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2815 switch card analog bus 0 has extraneous voltage present

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We have a PXI chassis with a 2800 switchblock carrier and 4 2815 switch cards installed.  A user was getting

unexplained resistance measurement errors when switching signals through the 2815.  After some investigation it was found that analog bus 0 (AB0) has a voltage present on it, > 10V as measured with reference to chassis ground.  AB1, AB2 and AB3 do not exhibit this.  The measurement was taken at the connector on the face of the switch card.  The voltage appears as soon as the PXI chassis is powered on.  What is the cause of this voltage ?  Is it part of the self-test circuitry ?

We tried two different 2800 switchblock chassis and several other 2815 switch cards.  All showed the same thing.

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Hi Smully,

 

This voltage is being caused by a bug with the FPGA.This is what spurred adding the below caution to the specifications.

 

Caution: Always disconnect or turn off power sources before powering on achassis.

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I'll buy that and assuming you've added the information to the specs is it safe to say that it is not going to be corrected any time soon?

By the way another feature we've discovered is that when you enable the Fixture Interlock (500 ohms between the IE pin and Gnd on the 2815 ) the voltage goes to 0.

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Yeah I don't imagine that voltage behavior on startup will change.

 

It sounds like there might be an issue in connecting the safety interlock resistor.

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Perhaps I should give you a little background.  We have been putting these switch modules in a 2800 chassis into

our systems since before 2011.  We have probably shipped 16 or more of these systems. We have never had an issue with any of them.  The only reason this came up was because one programmer was getting strange resistance measurements when going through the matrix.

He was debugging his fixture when he came across the voltage on the Analog Bus 0 pin.  After adding the fixture interlock jumper the voltage went to 0 and his resistance measurements started behaving normally.

 

Is this the way these switch cards have acted all these years and we are just now seeing it?  There must be thousands of test programs running on these systems.  By the way we've sold all these systems exclusively to a large military contractor.

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Sorry I meant to include this link in my last post: https://www.ni.com/en-us/support/documentation/supplemental/10/ni-switchblock-hardware-architecture....

 

I wouldn't expect anything to have changed since then. It sounds like to troubleshoot the interlock was enabled which would require that jumper in place.

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