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I'm trying to get nav signals from my flight simulator to my hand held GPS. The GPS (an Aera 796) sits in a cradle with bare wires coming out that would normally be tied into devices and power behind the panel of a plane. Here's the install pinout diagram for hooking to a Garmin GTX 345 as an example: Hard wire installation in plane diagram

I have a USB to RS232 cable coming from the PC. The PC and sim software are configured to transmit nav data on COM1. The Blue and Yellow wires coming from the cradle are hooked to a DB9 connector and plugged in to the USB/232 coming from the PC.

Power to the cradle comes from a DC power supply plugged in to the wall, not from the PC. So the cradle Red and Black wires are connected to the power supply, not the 232 pins. Setup looks like this: RX and TX go to PC, but PWR/GND go to wall

The problem I'm having is that this worked great once, but every other time it either doesn't get a signal from the PC or gets it intermittently. For instance, yesterday I could get a one-time position update by manually touching the purple wire coming from the cradle to pin 6,7, or 8 on the DB9 terminator (the green one in the picture), but it didn't continually update. I would have to pull the wire out and reinsert, then the plane position would update on the Aera.

It did all work flawlessly one time wired as shown, which makes me think I got lucky and something isn't wired correctly. I'm a software guy, so all I know about electronics is that if I wire it too wrong something will smoke. Any thoughts/advice are greatly appreciated.

Eric Christoph
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RS-232 does not care if power comes from somewhere else.

However, data transmission needs a common ground reference to work.

The two wires might suffice if the devices had a common ground like through power supplies, but some power supplies have floating outputs.

You need to connect grounds together.

Justme
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  • Sounds awesome! Any idea how I would go about doing that? I need 5yo type instructions, like "connect the purple wire to pin 8 on the DB9". Thanks very much for your help!! – Eric Christoph Jan 29 '23 at 15:07
  • Also, this is the power supply:https://www.amazon.com/dp/B07VQGHSWY?ref=ppx_yo2ov_dt_b_product_details&th=1&pldnSite=1 – Eric Christoph Jan 29 '23 at 15:10
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    @EricChristoph You can connect a wire (ideally black but any colour will work) from pin 5 (GND = ground) on the DB9 breakout board (actually DE-9) to share the connection with the black wire in the power connector at the top right of your picture. You'll need to make sure both wires are held firmly in the power connector. – Graham Nye Jan 29 '23 at 15:48
  • @EricChristoph Note your Garmin diagram shows this ground connection by showing the ground or GND connections on each device being connected to an earth symbol. All the earth symbols in a diagram are assumed to be connected to each other. – Graham Nye Jan 29 '23 at 16:00
  • @GrahamNye ...what do you think happens if I disconnect the black wire from the power supply and instead attach it to Pin 5 on the DB9 terminator? – Eric Christoph Jan 29 '23 at 17:05
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    @EricChristoph Then your power supply will not power your device any more as it is missing a power supply. Don't disconnect it. – Justme Jan 29 '23 at 17:07
  • Cannibalized an old power cord for the ground, and hooked pin 5 up to the power supply. Absolutely glorious, thank you both! Last question: what do I do with the 5 wires just dangling around? Can I put those in the open pins to keep them out of the way, or will that cause a problem? Hate to cut them in case I ever want to use the bare wire cable for something else... – Eric Christoph Jan 29 '23 at 19:58
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    What open pins? Don't connect the unused wires to the RS232 adapter or you might fry something. Also don't cut the wires then if you need them. Tie them up neatly, maybe insulate the ends so they don't make contact. If you wiring bothers you that much, buy a plastic project box to have connectors on a box and wiring inside the box tied up. – Justme Jan 29 '23 at 20:05