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NI PXI 2800

Hi all,


I have a quick question / issue regarding the NI PXI 2800 block.  I have multiple 2834 cards loaded into the block and when I connect the cards together using the anolog buses on any of the rows I see almost 10 volts on the channel that is being placed there by the NI PXI Chassis backplane.  Is this correct or am I in-correctly connecting the cards via the anolog buses.  Should I be using something else to connect the rows for the switch matrix together?





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Message 1 of 13

Hi Andrew,


Can you describe exactly how you are making your connections?  A picture or diagram would be very helpful.



John R.
National Instruments
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Message 2 of 13

Found this thread because I just discovered the same problem (maybe).  With a 2834A in a switchblock and no cables connected, I can measure 11Vdc on AB0W0 - AB0W1 (pins E1 and D1).  I noticed this problem because a new test setup has a DMM connected to AB0. And if no other loads are switched onto AB0, the volt meter reads 11V.  I've swapped 2834A cards and 2800 carriers, same issue.


So, why is the 2834A driving 11Vdc onto AB0W0/1?

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Message 3 of 13

Hi bholsinger,


Even though you're correct that this problem seems similar to the OP, you may get more visibility from creating a new thread and just linking this one there. That being said, that does seem strange, those are all the analog bus lines, it seems for some reasons there's 11V on the analog bus somewhere and then that's getting shared with all of those pins. Does this go away when you switch another load onto AB0? You've probably seen these before but these talk a bit about how to configure the switchblock:





And this talks a bit about the analog bus lines:



The question is why is the switchblock being connected to 11V on the carrier without you having configured it that way. Do you have any other switchblocks? If so do they exhibit similar behavior in that slot?



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Message 4 of 13

I've checked three different switch block cards in three different carriers in two different PXIe chassis, different slots in the PXI chassis... All show same 11v on the AB0 line.


And yes, if I switch another voltage source onto AB0, the 11v goes away. Or at least, is overwhelmed. A non powered DUT net switched onto this bus will see bypass caps charging from the 11v, and prevents an accurate resistance measurement will a DMM.

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Message 5 of 13

Thanks for that information, how are you setting up the analog bus line in software, and can you give me a wiring diagram of how you have the source connected onto AB0? 


From the Switches help, it says to connect your device to the analog bus you should connect a row to the analog bus channel, is this what you're doing? At this point you've tested this with such a thorough combination of hardware I think either something is configured wrong, or this voltage is expected, which brings up the question of why this isn't documented. 

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Message 6 of 13

Today I received an answer from NI support, basically they don't guarantee the state of the switch I/O connectors when the switch relays are open. If that's really the case, I'm disappointed in the switch design. When the relays are open, there shouldn't be any measurable voltage, and a resistance measurement should give the highest possible reading for the switch isolation (whatever it is, 100k or 10M, etc.).  

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Message 7 of 13

To clarify, are you saying that there is continuity across what should be open relay contacts in addition to the voltage you are seeing on the AB lines or that the voltage is indicative of continuity?


Also, do you have a safety interlock resistor attached to the front of the 2834 card?



Alex W.
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Message 8 of 13

The 11Vdc is a voltage measurement from the PXI-4065 DMM that is (was) connected to AB0.  After disconnecting the cable from the front of the 2834A, I used a handheld DMM (completely isolated from AC power and the PXI system) to measure the 11Vdc was still present. On two different systems.

I got a screwy reading of resistance on AB0 due to the 11V.  But with the PXI-4065 DMM connected to AB7, there is no undesired 11V (good!).  But there is a 100Kohm resistance which seems to be an inherent property of the 2834A.  I don't like it, but since I know it's there, I can adjust all resistance measurements by taking into account the 100K shunt.  So far I have not found anything in the NI documentation to explain the 100K. Do you know anything about it?


Almost forgot... NO interlock resistor installed.

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Message 9 of 13

Would it be possible to try the system with a 500 ohm safety interlock resistor? It appears that the switch should not access the AB lines unless a resistor is installed. The link below has a bit more information on it.





Alex W.
Message 10 of 13