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Using molded PC style D sub cables?

stevenclin

I'm New Here
I've actually just ordered a kit (not an RV, sorry!) and am starting to think about avionics, while I wait for the sheet metal to arrive.

From past non-aviation experience, it seems that the normal method of crimping each individual pin in the large d-sub connectors used by most avionics can be (a) tedious, (b) prone to some subtle intermittent connections that can be a bear to debug, and (c) kind of a hassle to change around, if you change your mind about things or install additional equipment later.

I have some experience with PCB design/layout, so I had a crazy thought. What if I designed one or more hubs myself (like the Approach Fast Stack but customized to exactly the equipment and interconnections I want). But then rather than getting custom harnesses made, or making them myself, why not use something like these cables for all the various D-Sub connectors (DB78/HD78 example here)?

https://cablesondemand.com/78-pin-h...cable-copper-shielded-male-male-cs-dsdhd78mm0

Seems to me there are pros & cons:

Pros:
* Relatively cheap and easy to obtain / replace if needed, available in multiple lengths.
* The cable will likely be much better made than I can fabricate myself (i.e. seems unlikely to have any intermittent connections, poor crimps, loose pins, etc.).
* In the cables linked above, each wire is individually shielded and then the drain wire is connected to the dsub housing, which seems acceptable.
* The cables linked above are also UL94-V0 flame resistant.
* If I need to change interconnections or add equipment later, I just need to re-layout the PCB in the "hub"; no need to be tearing apart wire harnesses.

Cons:
* The wire in the linked cables above is AWG28. Garmin (for example) recommends AWG22 or heavier.
* For some connections - CAN bus and thermocouples for example - I will still need to break them out close to the avionics, since these signals aren't suitable to run the length to the hub and back.
* It's not clear to me whether the shielding of each wire individually would be worse off than a single shield around a pair or 3 wires, as is normally done in avionics wiring. It doesn't _seem_ like most of the signalling is differential pairs or anything that would absolutely require twisted pairs.
* Flame resistant is different from non-toxic. Maybe an undue safety hazard in case of any electrical fire?

For the CAN/thermocouple issue, I was thinking of making a very small interconnect PCB with male Dsub on one side and female on the other, that brings the CAN/thermocouple leads out to spade connectors. That way I can connect the CAN bus and thermocouples near each piece of avionics, but the vast majority of other connections would go through the molded cables to the hub.

Is this crazy talk? Or would using these kind of computer-industry pre-fabricated cables actually be viable and useful? Thanks for any thoughts!
 
other plug types

I think the main concern will be the suitability of the wire type for aircraft use. Tefzel is common because it is abrasion and heat resistant. PVC cable gives off toxic fumes when heated though I don't know about the material in the harnesses you mentioned.

Rather than use D-sub type plugs, have you considered other plug types for the interconnect PCB? Mine has 0.1 inch pitch connectors and the crimping is relatively straightforward, with the right tools.
 

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You're totally overthinking this. Crimping D subs is really quite an enjoyable part of the build. Compared to all the grinding, deburring, edge finishing, priming etc. it really is quite pleasant. The problem with your pre-built cables is that you almost never have a single point to point connection. You come out of one device, and the wires go to a dozen other points. You need to build something custom. Pick up a DMC crimper (expensive, but you can always sell it at the end and get almost all your money back), a few positioning dies and a bunch of pins/housing components from Steinair and you'll be set.

T.
 
Extra connections mean more chances for failures.
Future changes might not happen.
If they do, it is not that hard to reposition D-Sub pins.
 
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