How do you terminate shields to the SCXI-1520

[Exp_Design]

We will be using SCXI-1520, SCXI-1503 and SCXI-1540 and the appropriate terminal blocks.  I can not see any place to terminate the shields for each of these modules.  The modules will be connected to a PXI-1075 chassis with a PXIe-6259 card.

The SCXI-1121/SCXI-1321 had J10 which was used for shields at a company I previously worked at.

[Ali M]

Hello Exp_Design,

Will you be using the NI 1314 Terminal block with the NI SCXI-1520? Also, since the NI PXIe-1075 is not a combo chassis and only supports the PXI specification for both PXI and PXI Express modules, please clarify the chassis information.  

Best regards,

Ali M

Applications Engineer

National Instruments

www.ni.com/support

[Exp_Design]

HI Ali

Yes it will be a SCXI-1314.  I should have included the fact the PXIe-6259 will be connected to an SCXI-1001 chassis where the modules will be installed.

James

[Ali M]

As per the NI SCXI-1314 Universal Strain Terminal Block Installation Guide, you can use the Safety-Ground Lug. Please review page 3 of this user guide for the part location.

 Best regards,

Ali M

Applications Engineer

National Instruments

[Exp_Design]

Hi Ali,

Could you explain the meaning of "safety ground lug"?  I thought it would for hazardous voltages rather than for EMI or RFI noise.

I've tried using the "safety ground lug" on a SCXI-1327 which made my signal noisier.  Please explain why this happened on the SCXI-1327 and include an explanation why this is different on the SCXI-1314.

Thanks,

James

[Ali M]

The safety ground lug is connected to the chassis ground. A possible reason for the noise is if your signal is also connected to ground which would cause a ground loop. The NI SCXI 1314 is a strain gauge input terminal block, mainly used with floating signals (the voltage signal is not referred to an absolute reference, such as earth or building ground); if this is how you have signals connected here versus the NI SCXI-1327, then it would explain the behavior difference. Also, please review the following NI Developer Zone Tutorial for more information regarding field wiring and noise considerations for analog signals.

Best regards

Ali M

Applications Engineer

National Instruments

[Exp_Design]

Hi Ali,

I apologize for not stating at the beginning that all signals received will be floating source types.  It would not be possible for me to terminate the shields at the signal source.  I will terminate it at or near the chassis.

The SCXI-1327 has a safety hazard lug and a chassis ground.  The SCXI-1321 is the same.  I tried the safety hazard as an experiment and switched the cable back to the chassis ground.  I can only guess that the safety hazard lug did not follow the same path to ground as the chassis ground.  Was there a reason for the change from SCXI-1327 to SCXI-1314?

It would help my understanding if you can explain the ground path for the analog, digital and Safety Hazard Lug for the SCXI-1314.  Please do the same for the SCXI-1327.

Thanks,

James

[Exp_Design]

Hi Ali,

[Exp_Design]

Hi Ali,

I was hoping to get a reply on this.  It also sounds like this setup depends upon the PXI chassis and the SCXI chassis sharing the same power ground.  What problems can occur if they don't share the same outlet?

Thanks,

James

[Ali M]

Hello Exp_design,

Thank you for your response. For example, when you connect two terminals to two different household power outlet grounds, and a slight potential difference exists between the grounds, significant random noise can be observed in your measured signal due to ground loops. The resulting noise is due both to induced AC voltage (due to lead resistance between this potential difference), and to electromagnetic interference being picked up by the wire loop that acts as an antenna. Ground lugs can be used to attach shield wires to ground. In your statement, did you mean that you were observing a difference when the the PXI and SCXI chassis were connected using the same power outlet vs. different outlets?

Best regards,

Ali M

Applications Engineer

National Instruments