1520 only takes a single sample until reset

[Eric_G]

I am using a M-series 6251 PXI card with a SCXI 1520 card.  I am inputting <100mV directly into the S+ & S- on the 1314 adapter block.  I am then reading this signal with MAX test panel.  It appears that only a single initial data point is read when I hit "start" on the test panel since I change my input voltage, with no change on the signal being read.  The graph shows the initial voltage value, then will drift up over time.  For example, I'll send 20mV to the 1520 and hit "start".  The graph will display 20mV, then slowly start drifting up.  Without stopping, I'll increase the voltage to 40mV, with no affect on what is being displayed.  I can then stop the test panel & restart it, at which time it will read 40mV, with the same upward drift.  I am selecting "continuous" in the test editor.  I see this same behaviour in the Labview program that I am using to read the 1520.

Any thoughts on why I need to reset the card for a new data point to be taken?  My guess is that only a single sweep is being made upon reset instead of being continually updated. 

Thanks!

[TheWoost]

Hi Eric,

This is certainly strange behavior you are seeing.  The way in which the signals appear to float up from the first reading makes me think there is an issue with the referencing of your signal.  Are you using a benchtop power supply to provide these voltages?  Is the wire connected to the negative terminal on the 1314 terminal block referenced to the ground of your source?  There is another discussion forum post here with a closely related issue.  Let us know if this helps.

Regards,
Andrew W
National Instruments

[Eric_G]

Thanks Andrew.

We are using a Fluke Calibrator to input a mV signal across the S+ & S- input pins.  We have tried multiple channels as well as  different 1520 cards.  The input voltage doesn't seem to make a difference, as we can remove the voltage and not see a change in voltage.  We are also no longer able to read even the initial voltage, which we could previously do on 1 of the channels.  The channels that aren't receiving the voltage input are reading a solid ~80mV, uncalibrated.  Only the channel that we started inputting a signal demonstrates the noise & flotation issue, though when the signal is taken away the noise signal is unaffected.  We've also tried grounding the S- pin to the negative terminal pin on the 1314, also to no effect.  Do you have any recommendations of something else that we could try?

We are able to successfully use 1102 & 1112 cards, so we suspect the SCXI & PXI chassis are ok.  We are using a new 6251 PXI card to read the SCXI chassis.

[Logan_K]

You might want to elaborate on your system setup for us - what SCXI chassis and PXI chassis are you using, and which SCXI module is connected to your 6251? What other SCXI modules are in the chassis with the 1520? Have you tried removing all modules from the chassis except for the SCXI-1520 to see if any of them are affecting it? You might want to start with the SCXI-1112 - I suspect it may be part of the problem.

Regards,
Logan