to measure 2 wire PT1000, or how could I remove the Ethernet Bug?

Thank you all in advance!

Best Regards,

iamnewhere

[nathand · Accepted solution]

I would check for a grounding problem. Sounds like your ethernet cable is acting as a ground, which is not good. Is the cRIO otherwise grounded somehow?

[iamnewhere]

Thank you so much, it was a grounding problem. The ground wire has worn out! I replaced the groud wire head, it worked fine now. 

I never though of Ethernet Cable could act like a ground. I also noticed a small difference between PC-Ethernet ground and CompactRio wired ground. When they are both connected, PC-ethernet groudn dominates.  That means all my measurements when CompactRio was connected to PC were off!