Internal Resistance on 2800 Backplane?

[chall]

I have a new NI 2800 switchblock with 2811A cards that I am connecting through a Virginia Panel ITA.  Before using the switches for my project I was testing the switches and I found some behavior that I did not expect.  I have a PXI 4072 Digital Multimeter connected to SwitchBlockDev5 c16 and c17.  When I connect c16 and c17 through another line as in the first attachment (c5 is the other line in this example) then I measure a ~200 ohm resistance (I was expecting something closer to 0).  When I run the lines through the backplane and connect the DMM leads there as in the second attachment (where the lines are connected on SwitchBlockDev6 c18) then I measure a ~168 Mohm resistance (I was expecting something closer to 0 or 200 ohms), if I switch the DMM leads on the second setup then I get -4.45 Mohms as my measurement.  Can someone explain what is happening and what the circuit looks like?  Is there some kind of setting I can make in software that will give me my desired values when I'm making these DMM measurements?

Thanks,

chall

[Frank-L · Accepted solution]

Chall,

The 200 Ohm measurement that you are receiving is likely because you are using the "SH96F-96M-RES" cable. There are two NI SwitchBlock cables, a "normal" cable and a cable with additional 100 Ohm resistors tied internally to the cable per column [two columns together  = 200 Ohms]. This additional 100 Ohm resistance is used to prevent inrush current. If you are using the "SH96F-96M-RES" cable and want to read 0 Ohms you need to exchange the cable with the "normal" "SH96F-96M" cable.

The 168 M Ohm resistance you are measuring is likely due to the Safety Interlock Resistor missing on your test fixture. If the safety interlock resistor is missing, connections between relay cards in your 2800 carrier will never connect.

[chall]

Thanks!  You were right, I had read about the Safety Interlock Resistor, but I had thought it would be built into the Mass Interconnect.  Your explanation makes a lot more sense, I added resistors to my fixture and everything is now working as expected.