Understanding cFP gounding and isolation

[Anton]

From the documentation provided I have been unable to get a full understanding of the internal wiring of the cFP system, especially the relationship between grounding and isolation. Is there a diagram available that show the whole system from end to end illustrating this?

Some specific questions I have are: if I connect the shield of my thermocouple cable to the "c" terminal, where does the "c" terminal go? Is the noise somehow coupled to ground or do I need another wire to ground? And if I don't need the isolation on this particular module, will I get less noise if I do connect the "c" to ground?

Answers to these questions would be helpful, but I really do want to get a broader understanding as well.

[Mike S. @NI]

Anton,

The best diagrams I can offer are figures 3 and 5 of the operating instructions.

All of the common pins on the module are connected to the same reference. If you ground this reference, you would be limiting the common mode voltage rejection of the module. Because of this, you might start seeing erratic readings on all channels.

Regards,
Michael Shasteen
Applications Engineering
National Instruments
www.ni.com/ask
1-866-ASK-MY-NI

[Aaron_G]

Anton,

Let me try and provide a better understanding of the grounding and isolation in FieldPoint and Compact FieldPoint systems. Please start out by reading the KB article linked here: http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/597c2b4cecef6bb586256831005b913f?OpenDocument . Although this document was written prior to the release of Compact FieldPoint, it still provides a basis for the terminology used in the manuals.

FieldPoint & Compact FieldPoint systems generally offer isolation on a module per module basis. The current exceptions are the Dual Channel Discrete IO Modules; the discrete modules are isolated on a channel to channel basis (Dual Channel analog modules are module per module). For all IO modules, the IO stage of the circuitry (with electrical connections through pins 1-32, V & C) is optically and/or galvanically isolated from the digital circuitry that interfaces with the FieldPoint data bus that is shared between terminal bases/backplanes and controllers.

On FieldPoint systems, the controller and terminal bases (shared bus circuitry) float and do not provide the user the ability to easily ground the entire shared bus (along the length of the bus). Grounding the common of any IO stage can be accomplished through the use of properly rated isolation capacitors or varistors connected between ground and the common (C or Com) terminals. Depending upon the components used, you may or may not bypass the isolation barriers in grounding the C & Com terminals of a module.

On Compact FieldPoint systems, the cFP-BP-4 and cFP-BP-8 backplanes have screw terminals for the express purpose of grounding the system. Grounding the cFP-BP-X chassis ground, capacitively grounds the shared bus to ground. Unlike FieldPoint which requires external wiring to ground any IO module, the Compact FieldPoint backplane also grounds the IO stage of each module through properly rated isolation components. Thus, as long as you appropriately ground the backplane (per the instruction manual), you are grounding the IO modules as well. The isolation barrier between the IO stages of each module is maintained in a Compact FieldPoint system.

In answer to your specific questions:
1) The C terminal is connected to the ground terminal through components that are designed to maintain isolation barriers.
2) You should not need another wire. Proper grounding of the backplane is sufficient, please follow the Operating Instructions for proper wire gauge.
3) When a Compact FieldPoint backplane is grounded, the IO modules are also grounded via isolation components, thus this question is moot.

Regards,
Aaron

[Aaron_G]

Oops, I need to correct some mistakes and provide a little more clarification.

Correction 1:
Compact FieldPoint systems have the DSUB shells chassis grounded through isolation components, no thte C and Com terminals.

Clarification 1:
On FieldPoint system, you can ground the C terminal of the network module which will ground the bus. However, since the bus is only grounded in the network module and is not grounded along the length of the bus, there still exists the possibilities for the bus to develop minor potential differences along the length of the bus (through the 1-9 terminal bases).

Correction 2:
In answer 1, I stated that the C terminal is connected to ground, per correction 1 above, that is incorrect.

Clarification 2:
In regards t
o questions 1&3, you do not need to ground the IO module C or Com terminals. The IO modules are designed to have a floating ground which allows them to adjust to different potentials so long as the potentials are less than 250 Vrms sustained, 2300 Vrms transient.

Regards,
Aaron

[Aaron_G]

Oops, I need to correct some mistakes and provide a little more clarification.

Correction 1:
Compact FieldPoint systems have the DSUB shells chassis grounded through isolation components, no thte C and Com terminals.

Clarification 1:
On FieldPoint system, you can ground the C terminal of the network module which will ground the bus. However, since the bus is only grounded in the network module and is not grounded along the length of the bus, there still exists the possibilities for the bus to develop minor potential differences along the length of the bus (through the 1-9 terminal bases).

Correction 2:
In answer 1, I stated that the C terminal is connected to ground, per correction 1 above, that is incorrect.

Clarification 2:
In regards to questio
ns 1&3, you do not need to ground the IO module C or Com terminals. The IO modules are designed to have a floating ground which allows them to adjust to different potentials so long as the potentials are less than 250 Vrms sustained, 2300 Vrms transient.

Regards,
Aaron

[Anton]

Hi Aaron,

In your first answer you indicated that the C terminal is connected to ground (thought not directly). In your clarification you indicated that that is incorrect, however you did not provide the correct answer.

The apparent complexity of the issue leads me to repeat my suggestion to produce a clear and complete diagram for clarification.

Regards,

Anton