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We are implementing an aeronautical system that's powered with a 28 VDC source and consumes around 20 W. There is an FPGA and many communication interfaces, such as CAN or RS485. We are a bit confused about whether to connect mounting holes to GND, CHASSIS or to simply leave them floating.

The chassis is going to be a metal enclosure. I have thought of connecting the four mounting holes in the corners to the chassis and then place a 0 Ω resistor between CHASSIS and GND next to a connector, so that the likely interference wouldn't affect the system much.

However, this seems like too easy for me, and I'm sure that I'm missing some concepts and key details.

Could you provide us some knowledge about this? I have read tons of different things on the internet.

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The chassis is normally considered as part of the shielding, but not connected to the active circuitry. But you also don't generally want "chassis" and "gnd" floating arbitrarily with respect to each other. A common technique is to connect them together at a single point with the parallel combination of a capacitor (10 to 100 nF, to couple RF signals) and a resistor (1 Mohm, to bleed off static charges and leakage current). The PCB and connector mounting holes are connected to "chassis", but nothing else is.

If you have any unbalanced analog connectors (coaxial), you'll probably need additional capacitors near their ground terminals to conduct the return current with minimal loop area.

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If you are designing a device which will be airborne during operation as your question implies ("aeronautical"), you should be relying on ARINC, or similar specifications. ARINC stands for "Aeronautical Radio, Inc", which is a long gone company. However ARINC is still an active organization which publishes many aircraft industry specifications, mostly related to airborne electronics and software. I have found these specs to be very well written and easy to understand (for people familiar with the topics). They often contain "COMMENTARIES" which delve into the background of the design & test requirements presented in the body of the specifications. I'm sure there is one related to grounding and wiring practices.

For your specific question, it really depends on what GND is connected to inside your enclosure. E.g. are there other metallic, non-circuit elements within the main enclosure like a chassis of an open frame power supply, shielding between circuit sections, and the like.

Also, there is a very common misuse of terms wherein "Ground" is often confused with "Common", sometimes with "Chassis" which can be short for "Chassis Ground". What are the names of the two 28VDC supply voltage feed wires? Is one of them is COM or COMMON, it should not be interpreted as "Ground" (aka GND).

If you ae designing this device according to a product specification or requirements document, the grounding topology should be spelled out clearly in that document. If there is no guidance in the design specification, you need to ask a lot of questions about the electrical environment in which your device will operate, including clarification and standardization of power signal names and functions.

The topic of grounding airborne equipment is too broad to discuss meaningfully in this context, a specific question such as yours could possibly be answered but more information is needed related to the signal names and functions (e.g. DC Common versus Chassis Ground, etc.) and whether there are ARINC or similar design standards referenced in the equipment specification or requirements document.

There is a pretty good overview of ARINC on Wikipedia and ARINC , of course, has its own website. For your situation, I would start by getting a copy of "ARINC 404 - Air Transport Rack" as I believe it deals with grounding of airborne equipment.

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Based on the information provided I’d suggest keeping the case/chassis isolated from the circuit. The reason for this is that it allows the 28V supply return current to flow through a designated path rather than whatever part of the airframe happens to provide a favourable path. In that context, pay attention to other ground connections that link to other devices as these could end up passing a significant return current even though they’re not intended to do so.

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