Harness assembly part 9 (more connections!)

I don’t have a lot in the way of introductory text to put in here, much as I might like to do my usual summation. I started off today by moving on up the harness to the first of the Skyview display connectors. One new thing to work with here vs the connectors from yesterday was the need for split power/ground leads. The Skyview displays have two pins each for power and ground; not for redundancy or anything, I guess just to distribute the current a bit or something.

Whatever the case, when I pulled the harnesses, I just pulled a single power/ground pair to each connector; that meant that today I needed to do a couple window splices for each connector to get the split. Since I was already using printable heat shrink to mark each wire, I just positioned the tube for these wires to cover up the splits.

Hey look, a nice little forest of D-sub sockets!

These connectors are also unique in that they’ve got a couple prefab items attached. From the prefab harnesses that came with the displays, I’m reusing the backup battery connectors, as well as the USB connectors (these are used for software updates and such). So I had to de-pin those from the original connectors and add them to the new ones I was building up. For now, those leads are just sort of hanging out on the table. The battery leads will stay that way (though I may lace them together or something), but I’m not quite sure about the USB leads. My general intent is to just have them hanging behind the panel where I can grab them if needed, so I’ll figure out the details of that once I get this harness into the fuselage.

Also, I probably should have gotten a photo of a completed connector, but oh well. Just imagine the ones from yesterday, but bigger.

Next up, the real fun: the connector for the Garmin GPS-175. This one was its own challenge for a couple reasons. For one, while I’m intimately familiar with the Skyview installation manual, the GPS-175 manual I need to read a bit more carefully. For another, unlike the relatively simple Skyview connectors, the GPS-175 uses a 62-pin high-density D-sub connector.

However, that makes it sound more intimidating than it really is. Out of those 62 pins, I only need to use 17 of them, and six of those have connections that are entirely internal to the connector. Everything else is for external instruments I don’t have, like an analog CDI/HSI, or to integrate with a larger Garmin suite.

Also, the Garmin connector backshells are really nice (and they should be for what Garmin charges for them). I especially like that all the wires are forced into a small central exit from the backshell; instead, you get a nice full-width strain relief clamp, even if it’s pretty empty, like in my case:

The completed backshell in this case mates with a bracket that will attach to the back of the rack mount. Unlike the Dynon units, where the connector is removed from the device to disconnect it, the GPS-175 mates with the connector simply by virtue of being installed into the rack. Here’s the completed connector:

After this, I got into the real challenge of this harness, which is the intercom connector. And immediately I was faced by a question I was unsure about: was the connector I bought for the SDS CPI2 audio output going to work? The CPI2 came with what amounts to a regular old audio aux cable, with TRS connectors on each end. That’d be great if I was going to plug it into an audio jack, but I’m not. Additionally, the CPI2 manual specifies that it will only output audio to one channel through this cable, though the channels can be tied together inside the CPI2 by way of a jumper on the control board.

Well, I figured that I could just buy a cable with a TS connector on one end and bare wires on the other, and that TS connector ought to effectively perform the same function as that control board jumper SDS talks about. Except today I started to doubt that I was right about this. It all depends on whether the CPI2 is using the tip or the ring terminal for the audio output; if it’s the former than my plan will work fine, if the latter, nooope. So I fired off an email to SDS to ask about this; hopefully I’ll get an answer on that for tomorrow.

I also realized that I needed to work out how I’d use the color-coding in the shielded bundles for the audio system. So I pulled apart a couple samples to see what color wires were inside, then sat down with my schematics a little more to work out which colors went where, and particularly to make sure my decisions were recorded. After all, I’m only going to do about half of these terminations right now…

Finally, I got started with the intercom plug terminations. The single wires were easy enough, and I got the shielded cables stripped back, with the solder sleeves in place to allow me to tie all the shields to ground at the intercom. I still have to figure out how I’m going to do that, though. So far my plan is to make my own ground bus out of some thick aluminum stock I have, and use small ring terminals to tie together the various grounds. I still have to figure out what that’s going to look like, though. I suppose that’s where I’ll pick up tomorrow.

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