PCI 6229 acquiring inaccurate voltage measurement

[WindResearcher]

Hi All,

I am attempting to collect some meteorological data.  My setup is a PCI 6229 connected to two SCB 68s which are of course going into connector 0 and 1. On connector zero I taking data on seven different instruments each setup as a differential.  However, the voltage being shown in LABView does not match the voltage coming from the instrument.  One instrument is outputting a four volt differential which is what is across the connections in the SCB 68.  However, when I run the LABView code I am acquiring a varying signal that is usually around one volt plus or minus .25 V.   This instrument has its positive terminal to AI7 and AI15 is grounded. 

I am using LABView 7.1 with DAQmx 8.9.  My current code is using the compatibility VIs, specifically AI Acquire waveforms.  We are attempting to gather data on 12 instruments at the same time, which is ran in a loop to save the data into temporary files which are then made into permanent files each night. 

I have already attempted to rewrite the code using the DAQmx subVIs and still acquire the same 1V signal.  I have done the self calibration in MAX and it passes the device self test.  When I run test panels in MAX setup as a differential on continues it shows about a -1.2 signal.

I have already tripled checked the wiring and I have checked the ground.  I am out of ideas as to why I cannot get this reading.

Thanks for any help,

WindResearcher

[Brian_P]

WindResearcher,

Thank you for using the forums.  I am sorry that you are reading an incorrect voltage on your PCI 6229.  I have couple of questions that would greatly help my understanding of the actual problem.

Are you only getting incorrect readings on AI7 or on all of your instruments? If it is only that particular channel that is getting the incorrect readings, have you been able to try that particular instrument on another channel?

A couple of over troubleshooting suggestions that I would try would be first to do a loopback test.  I would try to connect an anolog output channel to AI7 and then ground AI15.  I would then use a test panel to output a known voltage and then read that voltage back in on AI7.  Another suggestion to try is to bi-pass the SCB-68, if it is possible try to connect the instrument directly into the cable and take readings from there.

Hopefully this will help to narrow down what exactly is going wrong in the measurements.  Please let me know how the tests go.

Regards, 

Brian P.