Multifunction DAQ

cancel
Showing results for 
Search instead for 
Did you mean: 

cDAQ Chassis Power C Terminal

Hello,

 

The user manual for the cDAQ-9185/9189 Chassis says on page 10 that "the C terminal is not connected to chassis ground," referring to the power supply connector for the chassis. This same sentence is included in other documents for the chassis as well. We did a resistance measurement on the C pin and we found that this sentence is incorrect; the C pin is shorted to the chassis ground screw (at 0.2 ohms). We did this measurement on multiple chassis of both the 9185 and 9184 varieties. We even removed the PCB from one of the chassis and the C pin was still shorted to chassis ground. Whether intentional or not, it seems like this connection is present, making the user manual and other documentation incorrect. We did base our design on the fact that the C terminal is isolated from the chassis ground, and now we'll need to do some redesign. Just want to point this out to others to prevent inadvertent grounding in future designs.

 

Can NI confirm that the C pin is connected to Chassis Ground and can NI update the documentation?

 

Thanks,

Gabe

0 Kudos
Message 1 of 6
(2,547 Views)

Just checked on my 9185, there is no continuity between C and the chassis.

Make sure all wiring is disconnected from the chassis and modules, perhaps you are introducing a ground loop in your wiring.

Note that there is continuity between C and the Ethernet/PFI 0 outer metal housings.

 

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
~~~~~~~~~~~~~~~~~~~~~~~~~~
0 Kudos
Message 2 of 6
(2,529 Views)

If you measure between the C pin and the chassis metal, you won't read a short because the metal is painted over and anodized. If you measure from the C pin to the chassis grounding screw, it is shorted. It is also shorted to the DB9 shells. I did this measurement with no modules or cables connected. In fact, I did this measurement on the bare PCB!

 

Can you  please confirm this, AK2DM?

 

Thanks,

Gabe

0 Kudos
Message 3 of 6
(2,525 Views)

I just checked continuity between the chassis input power supply 'C' (Negative) input pin and the chassis ground screw on a cDAQ-9188XT chassis and found them to be shorted.  The 9188XT User manual explicitly states they are NOT connected.  From page 1-8 (last paragraph) of the manual "The C terminal is not connected to chassis ground.  You can connect the C terminal to chassis ground externally".  I took this measurement with the chassis completely isolated from my system i.e. no other external connections.  This is not what we expected!  Can anyone else confirm?

Thanks.

Message 4 of 6
(2,518 Views)

Gabe:

 

Ohm my, I stand corrected. I had lost the ground screw so I probed the case in which you are correct- the anodized prevent continuity.

So I probed the female threads and also pierced the anodized- continuity was confirmed to the C terminal.

 

Thanks for the follow up.

 

-AK2DM


@gausfresser wrote:

If you measure between the C pin and the chassis metal, you won't read a short because the metal is painted over and anodized. If you measure from the C pin to the chassis grounding screw, it is shorted. It is also shorted to the DB9 shells. I did this measurement with no modules or cables connected. In fact, I did this measurement on the bare PCB!

 

Can you  please confirm this, AK2DM?

 

Thanks,

Gabe



@gausfresser wrote:

If you measure between the C pin and the chassis metal, you won't read a short because the metal is painted over and anodized. If you measure from the C pin to the chassis grounding screw, it is shorted. It is also shorted to the DB9 shells. I did this measurement with no modules or cables connected. In fact, I did this measurement on the bare PCB!

 

Can you  please confirm this, AK2DM?

 

Thanks,

Gabe


 

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
~~~~~~~~~~~~~~~~~~~~~~~~~~
0 Kudos
Message 5 of 6
(2,516 Views)

Thanks for the confirmations. I opened up a service request and submitted a bug report to NI on the cDAQ chassis documentation.

Gabe

Message 6 of 6
(2,456 Views)